Sertoli cell-specific expression of rat androgen-binding protein in transgenic mice: effects on somatic cell lineages

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.

Impact of low molecular weight phthalates in inducing reproductive malfunctions in male mice: Special emphasis on Sertoli cell functions

Phthalates are commonly used as plasticizers in a variety of products. Since they have been identified as endocrine-disrupting chemicals (EDCs), effect of phthalates on human health is a major concern. In this study, we evaluated individual as well as combined mixture effects of three low molecular weight phthalates on the reproductive system of male mice, specifically on the Sertoli cell structure and function. In order to analyze the blood testes barrier (BTB) dynamics, primary culture of Sertoli cells from 3-weeks old male mice was used for mimicking typical tight junction structures. Male mice were exposed to long-term (45 days) and combined mixture of three phthalates, diethyl phthalate (DEP), diphenyl phthalate (DPP), and dimethyl isophthalate (DMIP) between pre-pubertal to adult stage. Our data showed significant decrease (p < 0.05) in the rates of transcription of certain prominent Sertoli cell specific genes like transferrin, testin and occludin. Moreover, we also observed significant decreases in the expression of proteins like 3β-HSD, connexin-43 and occludin in testicular lysates of treated animals (p < 0.05). The transmission electron microscopic analysis revealed that the test compounds significantly altered the structural integrity of Sertoli cells. The significant changes of Sertoli cell tight junction structure by test compounds were associated with phosphorylation of ERK. Taken together, our study suggests that low molecular weight phthalates may affect male fertility by altering both structural and functional integrity of Sertoli cells in testes.

Temporal role of Sertoli cell androgen receptor expression in spermatogenic development

Sertoli cell (SC) androgen receptor (AR) activity is vital for spermatogenesis. We created a unique gain-of-function transgenic (Tg) mouse model to determine the temporal role of SCAR expression in testicular development. The SC-specific rat Abpa promoter directed human Tg AR [Tg SC-specific AR (TgSCAR)] expression, providing strong premature postnatal AR immunolocalized to SC nuclei. Independent Tg lines revealed that TgSCAR dose dependently reduced postnatal and mature testis size (to 60% normal), whereas androgen-dependent mature seminal vesicle weights and serum testosterone levels remained normal. Total SC numbers were reduced in developing and mature TgSCAR testes, despite normal or higher Fshr mRNA and circulating FSH levels. Postnatal TgSCAR testes exhibited elevated levels of AR-regulated Rhox5 and Spinlw1 transcripts, and precocious SC function was demonstrated by early seminiferous tubular lumen formation and up-regulated expression of crucial SC tight-junction (Cldn11 and Tjp1) and phagocytic (Elmo1) transcripts. Early postnatal Amh expression was elevated but declined to normal levels in peripubertal-pubertal TgSCAR vs. control testes, indicating differential age-related regulation featuring AR-independent Amh down-regulation. TgSCAR induced premature postnatal spermatogenic development, shown by increased levels of meiotic (Dmc1 and Spo11) and postmeiotic (Capza3 and Prm1) germ cell transcripts, elevated meiotic-postmeiotic germ:Sertoli cell ratios, and accelerated spermatid development. Meiotic germ:Sertoli cell ratios were further increased in adult TgSCAR mice, indicating predominant SCAR-mediated control of meiotic development. However, postmeiotic germ:Sertoli cell ratios declined below normal. Our unique TgSCAR paradigm reveals that atypical SC-specific temporal AR expression provides a direct molecular mechanism for induction of precocious testicular development, leading to reduced adult testis size and decreased postmeiotic development.

DOES ORCHIDOPEXY REVERT THE HISTOLOGICAL ALTERATIONS IN EPIDIDYMAL AND VAS DEFERENS CAUSED BY CRYPTORCHIDISM?

Cryptorchidism is a pathological condition in which the testicles are retained in the abdominal cavity, resulting in atrophic seminiferous tubules. Some gross structural abnormalities and histological altercations have been described in the epididymis and vas deferens in humans with cryptorchidic testes. Orchidopexy surgery restores testicular spermatogenesis in experimental and clinical procedures, but it is still unclear whether histological changes in the epididymis and vas deferens caused by cryptorchidism may be reverted by orchidopexy. The aim of this study was to evaluated the histological changes in the epididymis and vas deferens following experimental uni- bilateral cryptorchidism in mature and immature mice, and to determine whether altercations could be reversed by orchidopexy. Young and adult C57 BL6 mice were randomized into three groups: control mice, bi/unilaterally cryptorchidic mice and bilaterally cryptorchidic mice with orchidopexy. After evaluation of testis, epididymis and vas deferens, there were no histological alterations in contralateral epididymis of mice unilaterally cryptorchidic. Ipsilateral epididymis of unilaterally cryptorchidic mice and epididymis from bilaterally cryptorchidic mice showed significant histological alterations. Orchuidopexy resorted normal spermatogenesis and the histological features of epididymis. It would appear that persistent male infertility clinically observed after orchidopexy could not be related to histological alteration in the testis and epididymis. Development and maintenance of the vas deferens seems to be controlled independently of the epididymis since it was not altered by cryptorchidism condition.

Estrogen Receptor β Expression and Apoptosis of Spermatocytes of Mice Overexpressing a Rat Androgen-Binding Protein Transgene1

Progression of the first meiotic division in male germ cells is regulated by a variety of factors, including androgens and possibly estrogens. When this regulation fails, meiosis is arrested and primary spermatocytes degenerate by apoptosis. Earlier studies showed that overexpression of rat androgen-binding protein (ABP) in the testis of transgenic mice results in a partial meiotic arrest and apoptosis of pachytene spermatocytes. In view of the recent localization of estrogen receptor beta (ERbeta) in primary spermatocytes and data suggesting the ability of ERbeta to repress cellular proliferation, we tested the hypothesis that variations in the testicular steroid microenvironment caused by excess ABP produce changes in ERbeta expression in this cellular type that could be associated to the meiotic arrest and, eventually, to the induction of germ cell apoptosis observed in the ABP transgenic mice. Increased levels of ERbeta mRNA and protein were demonstrated in the testis of rat ABP transgenic mice compared with nontransgenic littermates by reverse transcriptase-polymerase chain reaction (RT-PCR) experiments, Northern blotting, and Western Blotting. The major differences were found when isolated germ cells of transgenic and nontransgenic littermates were analyzed by RT-PCR. In keeping with this finding, ERbeta was strongly immunolabeled in pachytene spermatocytes of rat ABP transgenic mice and localized in tubular stages in which TUNEL labeling was maximal. Confocal microscopy analysis of a fluorescent TUNEL assay and ERbeta immunohistochemistry revealed that degenerating pachytene spermatocytes overexpressed ERbeta. The present results are consistent with the interpretation that ERbeta is associated with the events that regulate negatively the progression of meiosis or that lead to spermatocyte apoptosis.

Structural and functional modifications of sertoli cells in the testis of adult follicle-stimulating hormone receptor knockout mice

Follicle stimulating hormone (FSH) plays important roles during testicular development and in the maintenance of spermatogenesis in the adult. However, the cellular events or pathways that FSH regulates to achieve these effects in Sertoli cells, where the FSH receptors (FSH-R) are located, is still not fully elucidated. The development of FSH-R knockout (FORKO) mice provides a model to examine alterations in testicular structure and function in its absence. To this end, light (LM) and electron microscopic (EM) analyses of perfusion-fixed testes of wild-type and FORKO mice of different ages were performed. Under the LM, a significant reduction was noted in the profile area of seminiferous tubules of FORKO mice compared with their wild-type counterparts at different ages. In addition, FORKO testes revealed large irregularly shaped spaces within the seminiferous epithelium, extending from the base to the lumen. Such spaces were often separated by anastomotic cords of spherical germ cells or completely surrounded elongating spermatids. This phenotype was restricted to half or less of the circumference of only some tubules, but was seen at all stages. EM analyses revealed that the spaces corresponded to an apparent accumulation of fluid in the Sertoli cell cytoplasm, coincident with an absence of the fine flocculent ground substance seen in wild-type mice. However, the Sertoli organelles, while less prominent, appeared intact and to be floating in the enlarged fluid-filled cytoplasm. Functionally, androgen-binding protein (ABP), a major secretory protein of Sertoli cells, was dramatically reduced in FORKO mice. These results suggest that FSH-R signaling normally maintains water balance in Sertoli cells in addition to regulating ABP production.

gamma-Tubulin overexpression in Sertoli cells in vivo. II: Retention of spermatids, residual bodies, and germ cell apoptosis

The degree of germ cell dependence on Sertoli cell-mediated activities has been a subject of considerable attention. Sertoli cell secretory pathways have been extensively studied both in an effort to understand their normal physiologic roles and as targets for pharmacologic and toxicant activity. To determine the degree to which normal spermatogenesis depends on key functions of the Sertoli cell microtubule network, adenoviral vectors that overexpress the microtubule nucleating protein, gamma-tubulin, were delivered to Sertoli cells in vivo. gamma-Tubulin overexpression disrupts the Sertoli cell microtubule network (as described in the companion article); leads to gross disorganization of the seminiferous epithelium, inducing retention of spermatids and residual bodies; and causes germ cell apoptosis. These data are consistent with earlier studies in which toxicants and pharmacologic agents were used to disrupt microtubule networks. These data confirm that Sertoli cell microtubule networks play an important role in maintaining the organization of the seminiferous epithelium and that in the absence of an intact Sertoli cell microtubule network, germ cell viability is impaired.

GATA-1 testis activation region is essential for Sertoli cell-specific expression of GATA-1 gene in transgenic mouse

BACKGROUND

The erythroid transcription factor GATA-1 is also expressed in Sertoli cells of the testis. The testicular expression of GATA-1 is regulated in a developmental and spermatogenic stage-specific manner. To further clarify the regulatory mechanisms of testicular GATA-1 gene expression, we carried out transgenic reporter gene expression analyses.

RESULTS

We found that GATA-1 expression in Sertoli cells is markedly decreased concomitant with the emergence of elongated spermatids in the seminiferous tubules. Transgenic reporter mouse analyses revealed that a 15 kb GATA-1 genomic region is sufficient to recapitulate the gene expression profile in Sertoli cells. While the GATA-1 haematopoietic enhancer and the proximal first exon are included within the 15 kb genomic region, these regulatory elements are not essential for GATA-1 expression in Sertoli cells. Further analyses using deletion constructs revealed that a 1.5 kb region 5' to the GATA-1 haematopoietic enhancer is essential for gene expression in Sertoli cells and this region is referred to as the GATA-1 testis activation region.

CONCLUSION

These results thus demonstrated that the GATA-1 testis activation region is essential for Sertoli cell-specific expression of GATA-1 gene. The 15 kb genomic region is applicable and useful for the expression vector system specific for adult Sertoli cells in stage VII to IX.

Dynamics of testicular germ cell apoptosis in normal mice and transgenic mice overexpressing rat androgen-binding protein

The number and type of testicular germ cells undergoing apoptosis in different age groups of mice (from 7 to 360 days of age) was determined and compared in age-matched wild type (WT) control and in a transgenic (TG) mice homozygous to rat androgen binding protein (ABP) using flow cytometry. Flow cytometric quantification revealed that the total number of germ cells undergoing apoptosis did not differ significantly in WT and TG mice up to Day 14. From Day 21 to Day 60, the number of germ cells undergoing apoptosis was consistently higher in TG than in WT mice. Starting from Day 90, the number of germ cells undergoing apoptosis in TG mice was lower than controls until Day 360. In 21-60 days old TG mice, spermatogonia, S-Phase cells, and primary spermatocytes are the cell types undergoing apoptosis at significantly greater numbers than those in WT mice. However, starting from day 60, the total number of spermatids undergoing apoptosis was significantly lower in TG mice than in age-matched WT controls. TdT-mediated dUTP-biotin nick end labeling (TUNEL) in testicular sections from TG mice of 21 and 30 days of age confirmed the presence of increased numbers of apoptotic germ cells compared to their age matched controls. These data indicate that the continuous presence of greater than physiological concentrations of ABP in the mouse testis has a biphasic effect on the frequency of apoptosis in germ cells. The initial pre-pubertal increase in testicular germ cell apoptosis may result from direct or indirect actions of ABP and is likely to determine the subsequent life-death balance of germ cell populations in TG mice, whereas the subsequent reduction may result from maturation depletion. A wave of apoptosis during the pre-pubertal period is required for normal spermatogenesis to develop, and our data indicate that this apoptotic wave may be regulated by ABP and/or androgens.

An activated human follicle-stimulating hormone (FSH) receptor stimulates FSH-like activity in gonadotropin-deficient transgenic mice

FSH mediates its testicular actions via a specific Sertoli cell G protein-coupled receptor. We created a novel transgenic model to investigate a mutant human FSH receptor (FSHR(+)) containing a single amino acid substitution (Asp567Gly) equivalent to activating mutations in related glycoprotein hormone receptors. To examine the ligand-independent gonadal actions of FSHR(+), the rat androgen-binding protein gene promoter was used to direct FSHR(+) transgene expression to Sertoli cells of gonadotropin-deficient hypogonadal (hpg) mice. Both normal and hpg mouse testes expressed FSHR(+) mRNA. Testis weights of transgenic FSHR(+) hpg mice were increased approximately 2-fold relative to hpg controls (P < 0.02) and contained mature Sertoli cells and postmeiotic germ cells absent in controls, revealing FSHR(+)-initiated autonomous FSH-like testicular activity. Isolated transgenic Sertoli cells had significantly higher basal ( approximately 2-fold) and FSH-stimulated ( approximately 50%) cAMP levels compared with controls, demonstrating constitutive signaling and cell-surface expression of FSHR(+), respectively. Transgenic FSHR(+) also elevated testosterone production in hpg testes, in the absence of circulating LH (or FSH), and it was not expressed functionally on steroidogenic cells, suggesting a paracrine effect mediated by Sertoli cells. The FSHR(+) response was additive with a maximal testosterone dose on hpg testicular development, demonstrating FSHR(+) activity independent of androgen-specific actions. The FSHR(+) response was male specific as ovarian expression of FSHR(+) had no effect on hpg ovary size. These findings reveal transgenic FSHR(+) stimulated a constitutive FSH-like Sertoli cell response in gonadotropin-deficient testes, and pathways that induced LH-independent testicular steroidogenesis. This novel transgenic paradigm provides a unique approach to investigate the in vivo actions of mutated activating gonadotropin receptors.

Dynamics of testicular germ cell proliferation in normal mice and transgenic mice overexpressing rat androgen-binding protein: a flow cytometric evaluation

Transgenic mice carrying rat androgen-binding protein (ABP) genomic DNA express high amounts of testicular ABP and develop a progressive impairment of spermatogenesis. To understand the mechanism of these changes, we have studied the pattern of testicular germ cell proliferation from 7 to 360 days of age in wild-type (WT) control and transgenic homozygous (ABP-TG) mice by flow cytometry after labeling DNA in isolated germ cells with propidium iodide. At all ages studied, the body weight of the ABP-TG mice was lower than that of age-matched WT controls. Significantly reduced testicular weight and total germ cell number in the ABP-TG mice were evident from Day 30 and Day 60, respectively. Flow cytometric analysis of isolated germ cells revealed that the number of germ cells undergoing proliferation (S-phase cells) was identical in WT control and ABP-TG mice up to Day 14. Subsequently, the number of germ cells in S-phase was consistently higher in ABP-TG than in WT mice. The number of primary spermatocytes was significantly increased starting from Day 60, and the numbers of round and elongated spermatids were significantly reduced in the ABP-TG animals from Day 21 and Day 60 onwards, respectively. Immunocytometry for intracellular ABP at 90 days of age revealed that the percentage of ABP-containing germ cells was greater in ABP-TG than in WT mice. The continuous presence of ABP in mouse seminiferous tubules at greater than physiological concentrations facilitates the formation of primary spermatocytes but impairs subsequent transformation to round and elongated spermatids. Based on our observations and the analysis of the available literature, the most likely mechanism for production of these effects is sustained reduction in the bioavailability of androgens.

Reduced fertility and spermatogenesis defects in mice lacking chromosomal protein Hmgb2

High mobility group 2 protein (Hmgb2) is a member of the HMGB protein family, which includes the ubiquitous Hmgb1 and the embryo-specific Hmgb3. The three proteins are more than 80% identical at the amino acid level and their biochemical properties are indistinguishable. Hmgb1 is an abundant component of all mammalian nuclei and acts as an architectural factor that bends DNA and promotes protein assembly on specific DNA targets. Cells that lack Hmgb1 can survive, although mutant mice die shortly after birth. As Hmgb2 is present in all cultured cells and is abundant in thymus, the preferred source for HMGB proteins, it was considered a ubiquitous variant of Hmgb1. We show that in adult mice Hmgb2 is restricted mainly to lymphoid organs and testes, although it is widely expressed during embryogenesis. Mice that lack Hmgb2 are viable. However, male Hmgb2(−)(/)(−) mice have reduced fertility, that correlates with Sertoli and germ cell degeneration in seminiferous tubules and immotile spermatozoa. Significantly, Hmgb2 is expressed at very high levels in primary spermatocytes, while it is barely detectable in spermatogonia and elongated spermatids. This peculiar pattern of expression and the phenotype of mutants indicate that Hmgb2 has a specialised role in germ cell differentiation.

Meiotic arrest and germ cell apoptosis in androgen-binding protein transgenic mice

The fundamental role of androgen-binding protein (ABP) in spermatogenesis remains obscure after nearly 25 yr since its first characterization. In the present investigation, we used a transgenic mouse model that overexpresses rat ABP to examine the potential involvement of this protein in the regulation of processes occurring during spermatogenesis. Specifically, homozygous or heterozygous transgenic mice were analyzed in terms of spermatogenic progression, DNA fragmentation pattern, and germinal cell ploidy status. All animals homozygous for transgenic ABP exhibited an increased accumulation of primary spermatocytes and cells at metaphase with abnormal morphology and localization within the seminiferous epithelium. Analysis of DNA fragmentation by in situ techniques and agarose gel electrophoresis provided evidence for an increased occurrence of apoptosis in the transgenic animals, principally involving pachytene spermatocytes and cells at metaphase. Flow cytometric analysis of the DNA content of isolated germ cells revealed a reduction in the number of haploid cells, an increase in the number of tetraploid cells, and the appearance of a hypotetraploid cell population, consistent with degenerating primary spermatocytes. In mice heterozygous for the transgene, the effects were less prominent, and the degree to which spermatogenesis was compromised correlated with the levels of ABP messenger RNA in individual animals. The present results are interpreted to suggest that ABP can act as a modulator of spermatogenesis by regulating completion of the first meiotic division of primary spermatocytes.