This Article Abstract Full Text (PDF) An erratum has been published Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Burns, K. H. Articles by Matzuk, M. M. Search for Related Content PubMed PubMed Citation Articles by Burns, K. H. Articles by Matzuk, M. M.

Minireview: Genetic Models for the Study of Gonadotropin Actions

Department of Pathology (K.H.B., M.M.M.), Department of Molecular and Human Genetics (K.H.B., M.M.M.), and Department of Molecular and Cellular Biology (M.M.M.), Baylor College of Medicine, Houston, Texas 77030

Address all correspondence and requests for reprints to: Martin M. Matzuk, M.D., Ph.D., Professor and Stuart A. Wallace Chair, Department of Pathology, One Baylor Plaza, Baylor College of Medicine, Houston, Texas 77030. E-mail: . mmatzuk{at}bcm.tmc.edu

	Abstract

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
Fertility in both sexes relies on complex physiological and molecular processes with many levels of regulation, and our ability to alter the mammalian genome using transgenic technology has greatly enhanced our understanding of these processes. There are numerous commonalities in human and mouse physiology, and the list of mouse models recapitulating recognized and idiopathic human reproductive defects is growing at an ever-increasing rate. In this review, we focus on genetic models of gonadotropin actions, summarizing features of transgenic mice that phenocopy defects in gonadotropin production and gonadotropin receptor responses seen in patients. In addition, we provide examples of mouse models with genetic alterations influencing pituitary FSH and LH production and their effects. These include: 1) transgenic mice with aberrations in steroid hormone, inhibin, and activin feedback pathways; 2) knockouts that demonstrate specific in vivo functions of pituitary transcription factors; and 3) models with alterations in other pituitary hormones, IGF-1, and leptin signaling pathways, which affect both the central and peripheral endocrine axis. What we have to learn from these and other models will continue to revise our conceptions of physiology, identify new targets for contraception, and improve our tools for understanding, diagnosing, and treating cases of human endocrinopathies and pathologies of the reproductive tissues.

	Introduction

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 

"Whenever man comes up with a better mousetrap, nature immediately comes up with a better mouse."

	Mouse Models and Human Gonadotropin Signaling Pathologies

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
THE HYPOTHALAMIC-PITUITARY-GONADAL (HPG) axis is fundamental to the endocrine control of gametogenesis in mammals, and because of its long-recognized physiological importance, many human mutations disrupting the normal function of the axis have already been characterized, and several relevant mouse models have been engineered. The pituitary gonadotropins, FSH and LH, are central to this endocrine communication. FSH and LH are heterodimeric glycoproteins each comprised of a common -subunit and a unique ß-subunit; functional dimers are synthesized and secreted into the circulation in response to hypothalamic GnRH. The human common -, FSHß-, and LHß-subunits are glycosylated 92-, 111-, and 121-amino acid peptides, respectively (1). FSH and LH elicit intracellular signaling pathways by binding to their respective G protein-coupled transmembrane receptors, FSH receptor (FSHR) and LH receptor (LHR), in somatic gonadal cells to regulate follicular development, ovulation, and steroidogenesis in females, and spermatogenesis, testicular growth, and steroidogenesis in males (2, 3). The human FSHR and LHR proteins are 695- and 701-amino acid glycoproteins, respectively. Loss-of-function and gain-of-function human mutations in components of the HPG axis have been described previously (4, 5, 6), and examples are given in Table 1 and illustrated in Fig. 1.

View larger version (19K): [in this window] [in a new window]   Figure 1. The HPG endocrine axis is illustrated with several key components of the signaling relay. Within the central portion of the axis, the actions of hypothalamic GnRH mediate the production of the anterior pituitary heterodimeric gonadotropins, FSH and LH. These bind to FSHR and LHR in the periphery to regulate gametogenesis and steroidogenesis. At each level of the axis, mutations in human genes are known to cause defects in the development or function of the reproductive system; examples are listed on the right.

In contrast to humans, in which no GnRH mutation has been identified, a naturally occurring 33.5-kb deletion truncating the Gnrh gene in mice (11) results in a model of hereditary hypogonadism (hpg), which phenocopies the HHG of patients with defects in GnRH production or responsiveness. Hypogonadism is also a feature of a knockout mouse model harboring a null allele at the glycoprotein hormone common -subunit locus; besides reproductive defects, the knockout mouse is hypothyroid and displays proportional dwarfism owing to loss of TSH function (12). These and other relevant mouse models with reproductive phenotypes are shown in Table 2. No mutations altering the amino acid sequence of the common -subunit have been described in humans. It has been hypothesized that a deleterious mutation in the human -subunit gene (GLYCA) would result in embryonic lethality because in humans (and not in mice) the -subunit is also shared with human chorionic gonadotropin (hCG) (4). Embryo-derived hCG maintains ovarian luteal cells, and is thereby required for pregnancy during the first trimester. Substantiating this, the most highly expressed hCGß-subunit-encoding genes are highly conserved, with no individuals homozygous for a deletion of the CGß cluster (13).

Loss of FSH signaling causes infertility in women (19), and this condition can be modeled by disrupting either the FSHß (Fshb) or Fshr loci in mice. Women who are homozygous or compound heterozygous for inactivating ligand (frameshift/truncation and missense) or receptor (missense) mutations exhibit normal preantral follicle development, but no antral-stage follicles capable of ovulation form. These women typically present clinically as cases of primary amenorrhea and sexual infantilism. Both Fshb and Fshr knockouts in mice phenocopy the human mutations in this regard, displaying female infertility, uterine hypoplasia, and folliculogenesis blocks before antrum formation (20, 21, 22). Although estradiol is present in the serum of these mice, the mRNAs encoding P450 side chain cleavage and P450 aromatase estrogenic enzymes are markedly down-regulated in the ovaries of Fshb knockout mice (23). Both Fshb and Fshr knockout females also exhibit high levels of circulating LH (20, 21, 22). In both models, this is a progressive effect, with less LH seen in younger mice than in older mice. It is believed that LH and LH-induced androgen synthesis in the Fshr model contribute to the development of ovarian sex-cord stromal tumors, which are seen in 92% of these mice by 12 months of age. In these tumors, there is loss of granulosa cell proliferation control programs, accompanied by an up-regulation of Sertoli cell markers, Müllerian-inhibiting substance, and GATA-4 transcription factor (24). In men, FSHß and FSHR mutations have been associated with variable degrees of impairment of spermatogenesis and sometimes with delayed puberty (4). However, men with FSHR null mutations can father children (25). Fshb and Fshr knockout male mouse models are fertile but have lowered sperm counts and decreased testicular size. Therefore, it seems that FSH activity, while necessary for optimal sexual development and spermatogenesis, is dispensable for male, but not female, fertility in both humans and mice.

A transgenic mouse model overexpressing high levels of human FSH has also been developed (26). The transgenic males demonstrate stimulated Leydig cell function, elevated serum testosterone, and infertility; females exhibit infertility with hemorrhagic and cystic ovaries. These mice may prove valuable in modeling gonadal and more global physiological effects of FSH hyperstimulation. Consistent with the high conservation of FSHß genes in mice and humans, synthesis of human FSHß under its own promoter in the gonadotropes of mice lacking the endogenous FSHß-subunit restores normal fertility in male and female mice (27). Moreover, partial rescue of the knockout phenotype was observed in mice ectopically expressing human FSH (human - and FSHß-subunits) under the direction of the mouse metallothionein-I promoter (27). Together, these studies demonstrate functional conservation between mouse and human FSHß promoter elements, as well as between the two FSHß peptides in terms of their abilities to assume glycosylation patterns, dimerize with the mouse -subunit, route to secretory granules independent of LH, and bind and activate the mouse FSHR.

Loss of isolated LH signaling function can arise from defects in the unique LHß-subunit or LHR. One loss-of- function missense mutation in the hormone subunit was identified in an infertile man presenting with low testosterone levels, Leydig cell hypoplasia, and elevated immunoreactive but functionally ineffective LH (28). Several LHR mutations have been discovered to cause defects in receptor activity of variable severity. Phenotypes range from micropenis and hypospadias in the case of hypomorphic alleles to male pseudohermaphroditism and female infertility associated with a barrier to preovulatory follicle development, ovulation, and luteinization (4). These conditions are partially modeled by targeted deletion of the Lhr locus in mice; homozygotes demonstrate normal prenatal sexual development but are infertile (29). Male Lhr knockouts have defects in testicular growth and descent, Leydig cell hypoplasia, and a block in spermatogenesis at the round spermatid stage. Female knockouts have underdeveloped ovaries and uteri, and their ovaries do not contain preovulatory stage follicles or corpora lutea; thus, the Lhr knockout in female mice very closely models the pathology of women with LH resistance. It is notable that hpg (GnRH mutant) male mice closely phenocopy the Lhr mutant male mice, whereas the hpg female phenotype mimics that of Fshb and Fshr knockouts at early timepoints (<6 wk of age) but reflects a combined FSH/LH signaling defect at later points. This observation is consistent with studies showing that androgen administration to hpg male mice can restore spermatogenesis (30). Interestingly, the androgens produced by Leydig cells in response to LH may not be as crucial to spermatogenesis as estrogens that are then synthesized from androgen substrates. Provision of exogenous estradiol to hpg males increases testis weight and rescues qualitatively normal spermatogenesis in the absence of measurable androgens (31).

Gain-of-function mutations of LHR have been described in families with autosomal dominant male precocious puberty (32). Although no mouse models phenocopy this disorder, transgenics have been engineered to overexpress the bovine LHß-subunit with a C-terminal extension of the hCGß- subunit (bLHß-CTP); there is a prolonged serum half-life of the chimeric LH hormone (33). These in vivo studies corroborate findings of Boime and colleagues (34, 35), who demonstrate that the presence of the hCGß C terminus of several of the glycoprotein hormone ß-subunits (e.g. hCGß, FSHß) extends the circulating half-life. Interestingly, female bLHß-CTP transgenics have a 10-fold increase in circulating immunoreactive LH and exhibit impaired ovulation, a prolonged luteal phase, ovarian cysts, and granulosa/theca cell tumors on some genetic backgrounds (36). In addition, in immature transgenic females, enhanced LH and steroid hormones cause precocious follicular development and vaginal opening (37). Studies of these transgenic mice may shed light on the roles of LH in polycystic ovarian syndrome and the development of postmenopausal ovarian stromal tumors, as well as identify genetic modifiers of these phenotypes (36).

	Mouse Models of Aberrant Steroid Hormone Feedback to the Pituitary

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
Gonadotropins promote peripheral steroid production by inducing the expression of gonadal steroidogenic enzymes; steroid hormones then produced feedback to negatively regulate pituitary FSH and LH production (1, 38, 39). This paradigm is supported by studies of mouse models with alterations in steroid hormone receptors or steroid biosynthesis that develop secondary anomalies in gonadotropin production. For example, knockout mice lacking an estrogen receptor, ER, which is normally expressed in the hypothalamus, pituitary and gonads, exhibit female infertility associated with anovulation and the development of hemorrhagic, polycystic ovaries by 20–22 d of age (40, 41). The disruption to estrogen feedback signaling results in overexpression of gonadotropin subunit mRNAs in the pituitary (42) and chronically elevated serum LH (43). High levels of circulating LH are key to the ovarian pathogenesis in the ER knockout mice, and treatment with a GnRH antagonist precludes ovarian cyst formation (44). In contrast, female mice lacking ERß, which has a relatively restricted pattern of expression, are fertile, and preliminary studies indicate that there is no change in serum LH (45). ERß signaling plays some role, however, in the regulation of the hypothalamic-pituitary-gonadal endocrine axis, at least in the absence of ER activity. Double knockout females lacking ER and ERß have higher serum LH levels that do ER single knockouts, and exhibit a different ovarian phenotype in which granulosa cells take on a Sertoli cell-like morphology and express Sertoli cell markers, Müllerian-inhibiting substance, sulfated glycoprotein-2, and Sox9 (46).

Male ER knockout mice are infertile because of a disruption of fluid reabsorption from the lumen of the epididymis, which is important for concentration and maturation of spermatozoa; increased pressure from the accumulation of fluid leads to testicular atrophy and degeneration of the seminiferous epithelium in mature males (47). This phenotype does not have an obvious relationship to the only clinical case of an ER mutation, that of a man with estrogen resistance caused by a homozygous nonsense mutation in ER (48). The 28-yr-old patient presented with incomplete epiphyseal closure and continued linear growth into adulthood. Biochemical analyses provided evidence of increased bone turnover, and bone mineral density was low. Serum estradiol, FSH, and LH levels were elevated; serum testosterone was normal. Interestingly, the patient also had impaired glucose tolerance and hyperinsulinemia, and ER knockout male mice have increases in total body fat and in serum cholesterol and leptin levels after sexual maturity (49). These findings underscore the importance of estrogens in the bone and lipid metabolism, and have important implications for the use of hormonal contraceptions and estrogen replacement therapies in women, and appreciating the effects of exposure to environmental estrogens.

Estrogen production is abrogated in the P450 aromatase knockout mouse model, the female reproductive phenotype of which closely resembles that of ER knockout mice. Female P450 aromatase knockouts have high serum levels of LH and FSH, are infertile owing to a block in follicular development and ovulation defects, and develop hemorrhagic ovarian cysts by 21–23 wk of age (50, 51). Male P450 aromatase knockouts develop progressive infertility characterized by an arrest in early spermiogenesis, germ cell apoptosis, high circulating LH, and Leydig cell hyperplasia (52). Heritable aromatase deficiency has been described in patients with mutations at the aromatase locus (CYP19) (53). In the absence of fetal aromatase (and therefore placental estrogens), there is an elevation of testosterone and androstenedione in both maternal and fetal circulations. This causes pseudohermaphroditism in homozygote females, and virilization of the mother during her pregnancy. In the absence of ovarian aromatase activity, young girls exhibit hypergonadotropic hypogonadism and develop follicular cysts; with the onset of puberty, the ovaries become enlarged and further polycystic and there is progressive virilization. In males, the aromatase deficiency is associated with macro-orchidism, high circulating FSH, LH, and testosterone, as well as hyperinsulinemia and abnormal plasma lipids. In both women and men, aromatase deficiency is associated with delayed epiphyseal fusion and osteopenia (53).

An aromatase-overexpressing mouse model has been developed to study the effects of enhanced conversion of androgens to estrogens on male reproduction. These mice carry a transgene expressing human aromatase under the constitutive human ubiquitin C promoter, which is active in multiple mouse tissues by embryonic d 15. These mice have cryptochidism with Leydig cell hyperplasia and disrupted spermatogenesis, as well as underdevelopment and defects of accessory sex organs. Serum hormone assays reveal elevated estrogen, reduced testosterone, and reduced FSH, with no change in average LH levels but a reduction in LH level variation (54). The cryptorchidism is likely due to a disruption of steroid effects during prenatal development. The etiology of the Leydig cell hyperplasia is less clear, though both cryptorchidism and exposure to estrogens have been associated with testicular tumorigenesis in mice, and cryptorchidism is a risk factor in humans (55). Leydig cell tumorigenesis is reported in a second aromatase-overexpressing transgenic model in which the aromatase coding sequence is expressed under the mouse mammary tumor virus (MMTV) promoter. The expression leads to Leydig cell tumors that express high levels of ER, and up-regulation of a G1S phase cell cycle promoter, cyclin D1, known to be modulated by estrogen exposure (56). The MMTV promoter in this mouse model also drives aromatase expression in the mammary glands and this results in preneoplastic lesions (57). Interestingly, vitamin D has also been recently shown to be important for estrogen biosynthesis in both the ovary and testis. Studies of knockout mice lacking the vitamin D receptor reveal attenuation of gonadal P450 aromatase activity, histological abnormalities of the uterus, ovary, and testis consistent with estrogen deficiency, and elevated levels of LH and FSH (58). Together, the phenotypes of these models indicate important in vivo roles for the sex steroid hormones as mediators of the HPG endocrine axis and also as cell cycle controllers and differentiation factors in hormonally responsive peripheral tissues in both males and females.

	Peptide Hormone Feedback to the Pituitary

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
In addition to steroid hormones, peptide endocrine factors released from the gonads affect pituitary FSH and LH production. Inhibins (:ßA and :ßB heterodimers) and activins (ßA:ßA and ßB:ßB homodimers, and ßA:ßB heterodimers) are members of the TGFß superfamily named for their respective abilities to suppress and enhance FSH production. These peptides are synthesized in granulosa cells of the ovary and Sertoli cells of the testis and are also found in other tissues where they have been implicated in diverse biological processes (59). Male and female knockout mice lacking the -subunit (Inha^-/-), and therefore depleted of the biological effects of both inhibins, have high circulating activins and FSH, and develop steroidogenic sex-cord stromal tumors of the granulosa cell and Sertoli cell lineage. These tumors are associated with a cachexia-like wasting syndrome, which typically causes death between 8 and 18 wk of age (60, 61). In addition, superovulation experiments in younger knockout females indicate that there are defects in late stages of follicle development, and transplant experiments demonstrate important paracrine roles of ovarian inhibins in maintaining the granulosa cell phenotype and averting the formation of Sertoli tubule-like structures (62). FSH and LH are critical to the processes of tumorigenesis in these mice, as double mutant mice homozygous for the hypogonadal (hpg) mutation at the Gnrh locus and the Inha knockout allele do not develop tumors (63), and Fshb and Inha double knockouts develop tumors with later onset and a less aggressive course, a finding that is particularly pronounced in double knockout males (26). In contrast to the Inha^-/- model, mice that overexpress the rat inhibin coding sequence under the control of the metallothionein promoter have reduced FSH levels in both sexes and elevated LH levels in females. In addition, females exhibit subfertility owing to a decrease in the number of ovulated oocytes, a defect that can be corrected with the provision of exogenous gonadotropins (64).

Recent clinical evidence indicates that inhibins may regulate human gonadotropin production, and ultimately affect the duration of a woman’s reproductive potential. A point mutation in the human INHa gene, resulting in a nonconservative amino acid change (Ala²⁵⁷Thr), has been associated with premature ovarian failure (65). The mutation was found in three patients who presented with secondary amenorrhea, low serum estrogens, and elevated gonadotropins, at ages 16, 20, and 24 yr of age. The authors of the study suggest that a decrease in the bioactivity of the mutant inhibin led to persistently enhanced gonadotropins, and premature depletion of ovarian follicular reserves. Low levels of circulating inhibins and high levels of FSH have been associated with premature ovarian failure before, though this study is the first to implicate inhibins in the condition’s etiology. We anticipate that future studies of inhibin mutations, and clinical assessments of inhibin and FSH levels preceding ovarian failure, will be informative.

When inhibin knockout mice are castrated, they develop steroidogenic tumors of the adrenal cortex, indicating that inhibin signaling is key to proliferation control in the adrenal glands, as well as the gonads (61). Gonadal tumors and the development of adrenal cortical tumors upon castration are also features of a transgenic model expressing the SV40 T antigen (Tag) under the control of the mouse inhibin promoter (66, 67). Despite similarities in their phenotypes, the endocrine environments in which gonadal tumors develop in these two mouse models are quite distinct, with a progressive decrease in serum LH and FSH in the Tag overexpresser being noted as ovarian tumorigenesis proceeds (68). Nevertheless, there is a critical function of the gonadotropin hormones in this model, as is demonstrated by the findings that: 1) the hpg mutation prevents the Tag mice from developing both gonadal and adrenal tumors (69); 2) adrenal tumor cells express LHR and respond to LH by up-regulating steroid production in vitro (70); and 3) suppression of serum LH by exogenous testosterone administration precludes tumor development (69). Similarly, elevated serum LH in the bLHß-CTP transgenic model promotes LHR expression and induces steroidogenesis in the adrenal cortex (71). Such LH action may also be involved in tumorigenesis in the inhibin knockout models, where there is a striking induction of LHR and P450 aromatase mRNAs in adrenal tumors (our unpublished data). Together, these mouse models provide evidence for important regulatory roles of inhibins and LH on nongonadal steroidogenesis and tumorigenesis.

Homozygote mice with a null mutation engineered at the activin/inhibin ßA locus (Inhba^-/-) die neonatally due to craniofacial defects that prevent suckling, and until more recently this had precluded further studies pertaining to the role of activin subunit in reproduction. The null phenotype, however, can be rescued by replacing the activin/inhibin ßA coding sequence with that of the activin/inhibin ßB gene, conferring the activin/inhibin ßA expression pattern on this related sequence (63% amino acid identity). Knock-in mice demonstrate enlarged external genitalia, hypogonadism, and diminished female fertility, indicating unique and previously unrecognized functions of the activin/inhibin ßA protein product in reproduction (72). Serum FSH levels in these mice are slightly increased, though whether this reflects loss of pituitary inhibin :ßA signaling, enhanced activin ßB:ßB signaling, or is simply secondary to gonadal defects remains to be investigated. Knockout mice lacking a functional ßB locus (Inhbb^-/-) have developmental defects in eyelid closure, prolonged gestation, and a failure of mothers to nurse their litters (62, 73). The latter is also a primary characteristic of oxytocin knockout mice (74), and therefore, together these phenotypes substantiate that activin ßB is an important in the induction of this hypothalamic/posterior pituitary hormone (75).

Signaling pathways that mediate activin and inhibin effects are complex, and no receptor or downstream signaling protein mutations have been described that phenocopy the mutant models lacking functional ligands. In the case of activin signaling, however, transgenic mouse models with altered expression of activin-interacting proteins have elucidated specific aspects of activin function in FSH regulation. Activin receptor type II (ActRII) has been shown to relay activin-mediated induction of FSH, as knockout mice lacking ActRII have suppressed FSH levels in the pituitary and serum; LH levels are not affected. Mutant ActRII mice also exhibit gonadal pathologies at least in part due to the lack of FSH, including a delay in fertility and reduced testicular growth in males, and infertility, underdeveloped uteri, follicular atresia and reduced numbers of corpora lutea in females (76). Bioactivities of activin dimers are modulated by their association with a binding protein, follistatin; the interaction is believed to antagonize activin functions in the pituitary (62, 77). Follistatin knockout mice have numerous embryonic defects and die in the perinatal period (78), but the role of follistatin in controlling FSH levels can be appreciated from studies of transgenics overexpressing follistatin under the control of the metallothionein promoter. Lines of mice with widespread expression of the transgene exhibit suppression of serum FSH, and defects in gonadal growth and gametogenesis that can be partially ascribed to FSH deficiency (79).

	Transcriptional Control of the Gonadotropin Genes

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
Knockout mouse models have led to the functional characterization in vivo of several pituitary transcription factors important in mediating gonadotropin expression or providing for the survival of neuroendocrine cell populations in the hypothalamus and pituitary. One of these factors is steroidogenic factor-1 (SF-1), originally identified because of its role in directing the expression of cytochrome P450 steroid hydroxylases in the ovary, testis and adrenal gland. Knockout mice lacking SF-1 have profound defects in endocrine development that affect multiple levels of the HPG axis. Newborn SF-1 knockout mice have gonadal and adrenal agenesis, female sex traits, absence of the ventromedial hypothalamic nucleus, and impaired gonadotrope expression of GnRHR, FSH, and LH (80, 81, 82). Heterozygote mice are phenotypically normal. To circumvent the complexity of the SF-1 null phenotype and establish its role specifically in pituitary hormone production, tissue-specific SF-1 knockout mice have been engineered (83). In these mice, a portion of the endogenous SF-1 locus is marked for excision by Cre-recombinase enzyme by the introduction of tandem loxP sites, and a Cre-recombinase-encoding transgene is expressed under the -glycoprotein hormone subunit promoter specifically in cells of the anterior pituitary. In both males and females, the pituitary SF-1 loss-of-function caused sexual infantilism and hypogonadism, and gonadotrope cells failed to express appreciable levels of GnRHR, FSH, or LH (83). Essentially, all gonadotropic effects of the anterior pituitary are lost in these mice, the hypogonadism being as pronounced as when gonadotrope cells are ablated by targeted toxin expression (84), but there is no defect in gonadal or adrenal development during embryogenesis. In several respects, the SF-1 null mouse phenocopies the effects of a heterozygous mutation in the human FTZF1 gene encoding SF-1. The mutation has been described in a single patient (85) and is a loss-of-function 2-bp missense change affecting the DNA binding domain of the SF-1 protein and eliminating its recognition of the SF-1 canonical nucleotide sequence. The XY female patient presented with adrenal failure in the first 2 wk of life and had streak gonads with poorly differentiated tubule structures. Despite similarities to the knockout model, there was elevation of gonadotropin hormones in this patient in response to administration of GnRH. It is possible that FSH and LH production during this test relied upon the function of SF-1 transcribed from the normal allele.

The reason for the SF-1 haploinsufficiency seen in humans and not mice remains unclear, though dosage sensitivity is also a hallmark of two other human conditions involving related transcriptional regulators and their roles in gonadal development and sex determination. WT-1 (Wilms tumor-1) and DAX-1 (dosage-sensitive sex-reversal-adrenal hypoplasia, congenital critical region on the X chromosome 1, gene 1) proteins interact directly with SF-1 to promote and repress, respectively, its transactivation of target genes, including Müllerian-inhibiting substance (86). Heterozygote XY children with a dominant negative mutation of WT-1 have Denys-Drash syndrome, characterized by male pseudohermaphroditism, urogenital aberrations, and nephroblastoma (87). In contrast, XY sex reversal can result from duplication of the DAX-1 locus in the dosage-sensitive sex reversal region of the X chromosome (88). Aspects of the Denys-Drash syndrome phenotype are recapitulated in heterozygous mice with a truncation in WT-1 (89), whereas DAX-1 overexpressing transgenics create a dosage-sensitive sex reversal-like condition in mice with a hypomorpic Sry (sex determining region of chromosome Y) allele (90). These and other mouse models with sex determination phenotypes are reviewed by Whitworth and Behringer (91).

At birth and throughout adult life, a homeobox-containing transcription factor, OTX1 (orthodenticle homolog 1), is expressed in the pituitary and is involved in the transactivation of several glycoprotein hormone subunit genes. In addition to neurological abnormalities, Otx^-/- mice exhibit a prepubescent period of dwarfism and hypogonadism owing to decreases in GH, FSH, and LH; the condition corrects itself by 4 months of age and knockout mice then have restored growth and gonadal function (92, 93). The defect in these mice does not include alterations in the hypothalamic expression of GnRH or the pituitary expression of GnRHR. During the hypogonadotropic, hypogonadal period, knockout males had a block in spermatogenesis and females had ovaries devoid of antral follicles and corpora lutea. These histological findings were not seen in older fertile knockouts, in which there was recovery of all stages of sperm and follicle development (93). The Otx knockout phenotype is particularly intriguing because of the window in which it is apparent, this being the first example of a mouse model for investigating causes and effects of delayed growth and puberty, and the regulation of temporal-restricted molecular mechanisms in the onset of sexual maturity.

Mutant mouse models have defined in vivo functions of the early growth response (Egr) family of zinc finger transcriptional activators in regulating pituitary hormone production, hindbrain development, peripheral nerve myelination, muscle spindle morphogenesis, and spermatogenesis (94, 95, 96, 97, 98, 99). Two of these transcription factors are critical in the establishment of the HPG endocrine axis, EGR1 (also known as nerve growth factor 1A) and EGR4. Though the Egr1 gene is expressed widely during development, the phenotypes of Egr1 mutant mice are relatively restricted. The first Egr1 mutant mouse model was engineered by inserting a neomycin selection cassette into the DNA-binding region of the EGR1 coding sequence (Egr1^neo), and the primary defect is female sterility due to LH insufficiency (94). Homozygote mutant females show loss of estrous cyclicity, uterine hypoplasia, and no luteinized cells in their ovaries, though corpora lutea are evident upon pregnant mare’s serum gonadotropin and hCG treatment. FSH is produced in these mice and is up-regulated upon ovariectomy, but LHß-subunit mRNA expression is critically compromised. By contrast, Egr1^neo homozygote mutant male mice have no defects in fertility or spermatogenesis (94), though knockout males lacking EGR4 are infertile because of germ cell maturation defects (99). To examine functional redundancies between EGR1 and ERG4, Egr1^neo, Egr4^-/- double mutant mice were bred. Interestingly, double mutant males, unlike Egr1^neo or Egr4^-/- single mutants, demonstrated low levels of serum LH, low serum testosterone, and atrophy of androgen-responsive organs (100). Because steroidogenesis was restored by the addition of an LH receptor agonist, the defect in these mice appears to be in pituitary production of this gonadotropin (100). Therefore, though EGR1 is critical for LH production in the maintenance of female fertility, in Egr1^neo mutant males EGR4 compensates by establishing adequate LH to support androgen production. A second Egr1 mutant mouse line has been generated by targeted insertion of the lacZ (ß-galactosidase) gene into the Egr1 locus (Egr1^lacZ), and these mice have several notable phenotypic differences compared with Egr1^neo mice (95). In these mice, somatotrope development and production of GH in both sexes is impaired, male fertility is critically compromised (but can be rescued by LH administration), and female infertility cannot be rescued by LH administration and is associated with a down-regulation of LHR. These findings suggest singular and previously unexpected roles for EGR1 and its target genes in nongonadotrope pituitary cells, LH production in males, and LHR production in females. Whether the Egr^neo allele is a hypomorphic allele or whether the Egr^lacZ allele creates aberrations apart from abrogating EGR1 expression remains to be explored.

	Other Pituitary Hormones, IGF-1, and Leptin Affect Multiple Levels of the Endocrine Axis

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
The physiological complexities of the HPG axis can be further appreciated by studies of mouse models with disruptions of endocrine and paracrine factors known to affect central and peripheral components of the axis. Nongonadotrope anterior pituitary cell lineages and their product hormones may play important roles in establishing bidirectional gonadotrope-gonad communications. Ames and Snell dwarf mice [with mutations in Prop1 (paired like homeodomain factor 1; prophet of Pit1) and Pit1 (pituitary specific transcription factor 1) genes, respectively, encoding pituitary transcription factors] have TSH, prolactin (PRL), and GH deficiencies due to defects in thyrotrope, lactotrope, and somatotrope transcriptional regulation. In addition, both mouse models also display HHG, despite the absence of a direct role for the disrupted transcription factors in gonadotrope ontogeny or gene regulation (101, 102, 103). Interestingly, HHG and failure to respond to exogenous GnRH is a feature of patients with homozygous or compound heterozygous PROP1 mutations (with considerable variability depending upon the exact nature of the mutation), but not in patients with even complete loss-of-function mutations of the human PIT1 gene, POU1F1 (POU domain, class 1, transcription factor 1) (104).

To gain an understanding of the direct and indirect effects of nongonadotropin pituitary hormones on ovarian function represents an important challenge today for endocrinologists and researchers. Thyroid hormone has been implicated in the function of the lactotrope and somatotrope cell lineages in the anterior pituitary, which produce PRL and GH, respectively (105). Clinically, both hypothyroidism and hyperthyroidism in women have been associated with menstrual abnormalities, infertility, and complications in pregnancy (106, 107, 108). Moreover, analyses of a genetic mouse model of hypothyroidism (hyt) caused by TSH deficiency have demonstrated that thyroid hormone is important for peripheral gonadotropin hormone response and female fertility in mature animals (109). Postpartum hyperprolactinemia suppresses the HPG axis by inhibiting GnRH production (110), and defects in PRL regulation have been associated with reproductive abnormalities in patients, though the roles of PRL signaling outside of mammary tissue are still not well understood. Female knockout mouse models lacking PRL or PRL receptor are infertile due to defects in luteinization, and PRL receptor knockouts also exhibit irregularities in estrous cyclicity, as well as a failure to support oviductal embryogenesis (111, 112).

GH is crucial for growth and the onset of sexual maturity, both by its direct effects binding the GH receptor (GHR) and by the effects of downstream IGF-1. Knockout mice lacking the GHR have growth and reproductive defects. Females exhibit subfertility, and a delay in the onset of sexual maturity that can be corrected by administration of IGF-1 (113). GHR knockout males have low levels of circulating FSH and IGF-1, low plasma testosterone, and an attenuation in their steroidogenic response to exogenous LH owing to down-regulation of LHR in the testes (114). In contrast, transgenic males that overexpress a human GH transgene under the metallothionein promoter have enhanced transcription of FSH and LH mRNAs, increased serum levels of LH, and an increase in LH release in response to GnRH (103). Female transgenics in which bovine GH expressed by the phosphoenol pyruvate carboxykinase promoter, though unable to maintain pregnancies due to PRL deficiencies, have an increase in the numbers of preovulatory follicles and corpora lutea, and a decreased granulosa cell apoptosis in developing follicles (115, 116). This latter phenotype may in part reflect enhanced IGF-1 function in developing follicles (117). There is no readily evident clinical correlate for these phenotypes, and reports of reproductive defects in patients with alterations in GH regulation most commonly describe cases of hypogonadism in patients with acromegaly. GH overexpression in such patients has been associated with amenorrhea in women and testosterone deficiency in men, even in the absence of hyperprolactinemia (118).

Many biological effects of GH have been attributed to its induction of IGF-1 in peripheral tissues, and studies of Igf1 knockouts have revealed important functions of this growth factor in prenatal and postnatal growth, as well as within the gonads. Igf1 knockout males and females are infertile. In males, there is a marked reduction of plasma testosterone, and an inhibition of LH-mediated testosterone production in testicular organ culture experiments (119). In females, there are hypoplastic uteri and no ovulatory response to exogenous gonadotropins (119). Further examination of the Igf1 null ovaries revealed a block in follicular development reminiscent of that of the FSH knockout mouse and a lack of FSHR expression; this finding and the coexpression of Igf1 and Fshr mRNAs in healthy, growing follicles has led to the proposal that IGF-1 up-regulates FSHR and is needed for FSH induction of steroidogenesis (120). Consistent with a role for IGF-1 in steroidogenesis, both girls and boys with Laron syndrome (primary GH resistance) treated with IGF-1 develop elevated serum androgen levels and secondary effects of androgens (121, 122). Therefore, it seems that GH may influence gonadal functioning both by promoting central FSH and LH production, and by enabling gonadotropin response in the gonads, directly and through up-regulation of IGF-1. It should be noted that other physiological parameters influence IGF-1 levels and activities; metabolic defects in -glutamyl transpeptidase mice lead to decreases in IGF-1 levels and similar reproductive phenotypes (123).

Just as GH has pleiotropic effects on the HPG axis, leptin, a protein released from adipocytes, acts upon endocrine circuits at multiple locations. Leptin-deficient ob/ob mice are obese, infertile, and have characteristics of HHG with enhanced functioning of negative feedback pathways on gonadotropin production reminiscent of immature animals (124, 125). Similarly, leptin receptor-deficient db/db mice are obese and hypogonadal (126). In vitro studies have indicated that leptin normally functions at both the level of the hypothalamus (enhancing GnRH production), and at the level of the anterior pituitary (enhancing FSH and LH production) to promote the establishment of adult circulating gonadotropin levels (127). Interestingly, leptin may function dichotomously in HPG axis functioning, up-regulating GnRH, FSH, and LH production centrally, but down-regulating steroidogenesis in peripheral tissues (128, 129, 130, 131). To understand the effects of leptin and body fat content on the reproductive endocrine axis in humans represents a field of research with important implications for health care practices. Lean persons have decreased circulating leptin levels compared with obese persons (132). It may be speculated that low leptin levels contribute to the suppressed GnRH, low gonadotropins, abnormal glycosylation of gonadotropins, and delay of menarche or amenorrhea observed in young women athletes and anorexics (133, 134). Obese patients have elevated serum leptin levels but may exhibit leptin resistance so that some biological effects of leptin are attenuated. Roles that leptin signaling and other metabolic regulation pathways play in the development of the altered sex steroid profile and amenorrhea seen in obese women remain to be elucidated. Human mutations resulting in obesity have been described affecting both leptin (135, 136) and the leptin receptor (137). In the case of the leptin receptor mutation, two sisters homozygous for a nonsense mutation presented with early-onset obesity, attenuated levels of growth hormone and IGF-1, hypothalamic hypothyroidism, and did not undergo pubertal development. Their endocrine profile showed low levels of estrogens and gonadotropins that persisted without response to GnRH administration. This clinical case underscores the importance of metabolic pathways in the control of the reproductive axis and other endocrine communications, and the continued utility of the ob/ob and db/db mouse models to study leptin pathologies in humans.

	Conclusions

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 
Cases of human disease and transgenic mouse models offer powerful means for appreciating the molecular components of endocrine pathways, including the direct, indirect and compensatory results in vivo of their aberrant functioning. Loss-of-function mutations produce hypomorphic and null alleles in mice that recreate deficiency and resistance syndromes in humans. Transgenic overexpressers and targeted subtle mutations may mimic activating mutations or the effects of hypersecretion syndromes in patients. In some instances, mutant mice closely phenocopy human disorders, stand as important proofs-of-principle, and provide us with models for testing our understanding of etiologies and potential medicinal interventions. In some, unexpected phenotypes call our attentions to unrecognized endocrine relationships, and the pathophysiological bases for previously inexplicable clinical observations. These genetic models will continue to reveal to us aspects of the human endocrine system.

	Acknowledgments

 

	Footnotes

 
Abbreviations: ActRII, Activin receptor type II; CG, chorionic gonadotropin; DAX-1, dosage-sensitive sex-reversal-adrenal hypoplasia, congenital critical region on the X chromosome 1, gene 1; Egr, early growth response; ER, estrogen receptor; FSHR, FSH receptor; GHR, GH receptor; GnRHR, GnRH receptor; hCG, human CG; HHG, hypogonadotropic hypogonadism; HPG, hypothalamic-pituitary-gonadal; Inha, inhibin ; lacZ, ß-galactosidase; LHR, LH receptor; MMTV, mouse mammary tumor virus; OTX1, orthodenticle homolog 1; Pit1, pituitary-specific transcription factor 1; POU1F1, POU domain, class 1, transcription factor 1; PRL, prolactin; Prop1, paired like homeodomain factor 1, prophet of Pit1; SF-1, steroidogenic factor-1; Sry, sex determining region of chromosome Y; WT-1, Wilms tumor 1.

Received January 16, 2002.

Accepted for publication March 14, 2002.

	References

Top Abstract Introduction Mouse Models and Human... Mouse Models of Aberrant... Peptide Hormone Feedback to... Transcriptional Control of the... Other Pituitary Hormones, IGF-1,... Conclusions References

 

1. Bousfield GR, Perry WM, Ward DN 1994 Gonadotropins: chemistry and biosynthesis. In: Knobil E, Neill JD, eds. The physiology of reproduction. New York: Raven Press, Ltd.; 1749–1792
2. Greenwald GS, Roy SK 1994 Follicular development and its control. In: Knobil E, Neill J, eds. The physiology of reproduction. 2nd ed. New York: Raven Press, Ltd.; vol 1:629–724
3. Sharpe RM 1994 Regulation of spermatogenesis. In: Knobil E, Neill J, eds. The physiology of reproduction. New York: Raven Press, Ltd.; vol 1:1363–1434
4. Themmen APN, Huhtaniemi IT 2000 Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function. Endocr Rev 21:551–583[Abstract/Free Full Text]
5. Achermann JC, Weiss J, Lee EJ, Jameson JL 2001 Inherited disorders of the gonadotropin hormones. Mol Cell Endocrinol 179:89–96[CrossRef][Medline]
6. de Roux N, Milgrom E 2001 Inherited disorders of GnRH and gonadotropin receptors. Mol Cell Endocrinol 179:83–87[CrossRef][Medline]
7. Franco B, Guioli S, Pragliola A, Incerti B, Bardoni B, Tonlorenzi R, Carrozzo R, Maestrini E, Pieretti M, Taillon-Miller P, Brown CJ, Willard FH, Lawrence CB, Persico GM, Camerino G, Ballabio A 1991 A gene deleted in Kallmann’s syndrome shares homology with neural cell adhesion and axonal path-finding molecules. Nature 353:529–536[CrossRef][Medline]
8. Jackson RS, Creemers JW, Ohagi S, Raffin-Sanson ML, Sanders L, Montague CT, Hutton JC, O’Rahilly S 1997 Obesity and impaired prohormone processing associated with mutations in the human prohormone convertase 1 gene. Nat Genet 16:303–306[CrossRef][Medline]
9. Weiss J, Adams E, Whitcomb RW, Crowley Jr WF, Jameson JL 1991 Normal sequence of the gonadotropin-releasing hormone gene in patients with idiopathic hypogonadotropic hypogonadism. Biol Reprod 45:743–747[Abstract]
10. Seminara SB, Beranova M, Oliveira LM, Martin KA, Crowley Jr WF, Hall JE 2000 Successful use of pulsatile gonadotropin-releasing hormone (GnRH) for ovulation induction and pregnancy in a patient with GnRH receptor mutations. J Clin Endocrinol Metab 85:556–562[Abstract/Free Full Text]
11. Mason AJ, Hayflick JS, Zoeller RT, Young WS, Phillips HS, Nikolics K, Seeburg PH 1986 A deletion truncating the gonadotropin-releasing hormone gene is responsible for hypogonadism in the hpg mouse. Science 234:1366–1371[Abstract/Free Full Text]
12. Kendall SK, Samuelson LC, Saunders TL, Wood RI, Camper SA 1995 Targeted disruption of the pituitary glycoprotein hormone -subunit produces hypogonadal and hypothyroid mice. Genes Dev 9:2007–2019[Abstract/Free Full Text]
13. Miller-Lindholm AK, Bedows E, Bartels CF, Ramey J, Maclin V, Ruddon RW 1999 A naturally occurring genetic variant in the human chorionic gonadotropin-ß gene 5 is assembly inefficient. Endocrinology 140:3496–3506[Abstract/Free Full Text]
14. White RB, Eisen JA, Kasten TL, Fernald RD 1998 Second gene for gonadotropin-releasing hormone in humans. Proc Natl Acad Sci USA 95:305–309[Abstract/Free Full Text]
15. Neill JD, Duck LW, Sellers JC, Musgrove LC 2001 A gonadotropin-releasing hormone (GnRH) receptor specific for GnRH II in primates. Biochem Biophys Res Commun 282:1012–1018[CrossRef][Medline]
16. Cheon KW, Lee HS, Parhar IS, Kang IS 2001 Expression of the second isoform of gonadotrophin-releasing hormone (GnRH-II) in human endometrium throughout the menstrual cycle. Mol Hum Reprod 7:447–452[Abstract/Free Full Text]
17. Kang SK, Tai CJ, Nathwani PS, Leung PC 2001 Differential regulation of two forms of gonadotropin-releasing hormone messenger ribonucleic acid in human granulosa-luteal cells. Endocrinology 142:182–192[Abstract/Free Full Text]
18. Choi KC, Auersperg N, Leung PC 2001 Expression and antiproliferative effect of a second form of gonadotropin-releasing hormone in normal and neoplastic ovarian surface epithelial cells. J Clin Endocrinol Metab 86:5075–5078[Abstract/Free Full Text]
19. Layman LC, McDonough PG 2000 Mutations of follicle stimulating hormone-ß and its receptor in human and mouse: genotype/phenotype. Mol Cell Endocrinol 161:9–17[CrossRef][Medline]
20. Kumar TR, Wang Y, Lu N, Matzuk MM 1997 Follicle stimulating hormone is required for ovarian follicle maturation but not male fertility. Nat Genet 15:201–204[CrossRef][Medline]
21. Dierich A, Sairam MR, Monaco L, Fimia GM, Gansmuller A, LeMeur M, Sassone-Corsi P 1998 Impairing follicle-stimulating hormone (FSH) signaling in vivo: targeted disruption of the FSH receptor leads to aberrant gametogenesis and hormonal imbalance. Proc Natl Acad Sci USA 95:13612–13617[Abstract/Free Full Text]
22. Abel MH, Wootton AN, Wilkins V, Huhtaniemi I, Knight PG, Charlton HM 2000 The effect of a null mutation in the follicle-stimulating hormone receptor gene on mouse reproduction. Endocrinology 141:1795–1803[Abstract/Free Full Text]
23. Burns KH, Yan C, Kumar TR, Matzuk MM 2001 Analysis of ovarian gene expression in follicle-stimulating hormone ß knockout mice. Endocrinology 142:2742–2751[Abstract/Free Full Text]
24. Danilovich N, Roy I, Sairam MR 2001 Ovarian pathology and high incidence of sex cord tumors in follitropin receptor knockout (FORKO) mice. Endocrinology 142:3673–3684[Abstract/Free Full Text]
25. Tapanainen JS, Aittomaki K, Min J, Vaskivuo T, Huhtaniemi IT 1997 Men homozygous for an inactivating mutation of the follicle-stimulating hormone (FSH) receptor gene present variable suppression of spermatogenesis and fertility. Nat Genet 15:205–206[CrossRef][Medline]
26. Kumar TR, Palapattu G, Wang P, Woodruff TK, Boime I, Byrne MC, Matzuk MM 1999 Transgenic models to study gonadotropin function: the role of follicle-stimulating hormone in gonadal growth and tumorigenesis. Mol Endocrinol 13:851–865[Abstract/Free Full Text]
27. Kumar TR, Low MJ, Matzuk MM 1998 Genetic rescue of follicle-stimulating hormone ß-deficient mice. Endocrinology 139:3289–3295[Abstract/Free Full Text]
28. Weiss J, Axelrod L, Whitcomb RW, Harris PE, Crowley WF, Jameson JL 1992 Hypogonadism caused by a single amino acid substitution in the ß subunit of luteinizing hormone. N Engl J Med 326:179–183[Medline]
29. Zhang FP, Poutanen M, Wilbertz J, Huhtaniemi I 2001 Normal prenatal but arrested postnatal sexual development of luteinizing hormone receptor knockout (LuRKO) mice. Mol Endocrinol 15:172–183[Abstract/Free Full Text]
30. Singh J, O’Neill C, Handelsman DJ 1995 Induction of spermatogenesis by androgens in gonadotropin-deficient (hpg) mice. Endocrinology 136:5311–5321[Abstract]
31. Ebling FJ, Brooks AN, Cronin AS, Ford H, Kerr JB 2000 Estrogenic induction of spermatogenesis in the hypogonadal mouse. Endocrinology 141:2861–2869[Abstract/Free Full Text]
32. Shenker A, Laue L, Kosugi S, Merendino Jr JJ, Minegishi T, Cutler Jr GB 1993 A constitutively activating mutation of the luteinizing hormone receptor in familial male precocious puberty. Nature 365:652–654[CrossRef][Medline]
33. Risma KA, Clay CM, Nett TM, Wagner T, Yun J, Nilson JH 1995 Targeted overexpression of luteinizing hormone in transgenic mice leads to infertility, polycystic ovaries, and ovarian tumors. Proc Natl Acad Sci USA 92:1322–1326[Abstract/Free Full Text]
34. Matzuk MM, Hsueh AJ, Lapolt P, Tsafriri A, Keene JL, Boime I 1990 The biological role of the carboxyl-terminal extension of human chorionic gonadotropin ß-subunit. Endocrinology 126:376–383[Abstract]
35. Boime I, Ben-Menahem D 1999 Glycoprotein hormone structure-function and analog design. Recent Prog Horm Res 54:271–288
36. Keri RA, Lozada KL, Abdul-Karim FW, Nadeau JN, Nilson JH 2000 Luteinizing hormone induction of ovarian tumors: oligogenic differences between mouse strains dictates tumor disposition. Proc Natl Acad Sci USA 97:383–387[Abstract/Free Full Text]
37. Risma KA, Hirshfield AN, Nilson JH 1997 Elevated luteinizing hormone in prepubertal transgenic mice causes hyperandrogenemia, precocious puberty, and substantial ovarian pathology. Endocrinology 138:3540–3547[Abstract/Free Full Text]
38. Gore-Langton RE, Armstrong DT 1994 Follicular steroidogenesis and its control. In: Knobil E, Neill J, eds. The physiology of reproduction. 2nd ed. New York: Raven Press, Ltd.; vol 1:571–627
39. Hall PF 1994 Testicular steroid synthesis: organization and regulation. In: Knobil E, Neill J, eds. The physiology of reproduction. 2nd ed. New York: Raven Press, Ltd.; vol 1:1335–1362
40. Lubahn DB, Moyer JS, Golding TS, Couse JF, Korach KS, Smithies O 1993 Alteration of reproductive function but not prenatal sexual development after insertional disruption of the mouse estrogen receptor gene. Proc Natl Acad Sci USA 90:11162–11166[Abstract/Free Full Text]
41. Schomberg DW, Couse JF, Mukherjee A, Lubahn DB, Sar M, Mayo KE, Korach KS 1999 Targeted disruption of the estrogen receptor- gene in female mice: characterization of ovarian responses and phenotype in the adult. Endocrinology 140:2733–2744[Abstract/Free Full Text]
42. Scully KM, Gleiberman AS, Lindzey J, Lubahn DB, Korach KS, Rosenfeld MG 1997 Role of estrogen receptor- in the anterior pituitary gland. Mol Endocrinol 11:674–681[Abstract/Free Full Text]
43. Couse JF, Korach KS 1999 Estrogen receptor null mice: what have we learned and where will they lead us? Endocr Rev 20:358–417[Abstract/Free Full Text]
44. Couse JF, Bunch DO, Lindzey J, Schomberg DW, Korach KS 1999 Prevention of the polycystic ovarian phenotype and characterization of ovulatory capacity in the estrogen receptor- knockout mouse. Endocrinology 140:5855–5865[Abstract/Free Full Text]
45. Couse JF, Curtis Hewitt S, Korach KS 2000 Receptor null mice reveal contrasting roles for estrogen receptor and ß in reproductive tissues. J Steroid Biochem Mol Biol 74:287–296[CrossRef][Medline]
46. Couse JF, Hewitt SC, Bunch DO, Sar M, Walker VR, Davis BJ, Korach KS 1999 Postnatal sex reversal of the ovaries in mice lacking estrogen receptors and ß. Science 286:2328–2331[Abstract/Free Full Text]
47. Hess RA, Bunick D, Lee KH, Bahr J, Taylor JA, Korach KS, Lubahn DB 1997 A role for oestrogens in the male reproductive system. Nature 390:509–512[CrossRef][Medline]
48. Smith EP, Boyd J, Frank GR, Takahashi H, Cohen RM, Specker B, Williams TC, Lubahn DB, Korach KS 1994 Estrogen resistance caused by a mutation in the estrogen-receptor gene in a man. N Engl J Med 331:1056–1061[Abstract/Free Full Text]
49. Ohlsson C, Hellberg N, Parini P, Vidal O, Bohlooly M, Rudling M, Lindberg MK, Warner M, Angelin B, Gustafsson JA 2000 Obesity and disturbed lipoprotein profile in estrogen receptor--deficient male mice. Biochem Biophys Res Commun 278:640–645[CrossRef][Medline]
50. Fisher CR, Graves KH, Parlow AF, Simpson ER 1998 Characterization of mice deficient in aromatase (ArKO) because of targeted disruption of the cyp19 gene. Proc Natl Acad Sci USA 95:6965–6970[Abstract/Free Full Text]
51. Britt KL, Drummond AE, Cox VA, Dyson M, Wreford NG, Jones ME, Simpson ER, Findlay JK 2000 An age-related ovarian phenotype in mice with targeted disruption of the Cyp 19 (aromatase) gene. Endocrinology 141:2614–2623[Abstract/Free Full Text]
52. Robertson KM, O’Donnell L, Jones ME, Meachem SJ, Boon WC, Fisher CR, Graves KH, McLachlan RI, Simpson ER 1999 Impairment of spermatogenesis in mice lacking a functional aromatase (cyp 19) gene. Proc Natl Acad Sci USA 96:7986–7991[Abstract/Free Full Text]
53. Morishima A, Grumbach MM, Simpson ER, Fisher C, Qin K 1995 Aromatase deficiency in male and female siblings caused by a novel mutation and the physiological role of estrogens. J Clin Endocrinol Metab 80:3689–3698[Abstract]
54. Li X, Nokkala E, Yan W, Streng T, Saarinen N, Warri A, Huhtaniemi I, Santti R, Makela S, Poutanen M 2001 Altered structure and function of reproductive organs in transgenic male mice overexpressing human aromatase. Endocrinology 142:2435–2442[Abstract/Free Full Text]
55. Bosland MC 1996 Hormonal factors in carcinogenesis of the prostate and testis in humans and in animal models. Prog Clin Biol Res 394:309–352[Medline]
56. Fowler KA, Gill K, Kirma N, Dillehay DL, Tekmal RR 2000 Overexpression of aromatase leads to development of testicular Leydig cell tumors: an in vivo model for hormone-mediated testicular cancer. Am J Pathol 156:347–353[Abstract/Free Full Text]
57. Tekmal RR, Ramachandra N, Gubba S, Durgam VR, Mantione J, Toda K, Shizuta Y, Dillehay DL 1996 Overexpression of int-5/aromatase in mammary glands of transgenic mice results in the induction of hyperplasia and nuclear abnormalities. Cancer Res 56:3180–3185[Abstract/Free Full Text]
58. Kinuta K, Tanaka H, Moriwake T, Aya K, Kato S, Seino Y 2000 Vitamin D is an important factor in estrogen biosynthesis of both female and male gonads. Endocrinology 141:1317–1324[Abstract/Free Full Text]
59. Vale W, Bilezikjian LM, Rivier C 1994 Reproductive and other roles of inhibins and activins. In: Knobil E, Neill J, eds. The physiology of reproduction. 2nd ed. New York: Raven Press, Ltd.; vol 1:1861–1878
60. Matzuk MM, Finegold MJ, Su J-GJ, Hsueh AJW, Bradley A 1992 -Inhibin is a tumor-suppressor gene with gonadal specificity in mice. Nature 360:313–319[CrossRef][Medline]
61. Matzuk MM, Finegold MJ, Mather JP, Krummen L, Lu H, Bradley A 1994 Development of cancer cachexia-like syndrome and adrenal tumors in inhibin-deficient mice. Proc Natl Acad Sci USA 91:8817–8821[Abstract/Free Full Text]
62. Matzuk MM, Kumar TR, Shou W, Coerver KA, Lau AL, Behringer RR, Finegold MJ 1996 Transgenic models to study the roles of inhibins and activins in reproduction, oncogenesis, and development. Recent Prog Horm Res 51:123–157
63. Kumar TR, Wang Y, Matzuk MM 1996 Gonadotropins are essential modifier factors for gonadal tumor development in inhibin-deficient mice. Endocrinology 137:4210–4216[Abstract]
64. Cho BN, McMullen ML, Pei L, Yates CJ, Mayo KE 2001 Reproductive deficiencies in transgenic mice expressing the rat inhibin -subunit gene. Endocrinology 142:4994–5004[Abstract/Free Full Text]
65. Shelling AN, Burton KA, Chand AL, van Ee CC, France JT, Farquhar CM, Milsom SR, Love DR, Gersak K, Aittomaki K, Winship IM 2000 Inhibin: a candidate gene for premature ovarian failure. Hum Reprod 15:2644–2649[Abstract/Free Full Text]
66. Kananen K, Markkula M, Rainio E, Su JG, Hsueh AJ, Huhtaniemi IT 1995 Gonadal tumorigenesis in transgenic mice bearing the mouse inhibin - subunit promoters/simian virus T-antigen fusion gene: characterization of ovarian tumors and establishment of gonadotropin-responsive granulosa cell lines. Mol Endocrinol 9:616–627[Abstract]
67. Kananen K, Markkula M, Mikola M, Rainio EM, McNeilly A, Huhtaniemi I 1996 Gonadectomy permits adrenocortical tumorigenesis in mice transgenic for the mouse inhibin -subunit promoter/simian virus 40 T-antigen fusion gene: evidence for negative autoregulation of the inhibin -subunit gene. Mol Endocrinol 10:1667–1677[Abstract]
68. Rahman NA, Huhtaniemi IT 2001 Ovarian tumorigenesis in mice transgenic for murine inhibin subunit promoter-driven Simian Virus 40 T-antigen: ontogeny, functional characteristics, and endocrine effects. Biol Reprod 64:1122–1130[Abs