The androgen-binding protein gene is expressed in CD1 mouse testis

Dysregulation of Immature Sertoli Cell Functions by Exposure to Acetaminophen and Genistein in Rodent Cell Models

Sertoli cells are essential for germ cell development and function. Their disruption by endocrine disrupting chemicals (EDCs) or drugs could jeopardize spermatogenesis, contributing to male infertility. Perinatal exposure to EDCs and acetaminophen (APAP) disrupts male reproductive functions in animals and humans. Infants can be exposed simultaneously to the dietary soy phytoestrogen genistein (GEN) and APAP used for fever or pain relief. Our goal was to determine the effects of 10-100 µM APAP and GEN, alone or mixed, on immature Sertoli cells using mouse TM4 Sertoli cell line and postnatal-day 8 rat Sertoli cells, by measuring cell viability, proliferation, prostaglandins, genes and protein expression, and functional pathways. A value of 50 µM APAP decreased the viability, while 100 µM APAP and GEN decreased the proliferation. Sertoli cell and eicosanoid pathway genes were affected by GEN and mixtures, with downregulation of Sox9, Cox1, Cox2, and genes relevant for Sertoli cell function, while genes involved in inflammation were increased. RNA-seq analysis identified p53 and TNF signaling pathways as common targets of GEN and GEN mixture in both cell types. These results suggest that APAP and GEN dysregulate immature Sertoli cell function and may aid in elucidating novel EDC and drug targets contributing to the etiology of male infertility.

Single-cell RNA sequencing of adult rat testes after Leydig cell elimination and restoration

Spermatogenesis is an efficient, complex, and highly organized proliferation and differentiation process that relies on multiple factors including testosterone produced by the Leydig cells. Although the critical role played by testosterone in spermatogenesis is well recognized, the mechanism by which it works is still not completely understood, partially due to the inability to specifically and precisely monitor testosterone-dependent changes within developing germ cells. Here we present single-cell RNA sequencing data from10,983 adult rat testicular cells after the rats were treated with ethanedimethanesulfonate, which temporarily eliminates Leydig cells. The elimination and recovery of Leydig cells represented a complete testosterone depletion and restoration cycle. The dataset, which includes all developing germ cells from spermatogonia to spermatozoa, should prove useful for characterizing developing germ cells, their regulatory networks, and novel cell-specific markers. The dataset should be particularly useful for exploring the effects of the androgen environment on the regulation of spermatogenesis. As this is the first single-cell RNA-Seq dataset for rat testes, it can also serve as a reference for future studies.

TSPO ligand FGIN-1-27 controls priapism in sickle cell mice via endogenous testosterone production

Priapism, a prolonged penile erection in the absence of sexual arousal, is common among patients with sickle cell disease (SCD). Hypogonadism is also common in patients with SCD. While the administration of exogenous testosterone reverses hypogonadism, it is contraceptive. We hypothesized that the stimulation of endogenous testosterone production decreases priapism by normalizing molecular signaling involved in penile erection without decreasing intratesticular testosterone production, which would affect fertility. Treatment of SCD mice with FGIN-1-27, a ligand for translocator protein (TSPO) that mobilizes cholesterol to the inner mitochondrial membrane, resulted in eugonadal levels of serum testosterone without decreasing intratesticular testosterone production. Normalized testosterone levels, in turn, decreased priapism. At the molecular level, TSPO restored phosphodiesterase 5 activity and decreased NADPH oxidase-mediated oxidative stress in the penis, which are major molecular signaling molecules involved in penile erection and are dysregulated in SCD. These results indicate that pharmacologic activation of TSPO could be a novel, targetable pathway for treating hypogonadal men, particularly patients with SCD, without adverse effects on fertility.

Aberrant gene expression by Sertoli cells in infertile men with Sertoli cell-only syndrome

Sertoli cell-only (SCO) syndrome is a severe form of human male infertility seemingly characterized by the lack all spermatogenic cells. However, tubules of some SCO testes contain small patches of active spermatogenesis and thus spermatogonial stem cells. We hypothesized that these stem cells cannot replicate and seed spermatogenesis in barren areas of tubule because as-of-yet unrecognized deficits in Sertoli cell gene expression disable most stem cell niches. Performing the first thorough comparison of the transcriptomes of human testes exhibiting complete spermatogenesis with the transcriptomes of testes with SCO syndrome, we defined transcripts that are both predominantly expressed by Sertoli cells and expressed at aberrant levels in SCO testes. Some of these transcripts encode proteins required for the proper assembly of adherent and gap junctions at sites of contact with other cells, including spermatogonial stem cells (SSCs). Other transcripts encode GDNF, FGF8 and BMP4, known regulators of mouse SSCs. Thus, most SCO Sertoli cells can neither organize junctions at normal sites of cell-cell contact nor stimulate SSCs with adequate levels of growth factors. We propose that the critical deficits in Sertoli cell gene expression we have identified contribute to the inability of spermatogonial stem cells within small patches of spermatogenesis in some SCO testes to seed spermatogenesis to adjacent areas of tubule that are barren of spermatogenesis. Furthermore, we predict that one or more of these deficits in gene expression are primary causes of human SCO syndrome.

A young testicular microenvironment protects Leydig cells against age-related dysfunction in a mouse model of premature aging

Testicular Leydig cells (LCs) are the primary source of circulating androgen in men. As men age, circulating androgen levels decline. However, whether reduced LC steroidogenesis results from specific effects of aging within LCs or reflects degenerative alterations to the wider supporting microenvironment is unclear; inability to separate intrinsic LC aging from that of the testicular microenvironment in vivo has made this question difficult to address. To resolve this, we generated novel mouse models of premature aging, driven by CDGSH iron sulfur domain 2 (Cisd2) deletion, to separate the effects of cell intrinsic aging from extrinsic effects of aging on LC function. At 6 mo of age, constitutive Cisd2-deficient mice display signs of premature aging, including testicular atrophy, reduced LC and Sertoli cell (SC) number, decreased circulating testosterone, increased luteinizing hormone/testosterone ratio, and decreased expression of steroidogenic mRNAs, appropriately modeling primary testicular dysfunction observed in aging men. However, mice with Cisd2 deletion (and thus premature aging) restricted to either LCs or SCs were protected against testicular degeneration, demonstrating that age-related LCs dysfunction cannot be explained by intrinsic aging within either the LC or SC lineages alone. We conclude that age-related LC dysfunction is largely driven by aging of the supporting testicular microenvironment.—Curley, M., Milne, L., Smith, S., Jørgensen, A., Frederiksen, H., Hadoke, P., Potter, P., Smith, L. B. A Young testicular microenvironment protects Leydig cells against age-related dysfunction in a mouse model of premature aging.

Testosterone Retention Mechanism in Sertoli Cells: A Biochemical Perspective

Mechanism(s) involved in regulating Intratesticular Testosterone levels (iT) have assumed importance in recent years, from the point of view of hormonal contraception. Contraceptives using Testosterone (T) in combination with Progestins (P), for more effective suppression of pituitary gonadotropins thereby iT, are not 100% effective in suppressing spermatogenesis in human males, likely due to pesrsistence of Intratesticular Dihydrotestosterone (iD) in poor-responders. Several lacunae pertaining to the mechanism of action of principal male hormone T during spermatogenesis remain to be resolved. Notably, the mechanism through which T brings about the stage-specific differentiation of germ cells lacking Androgen Receptors (AR). Testosterone is a highly anabolic steroid with a rapid tissue clearance rate. T is intratesticular substrate for synthesis of Dihydrotestosterone (DHT) and Estradiol (E2) involved in spermtaogenesis. Therefore, it is important to delineate the mechanism(s) for retention of iT, in order to understand regulation of its bioavailability in testis. In depth studies, pertaining to the role of androgen-binding protein(s) in sequestration, retention and bioavailability of T/DHT are required to understand male fertility regulation. The appropriate approach to overcome this lacuna would be development of mice lacking functional testicular Androgen-Binding Protein (ABPKO), but not deficient T/DHT, Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH), in order to understand its physiological functions. Insights gained about androgen retention mechanism(s) from the ABPKO murine model will be of immense help in improving the efficacy of male hormonal contraceptives and infertility management.

Comparative contribution of the aryl hydrocarbon receptor gene to perinatal stage development and dioxin-induced toxicity between the urogenital complex and testis in the mouse

TCDD (2,3,7,8-tetrachlorodebenzo-p-dioxin) requires the presence of the aryl hydrocarbon receptor (Ahr) gene for its toxic effects, such as reproductive disorders in male offspring of maternally exposed rats and mice. To study the involvement of the Ahr gene in producing the toxic phenotype with respect to testicular development, we administered a relatively high dose of TCDD to mice with three different maternally derived Ahr genotypic traits, and then compared several Ahr-dependent alterations among male reproductive systems on Postnatal Day 14. Reduction in anogenital distance and expression of prostatic epithelial genes in the urogenital complex (UGC) were detected in Ahr(+/+) and Ahr(+/-) mice exposed to TCDD, whereas no difference was observed in Ahr(-/-) mice. In situ hybridization revealed the absence of probasin mRNA expression in the prostate epithelium, despite the obvious development of prostatic lobes in TCDD-exposed mice. In contrast to obvious prostatic dysfunction and induction of cytochrome P450 (CYP) family genes in the UGC by TCDD, no alterations in testicular functions were observed in germ cell/Sertoli cell/interstitial cell marker gene expression or CYP family induction. No histopathological changes were observed among the three genotypes and between control and TCDD-exposed mice. Therefore, mouse external genitalia and prostatic development are much more sensitive to TCDD treatment than testis. Further, the Ahr gene, analyzed in this study, does not significantly contribute to testicular function during perinatal and immature stages, and the developing mouse testis appears to be quite resistant to TCDD exposure.

Effects of FSH on testicular mRNA transcript levels in the hypogonadal mouse

FSH acts through the Sertoli cell to ensure normal testicular development and function. To identify transcriptional mechanisms through which FSH acts in the testis, we have treated gonadotrophin-deficient hypogonadal (hpg) mice with recombinant FSH and measured changes in testicular transcript levels using microarrays and real-time PCR 12, 24 and 72 h after the start of treatment. Approximately 400 transcripts were significantly altered at each time point by FSH treatment. At 12 h, there was a clear increase in the levels of a number of known Sertoli cell transcripts (e.g. Fabp5, Lgals1, Tesc, Scara5, Aqp5). Additionally, levels of Leydig cell transcripts were also markedly increased (e.g. Ren1, Cyp17a1, Akr1b7, Star, Nr4a1). This was associated with a small but significant rise in testosterone at 24 and 72 h. At 24 h, androgen-dependent Sertoli cell transcripts were up-regulated (e.g. Rhox5, Drd4, Spinlw1, Tubb3 and Tsx) and this trend continued up to 72 h. By contrast with the somatic cells, only five germ cell transcripts (Dkkl1, Hdc, Pou5f1, Zfp541 and 1700021K02Rik) were altered by FSH within the time-course of the experiment. Analysis of canonical pathways showed that FSH induced a general decline in transcripts related to formation and regulation of tight junctions. Results show that FSH acts directly and indirectly to induce rapid changes in Sertoli cell and Leydig cell transcript levels in the hpg mouse but that effects on germ cell development must occur over a longer time-span.

Effect of germ cell depletion on levels of specific mRNA transcripts in mouse Sertoli cells and Leydig cells

It has been shown that testicular germ cell development is critically dependent upon somatic cell activity but, conversely, the extent to which germ cells normally regulate somatic cell function is less clear. This study was designed, therefore, to examine the effect of germ cell depletion on Sertoli cell and Leydig cell transcript levels. Mice were treated with busulphan to deplete the germ cell population and levels of mRNA transcripts encoding 26 Sertoli cell-specific proteins and 6 Leydig cell proteins were measured by real-time PCR up to 50 days after treatment. Spermatogonia were lost from the testis between 5 and 10 days after treatment, while spermatocytes were depleted after 10 days and spermatids after 20 days. By 30 days after treatment, most tubules were devoid of germ cells. Circulating FSH and intratesticular testosterone were not significantly affected by treatment. Of the 26 Sertoli cell markers tested, 13 showed no change in transcript levels after busulphan treatment, 2 showed decreased levels, 9 showed increased levels and 2 showed a biphasic response. In 60% of cases, changes in transcript levels occurred after the loss of the spermatids. Levels of mRNA transcripts encoding Leydig cell-specific products related to steroidogenesis were unaffected by treatment. Results indicate (1) that germ cells play a major and widespread role in the regulation of Sertoli cell activity, (2) most changes in transcript levels are associated with the loss of spermatids and (3) Leydig cell steroidogenesis is largely unaffected by germ cell ablation.

Microarray analysis of androgen-regulated gene expression in testis: the use of the androgen-binding protein (ABP)-transgenic mouse as a model

BACKGROUND

Spermatogenesis is an androgen-dependent process, yet the molecular mechanisms of androgens' actions in testis are poorly understood. Transgenic mice overexpressing rat androgen-binding protein (ABP) in their testes have reduced levels of intratesticular androgens and, as a result, show a progressive impairment of spermatogenesis. We used this model to characterize changes in global gene expression in testis in response to reduced bioavailability of androgens.

METHODS

Total RNA was extracted from testes of 30-day old transgenic and wild-type control mice, converted to cRNA, labeled with biotin, and hybridized to oligonucleotide microarrays. Microarray results were confirmed by real-time reverse transcription polymerase chain reaction.

RESULTS

Three-hundred-eighty-one genes (3.05% of all transcripts represented on the chips) were up-regulated and 198 genes (1.59%) were down-regulated by at least a factor of 2 in the androgen-deficient animals compared to controls. Genes encoding membrane proteins, intracellular signaling molecules, enzymes, proteins participating in the immune response, and those involved in cytoskeleton organization were significantly overrepresented in the up-regulated group. Among the down-regulated transcripts, those coding for extracellular proteins were overrepresented most dramatically, followed by those related to proteolysis, cell adhesion, immune response, and growth factor, cytokine, and ion channel activities. Transcripts with the greatest potential impact on cellular activities included several transcription factors, intracellular signal transducers, secreted signaling molecules and enzymes, and various cell surface molecules. Major nodes in the up-regulated network were IL-6, AGT, MYC, and A2M, those in the down-regulated network were IL-2, -4, and -10, MAPK8, SOCS1, and CREB1.

CONCLUSION

Microarray analysis followed by gene ontology profiling and connectivity analysis identified several functional groups of genes and individual genes responding to sustained reduction of androgen levels in the mouse testis. These include genes whose products function as transcription factors, cell surface molecules including ion channels, extra- and intracellular signaling molecules, and secreted enzymes with the potential of regulating cell-to-cell attachment. The transcription factors CREB1 (down-regulated) and MYC (up-regulated) may mediate the most important initial phases of the testicular response to reduced levels of androgens. These results suggest specific avenues for further research that will lead to a better understanding of how androgens regulate spermatogenesis.

Expression of rabbit sex hormone-binding globulin during pregnancy and prenatal development and identification of a novel isoform

SHBG is a homodimeric plasma glycoprotein. It functions as a carrier for sex steroids in blood and regulates their access to target cells. In human and rabbit, SHBG is a single-copy gene comprised of eight exons and is expressed primarily in the liver and testis. In the present study, the ontogeny of rabbit SHBG (rbSHBG) gene expression was examined in both fetus and mothers. Trace amounts of rbSHBG mRNA were detected in fetal liver from d 11 to d 29 gestation. These levels increased dramatically at d 30 and remained high until parturition (d 33). In contrast, high levels of rbSHBG mRNA were detected in the maternal liver early during pregnancy, with maximal levels being attained by d 22 and declining markedly thereafter. A rbSHBG transcript lacking the exon 4 sequences was consistently expressed along with the rbSHBG mRNA. When expressed as a glutathione-S-transferase-fusion protein, this alternatively spliced rbSHBG transcript resulted in a product with almost no steroid binding activity, unlike the full-length rbSHBG-glutathione-S-transferase fusion protein, which bound 5alpha-dihydrotestosterone. Antibody specific to the novel rbSHBG isoform lacking the exon 4-encoding domain was raised, and a single immunoreactive protein of 33-35 kDa was detected by Western blot analysis in both fetal and maternal liver, and this indicates that the rbSHBG transcripts lacking exon 4 sequences are translated in vivo. An RT-PCR analysis further revealed that this alternatively spliced SHBG transcript is present in human HepG2 cells as well as human and mouse testes, indicating that exon 4 splicing in SHBG transcription is conserved among mammalian species. To our knowledge, this is the first report of the identification of a SHBG exon 4 splice variant that is translated. Because the SHBG isoform it encodes lacks appreciable steroid-binding activity, it may function beyond that of the widely accepted role of SHBG as a steroid-transport protein.

Regulation of Sertoli cell number and activity by follicle-stimulating hormone and androgen during postnatal development in the mouse

The roles of FSH and androgen in the postnatal development of Sertoli cell number and function have been investigated using mice that lack FSH (FSHbetaKO), FSH-receptors (FSHRKO), or androgen receptors (Tfm). At birth and d 5, Sertoli cell number was normal in FSHRKO and FSHbetaKO mice, but was significantly reduced on d 20 and in adulthood. In contrast, Sertoli cell number was reduced at birth in Tfm mice and remained significantly less than normal up to adulthood. Sertoli cell activity was determined through measurement of 11 different mRNA transcript levels. From birth to adulthood, the expression of most transcripts increased, with a significant rise occurring between d 5 and 10. In animals lacking FSH stimulation, mRNA expression (measured per Sertoli cell) was largely normal on d 5, but was reduced in seven transcripts on d 20 and in five transcripts at adulthood. In Tfm mice two transcripts showed reduced expression on d 5, and four were reduced on d 20, although expression in adult Tfm mice did not differ from that in normal cryptorchid controls. The results show that 1) testosterone, but not FSH, is required for Sertoli cell proliferation during fetal and early neonatal life; 2) FSH and testosterone both regulate the late stages of Sertoli cell proliferation; 3) FSH has a general trophic effect on Sertoli cell activity in the pubertal and adult mouse; and 4) androgens are required for specific transcript expression during prepubertal development. Specific effects of androgens were not seen in the adult, although these may be masked by the effects of cryptorchidism.

Effects of 3,3',4,4',5-pentachlorobiphenyl, a coplanar polychlorinated biphenyl congener, on cultured neonatal mouse testis

3,3',4,4',5-Pentachlorobiphenyl (PCB126), a congener with a planar configuration, has been established to have relatively strong toxicities similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via aryl hydrocarbon receptors. We investigated the effects of this coplanar PCB on mammalian early spermatogenesis and steroidogenesis in a mouse neonatal testicular organ culture system. Testes collected from newborn mice were subjected to organ culture in medium containing 0, 10, 100 or 1000 nM PCB126. Histochemical analysis revealed that the BrdU-labeling indices of both spermatogenic cells and Sertoli cells were unchanged in all testis specimens exposed to the coplanar PCB. CYP1A1 and steroidogenic enzymes (P450scc, P450c17, 3beta-HSD and 17beta-HSD) mRNA levels were determined by semiquantitative RT-PCR. The CYP1A1 mRNA level in cultured testis was significantly increased by PCB126 in a dose-dependent manner. Although mRNA levels of 3beta-HSD and 17beta-HSD were unchanged, the P450scc mRNA level was significantly down-regulated by PCB126 in a dose-dependent manner. In contrast, the P450c17 mRNA level was significantly higher in 1000 nM PCB126-exposed testis than in control testis. These results suggest that the coplanar PCB does not alter the proliferative activity of spermatogenic cells and Sertoli cells in neonatal testis, but that it directly affects the expression of steroidogenic enzyme genes.

Mice that express enzymatically inactive cathepsin L exhibit abnormal spermatogenesis

The finding of large, stage-specific changes in secretion of procathepsin L by rat Sertoli cells has led to the hypothesis that this proenzyme promotes the survival, replication, or differentiation of spermatogenic cells. Experiments described herein used a mouse model to test this hypothesis. To prove that mice are appropriate for this purpose, we first demonstrate that mature mouse Sertoli cells express cathepsin L mRNA in the same stage-specific manner as rat Sertoli cells and they also secrete procathepsin L. To test whether catalytically active cathepsin L is required for normal spermatogenesis, we examined the testes of 110- to 120-day-old furless mice, which express catalytically inactive cathepsin L. Morphologic examination of testes of furless mice revealed both normal and atrophic seminiferous tubules. Enumeration of atrophic tubules in furless and control mice demonstrates that lack of functional cathepsin L results in a 12-fold increase in seminiferous tubule atrophy. To determine whether lack of functional cathepsin L affects the production of male germ cells in apparently normal, nonatrophic tubules, we compared numbers in control and furless mice of preleptotene spermatocytes, pachytene spermatocytes, and round spermatids per Sertoli cell. Results demonstrate that the lack of functional cathepsin L causes a 16% reduction in formation of preleptotene spermatocytes and a 25% reduction in differentiation of these cells into pachytene spermatocyte. These results suggest that procathepsin L either directly or indirectly has two distinct functions in the testis. This proenzyme prevents atrophy of seminiferous tubules and promotes the formation of preleptotene spermatocytes and the differentiation of these meiotic cells into pachytene spermatocytes.

A human sex hormone-binding globulin isoform accumulates in the acrosome during spermatogenesis

Human sex hormone-binding globulin (SHBG) binds estradiol and testosterone with high affinity. Plasma SHBG is produced by hepatocytes, but the human SHBG gene is also expressed in the testis. Little is known about SHBG gene expression in the human testis, but human SHBG transcripts accumulate in a spermatogenic stage-dependent manner in the testes of mice containing an 11-kb human SHBG transgene. We have now found that human SHBG transcripts containing an alternative exon 1 sequence are located specifically in the testicular germ cells of these transgenic mice, whereas murine SHBG transcripts are confined to Sertoli cells. In addition, we have detected immunoreactive human SHBG in the acrosome during all stages of spermiogenesis in mice containing an 11-kb human SHBG transgene. Western blots of germ cell extracts from these transgenic mice and from human sperm indicate that the immunoreactive human SHBG in the acrosome composes electrophoretic variants, which are 3-5 kDa smaller than the major electrophoretic isoforms of human SHBG in the blood. This apparent size difference is due in part to differences in glycosylation of plasma and acrosomal SHBG isoforms. The function of the human SHBG isoform in the acrosome is unknown, but it binds steroid ligands with high affinity. This is the first demonstration that human SHBG transcripts encode an SHBG isoform that remains within a cellular compartment.

Rabbit sex hormone-binding globulin: expression in the liver and testis during postnatal development and structural characterization by truncated proteins

Although sex hormone binding globulin (SHBG) is found in the blood plasma of adult humans and rabbits and the gene is expressed in their livers, it is not detected in the plasma of adult rodents nor is it expressed in adult rodent livers. Thus the rabbit represents a good model to study the metabolism and function of SHBG in the blood. We have used a cloned rabbit SHBG cDNA to detect mRNA expression in rabbits during the postnatal period, and to construct truncated SHBG proteins for structure/function analysis. The SHBG mRNA appeared in the testis as early as 3 days after birth. The level increased gradually in abundance throughout postnatal development, and attained a maximum at 12 weeks of age when the gonads were fully matured. In contrast, SHBG mRNA in the livers of male and female animals increased to a maximum by 4 weeks of age, and were maintained at this level until 12 weeks before subsiding to the initial levels. The increase and decrease in SHBG mRNA levels in the liver were accompanied by similar changes in serum SHBG. This suggests that SHBG in the blood circulation comes from the liver and this might also provide a source of SHBG for the male reproductive tract before formation of the blood-testis barrier. To elucidate the minimal sequence of rabbit SHBG responsible for steroid-binding, a panel of 13 truncated SHBG proteins was constructed, expressed in Escherichia coli, and biochemically purified for study. It was shown that the complete protein sequence of rabbit SHBG was important for maintaining a stable steroid-protein complex. Unlike human SHBG for which a truncated protein of the first 206 residues of the 373 amino acid protein can still bind steroid, removal of 43 or more residues from the C-terminus of rabbit SHBG completely abolished steroid-binding.

Localization of 17beta-hydroxysteroid dehydrogenase/17-ketosteroid reductase isoform expression in the developing mouse testis--androstenedione is the major androgen secreted by fetal/neonatal leydig cells

The final step in the biosynthesis of testosterone is reduction of androstenedione by the enzyme 17beta-hydroxysteroid dehydrogenase/ 17-ketosteroid reductase (17betaHSD/17KSR). In this study, we have examined expression of the four known reductive isoforms of 17betaHSD/ 17KSR (types 1, 3, 5, and 7) in the developing mouse testis and have determined changes in the localization of isoform expression and testosterone secretion during development. Using RT-PCR isoforms 1, 3, and 7 were shown to be expressed in the seminiferous tubules of neonatal testis, whereas isoforms 3 and 7 were expressed in the interstitial tissue of the adult testis. The type 7 isoform is unlikely to be involved in androgen synthesis and further study concentrated on the type 3 isoform. Developmentally, isoform type 3 was expressed in the seminiferous tubules up to day 10, showed little or no expression on day 20 and from day 30 was confined to the interstitial tissue. In situ hybridization confirmed that the type 3 isoform was expressed only in the seminiferous tubules in fetal testes and in the interstitial tissue in adult testes. In accordance with the localization of enzyme messenger RNA expression 17-ketosteroid reductase enzyme activity was very low in isolated interstitial tissue from neonatal testes while interstitial tissue from adult testes showed high activity. Seminiferous tubules from both neonatal and adult testes showed high levels of enzyme activity. The major androgen secreted by the interstitial tissue of prepubertal animals was androstenedione up to day 20 while 5alpha-androstanediol and/or testosterone were the major androgens secreted from day 30 onwards. These results show that fetal Leydig cells do not express significant levels of a reductive isoform of 17betaHSD/ 17KSR and that androstenedione is the major androgen secreted by these cells. Production of testosterone up until puberty is dependent upon 17betaHSD/17KSR activity in the seminiferous tubules--a "two cell" requirement for testosterone synthesis. Expression of the 17betaHSD/17KSR type 3 isoform (the main reductive isoform in the testis) declines in the seminiferous tubules before puberty but then reappears in the developing adult Leydig cell population.

The rabbit sex hormone-binding globulin gene: structural organization and characterization of its 5-flanking region

Sex hormone-binding globulin (SHBG) transports sex steroids in the blood. In humans and rabbits, the gene encoding SHBG (shbg) is expressed primarily in the liver and testis, whereas the testis is the major site of shbg expression in rodents postnatally. Sequence analysis has revealed that rabbit shbg (rbshbg) spans 2.5 kb and comprises eight exons with consensus splice sites at all exon-intron junctions. The major transcription start site ofrbshbg is located 52 bp upstream from the translation initiation codon for the rabbit SHBG precursor. Unlike the situation in humans and rats, rbshbg transcripts contain no alternative exon 1 sequences in the liver or testis, and this suggests that the rbshbg 5'-flanking region plays an equally important role in controlling transcription of this gene in these tissues. Like the human and rat shbg promoter sequences, the rbshbg proximal promoter lacks a typical TATA box. It also contains several transcription factor-binding sites, but deoxyribonuclease I footprinting experiments indicated that the human and rabbit shbg proximal promoters interact quite differently with proteins extracted from rabbit liver nuclei. However, the predominant footprint on the rbshbg promoter is conserved at the same position within the human shbg (hshbg) promoter and includes consensus binding sites for the transcription factor nuclear factor- 1. Transient transfection studies of the rbshbg 5'-flanking sequence (893 bp) revealed regions that actively enhance and repress its activity in human hepatoblastoma and mouse Sertoli cells, but not in Chinese hamster ovary cells. Like the rat shbg proximal promoter, the rbshbg 5'-flanking sequence lacks a region that corresponds to a cis-element, designated footprinted region 4 in the hshbg proximal promoter. Furthermore, the hshbg promoter footprinted region 3 sequence is poorly conserved in rbshbg, and when mutated to resemble the corresponding human sequence it increased the transcriptional activity of the rbshbg promoter by 7-fold in hepatoblastoma cells. Thus, the rabbit and hshbg promoters appear to be controlled by a different set of transcriptional regulators. Further comparisons of their functional activities may shed light on species-specific differences in the spatial and temporal expression of this gene, the products of which play important roles in regulating sex steroid access to target cells.

Sex hormone-binding globulin: not only a transport protein. What news is around the corner?

The plasma Sex Hormone-Binding Globulin (SHBG) transports androgens and estradiol in the blood and regulates their bioavailable fraction and access to target cells. The recent advances in the knowledge of its structure and gene expression, and notabily the demonstration of a specific receptor (SHBG-R) located on membranes of sex steroid responsive cells, gave support to the thesis that SHBG has much more sophisticated functions at cell site. In particular, the receptor-mediated action of SHBG, which uses as a second messenger cAMP, has been linked to the effects of androgens and estradiol. It is conceivable that the SHBG/SHBG-R system works as an additional control mechanism which inhibits or amplifies the effects of DHT and estradiol in cells. In the prostate, it has been suggested that the estradiol-activated SHBG/SHBG-R complex cross-talks with the androgen receptor, and is able to activate AR even in the absence of DHT. Of great interest, for its potential clinical applications, is the observation that in estrogen-dependent breast cancer SHBG, through SHBG-R, cAMP and PKA, specifically inhibits the estradiol-induction of cell proliferation. This anti-proliferative, anti-estrogenic effect of human SHBG has not only increased and continues to increase our understanding of the molecular mechanisms involved in the biology of breast cancer, but could also be exploited as a future therapeutic strategy in the managing of estrogen-dependent tumours.

Complex gangliosides are essential in spermatogenesis of mice: possible roles in the transport of testosterone

Mice, homozygous for disrupted ganglioside GM2/GD2 synthase (EC 2.4. 1.94) gene and lacking all complex gangliosides, do not display any major neurologic abnormalities. Further examination of these mutant mice, however, revealed that the males were sterile and aspermatogenic. In the seminiferous tubules of the mutant mice, a number of multinuclear giant cells and vacuolated Sertoli cells were observed. The levels of testosterone in the serum of these mice were very low, although testosterone production equaled that produced in wild-type mice. Testosterone was found to be accumulated in interstitial Leydig cells, and intratesticularly injected testosterone was poorly drained in seminiferous fluid in the mutant mice. These results suggested that complex gangliosides are essential in the transport of testosterone to the seminiferous tubules and bloodstream from Leydig cells. Our results provide insights into roles of gangliosides in vivo.

The Pem homeobox gene. Androgen-dependent and -independent promoters and tissue-specific alternative RNA splicing

The Pem gene encodes an atypical homeodomain protein, distantly related to Prd/Pax family members, that we demonstrate is regulated in a complex transcriptional and post-transcriptional manner. We show that the rat Pem genomic structure includes three 5'-untranslated (5'-UT) exons and four coding exons, three of which encode the homeodomain. Several alternatively spliced transcripts were identified, including one that skips an internal coding exon, enabling this mRNA to express a novel form of the Pem protein. Other alternatively spliced mRNAs were characterized that possess different 5'-UT regions, including a muscle-specific transcript. The different 5'-UT termini present in Pem transcripts conferred different levels of translatability in vitro. Two promoters containing multiple transcription initiation sites were identified: a distal promoter (Pd) in the first 5'-UT exon and a proximal promoter (Pp) located in the "intron" upstream of the first coding exon. The Pd was active in placenta, ovary, tumor cell lines, and to a lesser extent in skeletal muscle. In contrast, transcripts from the Pp were only detectable in testis and epididymis and were only expressed in epididymis in the presence of testosterone. To our knowledge no transcription factors have previously been identified that exhibit androgen-dependent expression in the epididymis.

Hepatic expression of sex hormone-binding globulin associated with the postnatal surge of serum androgen-binding activity in the Djungarian hamster

Serum androgen-binding capacity in Djungarian hamsters, as in many other mammals, increases within days after birth and remains elevated until puberty. This increased activity has been attributed to a hepatic glycoprotein, sex hormone-binding globulin (SHBG), but expression of SHBG by the postnatal liver has not been demonstrated. Therefore, a full-length SHBG cDNA was cloned from the liver of neonatal hamsters and the expression of SHBG during development was examined. Hepatic SHBG RNA levels, as measured by both competitive RT-PCR and Northern analysis, were very low in fetal animals but increased significantly within 24 h of birth. Maximal values were maintained for 1 week after parturition, and then declined to basal adult levels. The developmental pattern in hepatic SHBG immunoactivity, as determined by Western analysis, mirrored that of hepatic SHBG mRNA. However, changes in serum SHBG immunoactivity and steroid-binding activity occurred approximately 1 week later. There were no sex differences in the levels of hepatic SHBG mRNA or protein during development, but serum immunoactivity tended to be higher in females at puberty. Sex- and age-related differences in the relative abundance of SHBG isoforms were also noted. Results of these studies demonstrate that Djungarian hamsters express an authentic SHBG and indicate that the postnatal surge in serum androgen-binding activity is due to perinatal up-regulation of SHBG expression.

Endocytosis of androgen-binding protein (ABP) by spermatogenic cells

To test whether Sertoli cell-secreted ABP could serve as steroid carrier to the germ cell (GC) lineage, radiolabeled ABP and SHBG and gold SHBG were used for binding studies and for internalization studies based on transmission electron microscope analyses and autoradiography of the radiolabeled samples. The data clearly showed that: (1) rat and human germ cells possess a single class of binding sites for rat ABP and human SHBG respectively (Kd of 0.78 and 0.56 nM); (2) 1.7 x 10(10) and 2.7 x 10(10) sites/mg protein was found in the corresponding plasma membrane preparations; (3) the receptor peak was eluted in the same position as dextran blue: 2000 kDa (M(r) = 2 x 106) for labeled rat ABP; (4) in the whole GC lineage, the labeled ligand was internalized through an endocytic pathway involving clathrin coated structures and the distribution was similar throughout the maturation step, however striking differences in the internalization rate were revealed with regard to the maturation step; and (5) this internalization occurred even in ligated seminiferous tubules, via the Sertoli cells cytoplasm. When isolated rat GC were incubated in the presence of ABP, a dose dependent increase in labeled secreted protein was observed for spermatocytes (50-250%) whereas ABP had no effect on spermatids. Addition of steroids and ABP caused a 200 and 50% increase in labeled secreted proteins for spermatocytes and spermatids respectively. 2-D SDS-PAGE analysis revealed that ABP alone increased the secretion of specific spermatocyte proteins whereas steroids in the presence of ABP resulted in the synthesis of new spermatocyte secreted proteins. Taken together these results strongly suggest that ABP may be required for spermatogenesis either as a steroid transmembrane carrier or on its own.

Androgen binding protein is tissue-specifically expressed and biologically active in transgenic mice

In view of the inconclusive data concerning the role of androgen-binding protein (ABP) in male reproductive physiology, we thought it would be pertinent to make several transgenic mouse lines overexpressing the rat ABP gene to unravel its role in Sertoli cell and epididymal homeostasis. Heterozygote transgenic mouse lines carrying the 5.5 kb ABP rat genomic DNA were produced by pronuclear microinjection. Northern blot analysis showed overexpression of rat ABP (rABP) mRNA in the testis of transgenic mice compared to rat testis control. rABP was appropriately expressed in Sertoli cells as demonstrated by in situ hybridization analysis. Sertoli cell number is increased in the seminiferous tubules of mice overexpressing rABP compared to non-transgenic littermates and scattered Sertoli cells present vacuolated-like cytoplasms, PAS and osmium negative. Compared to the wild type, the transgenic mice exhibited reduced fertility and focal damage in seminiferous epithelium characterized by morphological features compatible with programmed cell death.

Structure, function, and regulation of androgen-binding protein/sex hormone-binding globulin

Despite over 20 years of research, the functions of ABP and SHBG remain elusive. The major reason for this lack of knowledge has been the unavailability of natural mutants with clinical defects for study. There is strong evidence that these binding proteins do act to modulate the gene regulatory actions of nuclear sex steroid receptors by controlling the availability of androgens and estrogens. In plasma, SHBG controls the metabolic clearance rate of sex steroids. In addition there is strong evidence that they have a much broader function. The identification of plasma membrane receptors in target tissues and the finding of homologous domains in several developmental proteins support other functions. Moreover, other experiments suggest the proteins may actually be hormones or growth factors. These findings are not compatible with a model that has the proteins only regulating free steroid hormone levels. Obviously, much more experimentation will be necessary to reveal the functions of ABP and SHBG. The recent discoveries have offered several clues to their functions and open new routes for study. These experiments, coupled with newly developed techniques, such as gene knockout by homologous recombination, make one optimistic that the functions of these unique proteins will be deciphered in the near future.

Tissue-specific expression of the rat androgen-binding protein/sex hormone-binding globulin gene in transgenic mice

The testicular Sertoli cell produces an extracellular androgen-binding protein (ABP) that binds testosterone and dihydrotestosterone with high affinity. The ABP gene also encodes plasma sex hormone-binding globulin (SHBG), which is produced by the liver of most species. Unlike the human, adult rats and mice do not express SHBG. A 5.5-kb rat genomic DNA fragment was found to contain the entire coding regions of ABP and 1.5 kb upstream of the transcription start site. To aid in identification of the promoter and enhancer regions of the ABP/SHBG gene, we developed transgenic mice that express the rat gene. The 5.5-kb DNA was microinjected into the pronuclei of fertilized mice ova, which were transferred to the reproductive tract of pseudopregnant females. Three of the offspring were identified as carriers of the rat gene by Southern hybridization and these founders were bred with normal mice to establish heterozygous transgenic lines. Northern blot analysis, RNA-PCR and sequencing of the PCR products from the adult transgenic mice revealed extremely high levels of the rat ABP mRNA in the testis, but no detectable rat ABP mRNA in liver, kidney or brain. Primer extension experiments showed that the correct transcript ion start site is utilized in the transgenes. These data demonstrate that the 5.5-kb genomic DNA fragment contains an element(s) capable of directing ABP gene expression in the testis. This enhancer should prove useful for the targeting of specific gene products to the mature Sertoli cell in transgenic animals.

Characterization of Met-139 as the photolabeled amino acid residue in the steroid binding site of sex hormone binding globulin using delta 6 derivatives of either testosterone or estradiol as unsubstituted photoaffinity labeling reagents

Immunopurified human sex hormone binding globulin (SHBG) was photoinactivated and photolabeled by radioinert and radioactive photoaffinity labeling steroids delta 6-testosterone (delta 6-T) and delta 6-estradiol (delta 6-E2). The maximal levels of specific incorporation of these two reagents were 0.50 and 0.33 mol of label/mol of SHBG, respectively. Covalently labeled SHBG fractions were citraconylated, reduced, carboxymethylated, and cleaved by trypsin. Separation of tryptic digests by reverse-phase liquid chromatography gave single radioactive peaks at the same retention times with both steroid reagents. However, the two labeled peptidic fractions could be distinguished by capillary electrophoresis and immunodetection with anti-steroid antibodies, whereas the covalent attachment of radioactivity was confirmed by thin-layer chromatography on silica gel. Edman degradation of the two labeled peptides showed a single sequence His-Pro-Ile-([3H]X)-Arg corresponding to the pentapeptide His-Pro-Ile-Met-Arg 136-140 of SHBG sequence. The coincidence, in both cases, of the absence of an identifiable amino acid residue and of the elution of the most intense peak of radioactivity at the fourth cycle of Edman degradation suggests that the same Met-139 residue was labeled by delta 6-[1,2-3H2]T or by delta 6-[17 alpha-3H]E2. Liquid secondary ion mass spectrometry of the two peptides showed [M+H]+ ions at m/z 939.8 or 923.8, corresponding respectively to the addition of delta 6-T or delta 6-E2 to the pentapeptide. The presence of the steroid molecule in the delta 6-[3H]T-pentapeptide conjugate was confirmed by the difference of 2 mass units with the [M+H]+ peak of the delta 6-[4-14C]T-pentapeptide conjugate.

True hermaphrodites: an experimental model in the mouse

When the Y chromosome from the Mus musculus domesticus mouse strain is placed onto the C57BL/6J mouse background ovarian and testicular components develop in half of the XY progeny (B6.YDOM), providing an excellent model of true hermaphroditism. We examined the correlation between gonadal sex and development of the internal genital tract in the B6.YDOM mouse at puberty. Of 55 mice examined 20 had bilateral testes, 33 were true hermaphrodites and 2 had mixed gonadal dysgenesis. In all mice with bilateral testes male accessory sexual organs developed bilaterally. In the true hermaphrodites testes were found on either side but preferentially on the left side. When a male feature was present on the ipsilateral side of the ovary, the seminal vesicle was more frequently found than prostate or vas. Testicular mass was significantly different between the true hermaphrodites with and those without bilateral seminal vesicles. Similar difference was found in those with and without bilateral prostates. The serum testosterone level was not significantly different between these groups. We conclude that the testicular mass is a better discriminant than serum testosterone for the presence or absence of seminal vesicles and prostates at puberty.

Structure/function analyses of human sex hormone-binding globulin by site-directed mutagenesis

Human sex hormone-binding globulin (hSHBG) and rat androgen-binding protein (rABP) exhibit distinct affinities for sex-steroids. We therefore constructed and expressed a hSHBG/rABP hybrid cDNA encoding the N-terminal portion of hSHBG (205 residues) and the C-terminal portion of rABP (168 residues). The resulting chimera displayed similar steroid-binding characteristics as hSHBG and was recognised by a monoclonal antibody (S1B5) for hSHBG. We then created substitutions at Ser-133, His-136 and Met-139. The Asp-133 and Gln-136 mutants bound steroids in the same way as normal hSHBG while the steroid-binding affinity of Trp-139 was reduced. All three mutants cross-reacted similarly in a hSHBG radioimmunoassay, but Gln-136 was recognised poorly by the S1B5 antibody. These data imply that residues involved in steroid-binding are located within the N-terminal half of hSHBG and include Met-139, and that the S1B5 epitope is located in this region.

Complex structure and regulation of the ABP/SHBG gene

Extracellular androgen-binding proteins (ABPs) are thought to modulate the regulatory functions of androgens and the trans-acting nuclear androgen receptor. Testicular ABP and plasma sex hormone-binding globulin (SHBG), which is produced in the liver, are encoded by the same gene. We report here that the ABP/SHBG gene is also expressed in fetal rat liver and adult brain. Immunoreactive ABP was localized in the brain and fetal liver and mRNAs were identified in both tissues by northern blot hybridization. Analysis of brain and fetal liver cDNA clones revealed alternatively processed RNAs with sequence characteristics suggesting the encoded proteins could act as competitors of ABP/SHBG binding to cell surface receptors. One cDNA represented a fused transcript of the ABP/SHBG gene and the histidine decarboxylase gene that was apparently formed by a trans-splicing process. Gene sequencing experiments indicate that tissue-specific ABP/SHBG gene promoter-enhancer elements are utilized in testis, brain and fetal liver. These data demonstrate that the structure, RNA transcript processing and likely regulation of the ABP/SHBG gene are very complex.