Development of the adult-type Leydig cell population in the rat is affected by neonatal thyroid hormone levels

Thyroid Hormones Deficiency Impairs Male Germ Cell Development: A Cross Talk Between Hypothalamic-Pituitary-Thyroid, and—Gonadal Axes in Zebrafish

In vertebrates, thyroid hormones are critical players in controlling different physiological processes such as development, growth, metabolism among others. There is evidence in mammals that thyroid hormones are also an important component of the hormonal system that controls reproduction, although studies in fish remain poorly investigated. Here, we tested this hypothesis by investigating the effects of methimazole-induced hypothyroidism on the testicular function in adult zebrafish. Treatment of fish with methimazole, in vivo, significantly altered zebrafish spermatogenesis by inhibiting cell differentiation and meiosis, as well as decreasing the relative number of spermatozoa. The observed impairment of spermatogenesis by methimazole was correlated with significant changes in transcript levels for several genes implicated in the control of reproduction. Using an in vitro approach, we also demonstrated that in addition to affecting the components of the brain-pituitary-peripheral axis, T3 (triiodothyronine) also exerts direct action on the testis. These results reinforce the hypothesis that thyroid hormones are an essential element of multifactorial control of reproduction and testicular function in zebrafish and possibly other vertebrate species.

m6A mRNA methylation regulates testosterone synthesis through modulating autophagy in Leydig cells

Macroautophagy/autophagy is indispensable for testosterone synthesis in Leydig cells (LCs), and here we report a negative association between m6A modification and autophagy in LCs during testosterone synthesis. A gradual decrease of METTL14 (methyltransferase like 14) and an increase of ALKBH5 (alkB homolog 5, RNA demethylase) were observed in LCs during their differentiation from stem LCs to adult LCs. These events led to reduced mRNA methylation levels of N6-methyladenosine (m6A) and enhanced autophagy in LCs. Similar regulation of METTL14, ALKBH5, and m6A was also observed in LCs upon treatment with human chorionic gonadotropin (HsCG). Mechanistically, m6A modification promoted translation of PPM1A (protein phosphatase 1A, magnesium dependent, alpha isoform), a negative AMP-activated protein kinase (AMPK) regulator, but decreased expression of CAMKK2 (calcium/calmodulin-dependent protein kinase kinase 2, beta), a positive AMPK regulator, by reducing its RNA stability. Thus, m6A modification resulted in reduced AMPK activity and subsequent autophagy inhibition. We further demonstrated that ALKBH5 upregulation by HsCG was dependent on enhanced binding of the transcriptional factor CEBPB (CCAAT/enhancer binding protein [C/EBP], beta) and the TFEB (transcription factor EB) to its gene promoter. Moreover, HsCG treatment decreased METTL14 by reducing its stability. Collectively, this study highlights a vital role of m6A RNA methylation in the modulation of testosterone synthesis in LCs, providing insight into novel therapeutic strategies by exploiting m6A RNA methylation as targets for treating azoospermatism and oligospermatism patients with reduction in serum testosterone.Abbreviations: 3-MA: 3-methyladenine; ACTB: Actin, beta; ALKBH5: alkB homolog 5, RNA demethylase; AMPK: AMP-activated protein kinase; BafA1: bafilomycin A1; CAMKK2: calcium/calmodulin-dependent protein kinase kinase 2, beta; CEBPB: CCAAT/enhancer-binding protein (C/EBP), beta; ChIP: chromatin immunoprecipitation; FTO: fat mass and obesity associated; HsCG: human chorionic gonadotropin; HSD3B: 3β-hydroxysteroid dehydrogenase; LCs: Leydig cells; m6A: N6-methyladenosine; METTL14: methyltransferase like 14; METTL3: methyltransferase like 3; MTOR: mechanistic target of rapamycin kinase; PPM1A: protein phosphatase 1A, magnesium dependent, alpha isoform; PRKAA: 5'-AMP-activated protein kinase catalytic subunit alpha; SQSTM1: sequestosome 1; STK11/LKB1: serine/threonine kinase 11; TFEB: transcription factor EB; ULK1: unc-51-like kinase 1; WTAP: Wilms tumor 1-associating protein; YTHDF: YTH N6-methyladenosine RNA binding protein.

Thyroid hormones, Sertoli cell proliferation and differentiation in progenies from carbamazepine-treated rat dams during pregnancy and lactation

Carbamazepine (CBZ) is used in the control of seizure and affective disorders, causing hypothyroidism. Thyroid hormones regulate the Sertoli cell proliferation and differentiation. Clinical aspects must be considered since epileptic fertile women need to continuously use CBZ during pregnancy and lactation. This study aimed to evaluate the effects of CBZ on testis development of rat offspring from dams treated during pregnancy/lactation. Rat dams received CBZ (20 mg kg^-1  day^-1 ) or vehicle by intra-peritoneal route during gestation and lactation. Progenies were euthanised at 4, 14, 41, 63 and 93-days post-partum (dpp) for the evaluation of T3, T4 and TSH plasma total levels. Testicular cross sections were submitted to anti-Ki67, anti-PCNA, anti-p27^kip1 and anti-transferrin immunolabelling for the evaluation of Sertoli cells. There was a significant reduction in p27^kip1 -positive Sertoli cell numerical densities and an increase in TSH level at 14 dpp. CBZ exposure affected the volume density of transferrin-positive immunolabelling at 63 dpp. These results suggest that CBZ may cause a dysregulation of the controller system of thyroid hormones homeostasis leading to an increase in the proliferation rate at the neonatal phase and a differentiation delay of the Sertoli cell, culminating in an altered function at late puberty. The occurrence of hypothyroidism cannot be completely discarded.

Hypothyroidism induced by postnatal PTU (6-n-propyl-2-thiouracil) treatment decreases Sertoli cell number and spermatogenic efficiency in sexually mature pigs

Studies with 6-n-propyl-2-thiouracil (PTU) in laboratory rodents have shown that transient neonatal hypothyroidism leads to increased Sertoli cell (SC) number, testis size and sperm production. However, scarce and inconclusive data are available for farm animals. In the present study, Piau pigs received PTU in a gel capsule containing 8 mg/kg of body weight for 14 weeks starting from the first week of age, whereas control animals received only the vehicle. Blood samples were collected during the experimental period for hormonal evaluation in the serum. The animals were orchiectomized at adulthood and had their testes used for histomorphometric analysis. Indicating that the PTU concentration used was effective in promoting hypothyroidism, PTU-treated pigs showed a 30% lower body weight and reduced thyroxine levels (p < 0.05) during the treatment period. At adulthood, the body weight was similar in both groups but, surprisingly, PTU-treated pigs showed 30% lower testis weight (p < 0.05). In general, treated pigs presented increased follicle-stimulating hormone levels, whereas testosterone levels tended to be lower from 9 to 23 weeks of age. No significant differences were observed for estradiol, Leydig cell volume and number, tubular diameter, SC number per gram of testis, SC efficiency and meiotic index. However, seminiferous tubule occupancy, total tubular length, SC number per testis, and daily sperm production per testis and per gram of testis (DSP/g/T) were significantly lower (p < 0.05) in PTU-treated pigs. Therefore, in contrast to laboratory rodents, our results showed that SC proliferation and DSP/g/T (spermatogenic efficiency) in Piau pigs is diminished by postnatal PTU treatment.

A Novel Antigonadotropic Role of Thyroid Stimulating Hormone on Leydig Cell-Derived Mouse Leydig Tumor Cells-1 Line

Subclinical hypothyroid men characterized by a rise in only thyroid stimulating hormone (TSH) levels and normal thyroid hormone levels showed a fall in their serum progesterone and testosterone levels. This suggested a role of TSH in regulating Leydig cell steroidogenesis. Therefore, we investigated the direct role of TSH on steroid production and secretion using a mouse Leydig tumour cell line, MLTC-1. MLTC-1 cells were treated with different doses of TSH isolated from porcine pituitary as well as recombinant TSH. Steroid secretion was measured by radioimmunoassay. The mRNA levels of steroidogenic enzymes were quantitated by real time PCR whereas the corresponding protein levels were determined by Western blot. In MLTC-1 cells, pituitary TSH as well as recombinant TSH inhibited progesterone and testosterone secretion in a dose dependent manner. The inhibitory action of TSH on steroid secretion was unique and not mimicked by other anterior pituitary hormones including FSH and ACTH. Recombinant TSH showed no effect on StAR and CYP11A1, the enzymes catalysing the non-steroidogenic and steroidogenic rate-limiting steps of steroid synthesis respectively. Recombinant TSH was shown to inhibit steroidogenesis in MLTC-1 cells by inhibiting the 3β hydroxy steroid dehydrogenase mRNA and protein levels, the enzyme that catalyses the conversion of pregnenolone to progesterone. This inhibitory effect of TSH is probably direct as both mRNA and protein of the TSH receptor were shown to be present in the MLTC-1 cells.

Thyroid hormone actions on male reproductive system of teleost fish

Thyroid hormones (THs) play important roles in the regulation of many biological processes of vertebrates, such as growth, metabolism, morphogenesis and reproduction. An increasing number of studies have been focused on the involvement of THs in the male reproductive system of vertebrates, in particular of fish. Therefore, this mini-review aims to summarize the main findings on THs role in male reproductive system of fish, focusing on sex differentiation, testicular development and spermatogenesis. The existing data in the literature have demonstrated that THs exert their roles at the different levels of the hypothalamic-pituitary-gonadal (HPG) axis. In general a positive correlation has been shown between THs and fish reproductive status; where THs are associated with testicular development, growth and maturation. Recently, the molecular mechanisms underlying the role of THs in spermatogenesis have been unraveled in zebrafish testis. THs promote germ cell proliferation and differentiation by increasing a stimulatory growth factor of spermatogenesis produced by Sertoli cells. In addition, THs enhanced the gonadotropin-induced androgen release in zebrafish testis. Next to their functions in the adult testis, THs are involved in the gonadal sex differentiation through modulating sex-related gene expression, and testicular development via regulation of Sertoli cell proliferation. In conclusion, this mini-review showed that THs modulate the male reproductive system during the different life stages of fish. The physiological and molecular mechanisms showed a link between the thyroid and reproduction, suggesting a possibly co-evolution and interdependence of these two systems.

Age-related changes in testicular parameters and their relationship to thyroid hormones and testosterone in male Murrah buffaloes

Abstract. The present study aims to investigate the age-related changes in testicular parameters and their association with plasma triiodothyronine (T3), thyroxine (T4), and testosterone in male Murrah buffaloes. Testicular measurements and single blood samples were collected from male Murrah buffaloes (n= 103) aged between 6 months and 8 years. The correlation coefficients of average testicular length (ATL), paired testis width (PTW), and scrotal circumference (SC) in relation to age were 0.88, 0.91, and 0.90, respectively. The regression equation between testicular weight (TW) and age was Y=1.48×x0.005 (r= 0.90; R2= 0.79). Plasma T4 and testosterone increased significantly (p< 0.001) with age and their levels ranged between 12.9 and 41.8 and 0.05 and 1.48 ng mL−1, respectively. With respect to associations between testicular parameters and plasma hormone levels, we observed significant (p< 0.01) correlations between ATL, PTW, SC, TW, and plasma T4. A significant correlation (r= 0.31; p< 0.01) between plasma T4 and testosterone levels was also observed. However, the correlations between plasma T3 and testicular parameters and plasma T3 and testosterone were non-significant. From the present study, we conclude that plasma T4 is positively correlated with testicular parameters and plasma testosterone, indicating its role in testis development and steroidogenesis.

Effect of neonatal hypothyroidism on prepubertal mouse testis in relation to thyroid hormone receptor alpha 1 (THRα1)

Thyroid hormones (THs) are important for growth and development of many tissues, and altered thyroid status affects various organs and systems. Testis also is considered as a thyroid hormone responsive organ. Though THs play an important role in regulation of testicular steroidogenesis and spermatogenesis, the exact mechanism of this regulation remains poorly understood. The present study, therefore, is designed to examine the effect of neonatal hypothyroidism on prepubertal Parkes (P) strain mice testis in relation to thyroid hormone receptor alpha 1 (THRα1). Hypothyroidism was induced by administration of 6-propyl-2-thiouracil (PTU) in mother's drinking water from birth to day 28; on postnatal day (PND) 21 only pups, and on PND 28, both pups and lactating dams were euthanized. Serum T₃ and T₄ were markedly reduced in pups at PND 28 and in lactating mothers, while serum and intra-testicular testosterone levels were considerably decreased in pups of both age groups. Further, serum and intra-testicular levels of estrogen were significantly increased in hypothyroid mice at PND 28 with concomitant increase in CYP19 expression. Histologically, marked changes were noticed in testes of PTU-treated mice; immunohistochemical and western blot analyses of testes in treated mice also revealed marked decrease in the expression of THRα1 at both age groups. Semiquantitative RT-PCR and western blot analyses also showed reductions in both testicular mRNA and protein levels of SF-1, StAR, CYP11A1 and 3β-HSD in these mice. In conclusion, our results suggest that neonatal hypothyroidism alters localization and expression of THRα1 and impairs testicular steroidogenesis by down-regulating the expression SF-1, thereby affecting spermatogenesis in prepubertal mice.

Neonatal hypothyroidism does not increase Sertoli cell proliferation in iNOS−/− mice

Sertoli cell (SC) proliferation in mice occurs until two weeks after birth and is mainly regulated by FSH and thyroid hormones. Previous studies have shown that transient neonatal hypothyroidism in laboratory rodents is able to extend SC mitotic activity, leading ultimately to higher testis size and daily sperm production (DSP) in adult animals. Moreover, we have shown that due to higher SC proliferation and lower germ cell apoptosis, iNOS deficiency in mice also results in higher testis size and DSP. Although the cell size was smaller, the Leydig cells (LCs) number per testis also significantly increased in iNOS−/−mice. Our aims in the present study were to investigate if the combination of neonatal hypothyroidism and iNOS deficiency promotes additive effects in SC number, testis size and DSP. Hypothyroidism was induced in wild-type (WT) and iNOS−/−mice using 6-propyl-2-thiouracil (PTU) through the mother’s drinking water from 0 to 20 days of age, and were sacrificed at adulthood. Our results showed that, in contrast to the WT mice in which testis size, DSP and SC numbers increased significantly by 20, 40 and 70% respectively, after PTU treatment, no additive effects were observed for these parameters in treated iNOS−/−mice, as well as for LC. No alterations were observed in spermatogenesis in any group evaluated. Although we still do not have an explanation for these intriguing findings, we are currently investigating whether thyroid hormones influence iNOS levels and/or counterbalance physiological effects of iNOS deficiency in testis function and spermatogenesis.

Transient Hypothyroidism: Dual Effect on Adult-Type Leydig Cell and Sertoli Cell Development

Transient neonatal 6-propyl-2-thiouracil (PTU) induced hypothyroidism affects Leydig and Sertoli cell numbers in the developing testis, resulting in increased adult testis size. The hypothyroid condition was thought to be responsible, an assumption questioned by studies showing that uninterrupted fetal/postnatal hypothyroidism did not affect adult testis size. Here, we investigated effects of transient hypothyroidism on Leydig and Sertoli cell development, employing a perinatal iodide-deficient diet in combination with sodium perchlorate. This hypothyroidism inducing diet was continued until days 1, 7, 14, or 28 postpartum (pp) respectively, when the rats were switched to a euthyroid diet and followed up to adulthood. Continuous euthyroid and hypothyroid, and neonatal PTU-treated rats switched to the euthyroid diet at 28 days pp, were included for comparison. No effects on formation of the adult-type Leydig cell population or on Sertoli cell proliferation and differentiation were observed when the diet switched at/or before day 14 pp. However, when the diet was discontinued at day 28 pp, Leydig cell development was delayed similarly to what was observed in chronic hypothyroid rats. Surprisingly, Sertoli cell proliferation was 6- to 8-fold increased 2 days after the diet switch and remained elevated the next days. In adulthood, Sertoli cell number per seminiferous tubule cross-section and consequently testis weight was increased in this group. These observations implicate that increased adult testis size in transiently hypothyroid rats is not caused by the hypothyroid condition per se, but originates from augmented Sertoli cell proliferation as a consequence of rapid normalization of thyroid hormone concentrations.

THE EFFECTS OF MATERNAL HYPOTHYROIDISM ON THE IMMUNOREACTIVITY OF CYTOCHROME P450 AROMATASE IN THE POSTNATAL RAT TESTICLES

Introduction

Abnormal thyroid function affect spermato-genesis and male infertility. For men, the aromatase deficiency can cause infertility. In this study, the aim is to investigate the effect of maternal hypothyroidism on offspring testicular morphology and cytochrome-P450-aromatase (P450arom) immunoreactivity.

Materials and Methods

Eighteen Wistar albino pregnant rats were divided into three groups, namely A, B and K groups. Hypothyroidism was induced by adding 0.01% of propyl thiouracil (PTU) in drinking water. Hypothyroid mothers, group A: given PTU for 21 days during pregnancy, group B: given PTU for 21 days prior to pregnancy; control mothers, group K, given only water. Hypothyroid and control group mothers' pups at postnatal day (PND) 15 and 60 were sacrificed. We determined immunoreactivity intensity of P450arom and mRNA levels by RT-PCR performed in the testis tissues. ELISA method was used for thyroid function tests for T3, T4 and TSH. Structure of seminiferous tubule was evaluated by hematoxylin-eosin staining.

Results

It was seen that the aromatase expression in 15-day-old maternal hypothyroid groups was similar to the one in the control group while there was a decline in the aromatase expression of 60-day-old groups. As for mRNA, it was determined that it had a tendency to increase over time in all groups but this increase was not significant. The tubule diameter and Johnsen's Testicular Biopsy Score diminished in all hypothyroid groups in comparison to the control group.

Conclusion

The changes that occur in the early period of testis development due to maternal hypothyroidism negatively affect testis development in the next stages of life. This situation leads to a decline in aromatase expression in the following years.

A link between hypothyroidism, obesity and male reproduction

Hypothyroidism is a condition in which the serum levels of thyroid hormones are below that necessary to carry out physiological functions in the body. Hypothyroidism is related to obesity as an increase in body weight gain is seen in hypothyroid patients. Moreover, an inverse correlation between free thyroxine values and body mass index has been reported. Leptin, a polypeptide hormone produced by adipocytes, was originally thought to be an antiobesity hormone due its anorexic effects on hypothalamic appetite regulation. However, nowadays it is known that leptin conveys information about the nutritional status to the brain being considered a crucial endocrine factor for regulating several physiological processes including reproduction. Since the identification of thyroid hormone and leptin receptors on the testes, these hormones are being recognized as having important roles in male reproductive functions. A clear link exists among thyroid hormones, leptin and reproduction. Both hormones can negatively affect spermatogenesis and consequently may cause male infertility. The World Health Organization (WHO) estimates the overall prevalence of primary infertility ranging from 8 to 15%. The fact that 30% of couples' inability to conceive is related to a male factor and that the longer hypothyroidism persisted, the greater the damage to the testes, strongly suggest that more studies attempting to clarify both hormones actions directly in the testes need to be conducted specially in cases of congenital hypothyroidism. Therefore, the goal of this review is to highlight the relationship of such hormones in the reproductive system.

Endocrine-Disrupting Activity of Hydraulic Fracturing Chemicals and Adverse Health Outcomes After Prenatal Exposure in Male Mice

Oil and natural gas operations have been shown to contaminate surface and ground water with endocrine-disrupting chemicals. In the current study, we fill several gaps in our understanding of the potential environmental impacts related to this process. We measured the endocrine-disrupting activities of 24 chemicals used and/or produced by oil and gas operations for five nuclear receptors using a reporter gene assay in human endometrial cancer cells. We also quantified the concentration of 16 of these chemicals in oil and gas wastewater samples. Finally, we assessed reproductive and developmental outcomes in male C57BL/6J mice after the prenatal exposure to a mixture of these chemicals. We found that 23 commonly used oil and natural gas operation chemicals can activate or inhibit the estrogen, androgen, glucocorticoid, progesterone, and/or thyroid receptors, and mixtures of these chemicals can behave synergistically, additively, or antagonistically in vitro. Prenatal exposure to a mixture of 23 oil and gas operation chemicals at 3, 30, and 300 μg/kg · d caused decreased sperm counts and increased testes, body, heart, and thymus weights and increased serum testosterone in male mice, suggesting multiple organ system impacts. Our results suggest possible adverse developmental and reproductive health outcomes in humans and animals exposed to potential environmentally relevant levels of oil and gas operation chemicals.

The long-term effects of FSH and triiodothyronine administration during the pubertal period on Connexin 43 expression and spermatogenesis efficiency in adult rats

Follicle-stimulating hormone (FSH) and triiodothyronine (T3) are known regulatory factors of spermatogenesis initiation. Hyperstimulation of both hormones evokes regressional changes in connexin 43 expression and the seminiferous epithelium in young rats during testicular maturation. However, separate treatments with T3 reduce Sertoli cell number, which seems to be closely connected with the maturation of connexin 43 gap junctions. FSH elevates Sertoli cell number and function, but this effect may take place regardless of the presence of connexin 43-dependent intercellular communication. The aim of the study was to evaluate the later effects of such treatments. Newborn, male Wistar rats were divided randomly into experimental groups receiving daily subcutaneous injections of either 7.5 IU/animal FSH, or 100 mg/kg b.w. T3, or both substances or the same volume of vehicle (control group) until day 15 of life. The animals were sacrificed on day 50. Morphometric analysis and immunohistochemical reactions were performed using antibodies against Vimentin, Proliferating Cell Nuclear Antigen and Connexin 43 in the testis. Sertoli cell count, efficiency of spermatogenesis, and hormonal pattern were examined. Disturbances in the connexin 43 expression reduced the number of Sertoli cells, the efficiency of spermatogenesis and impaired endocrine function of testes in adult rats treated with FSH and T3 during puberty. Stimulation with FSH alone increased Sertoli cell number, but was associated with a negative effect on cell-to-cell connexin 43-dependent communication, with a consequential reduction of spermatogenesis efficiency. J. Exp. Zool. 323A: 256-265, 2015. © 2015 Wiley Periodicals, Inc.

Morphological and functional maturation of Leydig cells: from rodent models to primates

BACKGROUND

Leydig cells (LC) are the sites of testicular androgen production. Development of LC occurs in the testes of most mammalian species as two distinct growth phases, i.e. as fetal and pubertal/adult populations. In primates there are indications of a third neonatal growth phase. LC androgen production begins in embryonic life and is crucial for the intrauterine masculinization of the male fetal genital tract and brain, and continues until birth after which it rapidly declines. A short post-natal phase of LC activity in primates (including human) termed 'mini-puberty' precedes the period of juvenile quiescence. The adult population of LC evolves, depending on species, in mid- to late-prepuberty upon reawakening of the hypothalamic-pituitary-testicular axis, and these cells are responsible for testicular androgen production in adult life, which continues with a slight gradual decline until senescence. This review is an updated comparative analysis of the functional and morphological maturation of LC in model species with special reference to rodents and primates.

METHODS

Pubmed, Scopus, Web of Science and Google Scholar databases were searched between December 2012 and October 2014. Studies published in languages other than English or German were excluded, as were data in abstract form only. Studies available on primates were primarily examined and compared with available data from specific animal models with emphasis on rodents.

RESULTS

Expression of different marker genes in rodents provides evidence that at least two distinct progenitor lineages give rise to the fetal LC (FLC) population, one arising from the coelomic epithelium and the other from specialized vascular-associated cells along the gonad-mesonephros border. There is general agreement that the formation and functioning of the FLC population in rodents is gonadotrophin-responsive but not gonadotrophin-dependent. In contrast, although there is in primates some controversy on the role of gonadotrophins in the formation of the FLC population, there is consensus about the essential role of gonadotrophins in testosterone production. Like the FLC population, adult Leydig cells (ALC) in rodents arise from stem cells, which have their origin in the fetal testis. In contrast, in primates the ALC population is thought to originate from FLC, which undergo several cycles of regression and redifferentiation before giving rise to the mature ALC population, as well as from differentiation of stem cells/precursor cells. Despite this difference in origin, both in primates and rodents the formation of the mature and functionally active ALC population is critically dependent on the pituitary gonadotrophin, LH. From studies on rodents considerable knowledge has emerged on factors that are involved besides LH in the regulation of this developmental process. Whether the same factors also play a role in the development of the mature primate LC population awaits further investigation.

CONCLUSION

Distinct populations of LC develop along the life span of males, including fetal, neonatal (primates) and ALC. Despite differences in the LC lineages of rodents and primates, the end product is a mature population of LC with the main function to provide androgens necessary for the maintenance of spermatogenesis and extra-gonadal androgen actions.

Relative developmental toxicities of pentachloroanisole and pentachlorophenol in a zebrafish model (Danio rerio)

Pentachloroanisole (PCA) and pentachlorophenol (PCP) are chlorinated aromatic compounds that have been found in the environment and in human populations. The objective of this study is to characterize the effects of PCA in comparison to those of PCP on development at environmental relevant levels using a fish model. Zebrafish embryos were exposed to 0.1, 1, 10, 100, 500, 1000 μg/L PCA and PCP respectively for 96 h. Malformation observation, LC50 testing for survival rate at 96 hours post fertilization (hpf) and EC50 testing for hatching rate at 72 hpf indicated that the developmental toxicity of PCP was about 15 times higher than that of PCA. PCP exposure at 10 μg/L resulted in elevated 3, 3', 5-triiodothyronine (T3) levels and decreased thyroxine (T4) levels, whereas PCA had no effects on T3 or T4 levels. PCP and PCA exposure at 1 and 10 μg/L showed possible hyperthyroid effects similar to that of T3, due to increased relative mRNA expression of synapsin I (SYN), iodothyronine deiodinase type III (Dio3), thyroid hormone receptor alpha a (THRαa) and thyroid hormone receptor beta (THRβ), and decreased expression of iodothyronine deiodinase type II (Dio2). The results indicate that both PCA and PCP exposure can cause morphological deformities, possibly affect the timing and coordination of development in the central nervous system, and alter thyroid hormone levels by disrupting thyroid hormone regulating pathways. However, the developmental toxicity of PCA is at least ten times lower than that of PCP. Our results on the relative developmental toxicities of PCA and PCP and the possible underlying mechanisms will be useful to support interpretation of envrionmental concentrations and body burden levels observed in human populations.

Differential regulation of steroidogenic enzyme genes by TRα signaling in testicular Leydig cells

Thyroid hormone signaling has long been implicated in mammalian testicular function, affecting steroidogenesis in testicular Leydig cells. However, its molecular mechanism is not well understood. Here, we investigated the molecular action of thyroid hormone receptor-α (TRα) on mouse testicular steroidogenesis. TRα/thyroid hormone (T3) signaling differentially affected the expression of steroidogenic enzyme genes, mainly regulating their promoter activity. TRα directly regulated the promoter activity of the cytochrome P450 17α-hydroxylase/C17-20 lyase gene, elevating its expression in the presence of T3. TRα also indirectly regulated the expression of steroidogenic enzyme genes, such as steroidogenic acute regulatory protein and 3β-hydroxysteroid dehydrogenase, by modulating the transactivation of Nur77 on steroidogenic enzyme gene promoters through protein-protein interaction. TRα enhanced Nur77 transactivation by excluding histone deacetylases from Nur77 in the absence of T3, whereas liganded TRα inhibited Nur77 transactivation, likely due to interfering with the recruitment of coactivator such as the steroid receptor coactivator-1 to Nur77. Together, these findings suggest a role of TRα/T3 in testicular steroidogenesis and may provide molecular mechanisms for the differential regulation of steroidogenic enzyme genes by thyroid hormone.

Thyroid hormone function in the rat testis

Thyroid hormones are emerging regulators of testicular function since Sertoli, germ, and Leydig cells are found to express thyroid hormone receptors (TRs). These testicular cells also express deiodinases, which are capable of converting the pro-hormone T4 to the active thyroid hormone T3, or inactivating T3 or T4 to a non-biologically active form. Furthermore, thyroid hormone transporters are also found in the testis. Thus, the testis is equipped with the transporters and the enzymes necessary to maintain the optimal level of thyroid hormone in the seminiferous epithelium, as well as the specific TRs to execute thyroid hormone action in response to different stages of the epithelial cycle of spermatogenesis. Studies using genetic models and/or goitrogens (e.g., propylthiouracil) have illustrated a tight physiological relationship between thyroid hormone and testicular function, in particular, Sertoli cell differentiation status, mitotic activity, gap junction function, and blood-testis barrier assembly. These findings are briefly summarized and discussed herein.

Identification, proliferation, and differentiation of adult Leydig stem cells

Leydig cells, the testosterone-producing cells of the adult testis, rarely turn over. However, their elimination with ethane dimethanesulfonate (EDS) is followed by the appearance of new, fully functional adult Leydig cells. The cells that give rise to the new Leydig cells have not been well characterized, and little is known about the mechanism by which they are regulated. We isolated cells expressing platelet-derived growth factor receptor-α, but not 3β-hydroxysteroid dehydrogenase (3β-HSD(neg)) from the testes of EDS-treated adult rats. Depending on conditions, these cells proliferated indefinitely or differentiated and produced testosterone. To localize these cells and to determine the effect of the testicular environment on their function, the seminiferous tubules and testicular interstitium were physically separated and cultured. During the first 72 h in culture, 3β-HSD(neg) cells on the tubule surfaces underwent divisions. Some of these cells later expressed 3β-HSD and produced testosterone. Removal of the newly formed 3β-HSD(pos) cells from the tubule surfaces with EDS, followed by further culture of the stripped tubules, resulted in the reappearance of testosterone-producing cells. These results, taken together, suggest that the precursors for newly formed Leydig cells are stem cells, with many if not all situated on the surfaces of the seminiferous tubules. Although normally quiescent, the stem cells are capable of self-renewal and differentiation. The development of the tubule culture system should provide a valuable in vitro approach to assess the role(s) of niche components on the function of adult Leydig stem cells despite their residing in a complex mammalian tissue.

Connexins and pannexins: Coordinating cellular communication in the testis and epididymis

Gap junctions and connexins are critical for coordinating cellular functions in complex epithelia. In recent years there has been increased interest in understanding the regulation and function of gap junctions in both the testis and epididymis. Studies in transgenic mice in which connexin 43 (Cx43) is mutated or is knocked down only in Sertoli cells have demonstrated the essential role of Cx43 in spermatogenesis and differentiation of Sertoli cells. In the epididymis developmental studies have shown a role for numerous connexins in the differentiation of epithelial cells and communication between the basal cells and both principal and clear cells. In both tissues several factors, such thyroid hormones and androgens, are important in regulating expression and function of connexins. Pannexins, which form cellular channels but are structurally similar to gap junction proteins, have been identified in both testis and epididymis and, in the epididymis, are regulated by androgens. The objective of this review is to summarize the advances that have been made on the role and regulation of connexins and pannexins in the testis and epididymis and their implication in spermatogenesis and sperm maturation.

Stem Leydig cells: from fetal to aged animals

Leydig cells are the testosterone-producing cells of the testis. The adult Leydig cell (ALC) population ultimately develops from undifferentiated mesenchymal-like stem cells present in the interstitial compartment of the neonatal testis. Distinct stages of ALC development have been identified and characterized. These include stem Leydig cells (SLCs), progenitor Leydig cells, immature Leydig cells, and ALCs. This review describes our current understanding of the SLCs in the fetal, prenatal, peripubertal, adult, and aged rat testis, as well as recent studies of the differentiation of steroidogenic cells from the stem cells of other organs.

PROLONGED AND TRANSIENT NEONATAL HYPOTHYROIDISM ON LEYDIG CELL DIFFERENTIATION IN THE POSTNATAL RAT TESTIS

Hypothyroidism arrests the differentiation of adult Leydig cells (ALC) in the neonatal rat testis, and transient neonatal hypothyroidism produces a two-fold increase in the ALC numbers in the adult rat testis. We investigated 1) whether prolonged hypothyroidism beyond the neonatal period could continue to arrest the differentiation of the ALC, and 2) to understand how a two-fold increase in the number of ALC is produced in adult rats subjected to transient neonatal hypothyroidism. Three groups of Sprague Dawley rats were used; control, PTU-water group (transiently hypothyroid; added 0.1% propyl thiouracil/PTU to drinking water of lactating mothers at parturition until weaning of pups at day 21, pups were fed regular water thereafter), and PTU group (prolonged hypothyroid; mothers were fed 0.1% PTU in drinking water from parturition until pups were sacrificed at days 28 and 40 (pups had access to solid food after 21 days). Findings showed that PTU treatment continued to arrest ALC differentiation. Withdrawal of the PTU treatment at 21 days resulted in ALC differentiation by two-fold in number in PTU-water rats. Findings on luteinizing hormone (LH)-stimulated androgen secretory capacity per testis in vitro agreed with the morphological data. These results confirmed that 1) thyroid hormone is crucial to the onset of ALC differentiation in the postnatal rat testis, 2) increased numbers of mesenchymal cells present in the hypothyroid testes differentiate into ALC upon withdrawal of the PTU treatment to produce a two-fold number of ALC in adult rats subjected to transient neonatal hypothyroidism (i.e., PTU-water treatment), and 3) numbers of ALC and mesenchymal cells increase with age at a rate of 2:1 during the process of ALC differentiation in testes of control and PTU-water rats.

Androgen receptor roles in spermatogenesis and fertility: lessons from testicular cell-specific androgen receptor knockout mice

Androgens are critical steroid hormones that determine the expression of the male phenotype, including the outward development of secondary sex characteristics as well as the initiation and maintenance of spermatogenesis. Their actions are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily. AR functions as a ligand-dependent transcription factor, regulating expression of an array of androgen-responsive genes. Androgen and the AR play important roles in male spermatogenesis and fertility. The recent generation and characterization of male total and conditional AR knockout mice from different laboratories demonstrated the necessity of AR signaling for both external and internal male phenotype development. As expected, the male total AR knockout mice exhibited female-typical external appearance (including a vagina with a blind end and a clitoris-like phallus), the testis was located abdominally, and germ cell development was severely disrupted, which was similar to a human complete androgen insensitivity syndrome or testicular feminization mouse. However, the process of spermatogenesis is highly dependent on autocrine and paracrine communication among testicular cell types, and the disruption of AR throughout an experimental animal cannot answer the question about how AR in each type of testicular cell can play roles in the process of spermatogenesis. In this review, we provide new insights by comparing the results of cell-specific AR knockout in germ cells, peritubular myoid cells, Leydig cells, and Sertoli cells mouse models that were generated by different laboratories to see the consequent defects in spermatogenesis due to AR loss in different testicular cell types in spermatogenesis. Briefly, this review summarizes these results as follows: 1) the impact of lacking AR in Sertoli cells mainly affects Sertoli cell functions to support and nurture germ cells, leading to spermatogenesis arrest at the diplotene primary spermatocyte stage prior to the accomplishment of first meiotic division; 2) the impact of lacking AR in Leydig cells mainly affects steroidogenic functions leading to arrest of spermatogenesis at the round spermatid stage; 3) the impact of lacking AR in the smooth muscle cells and peritubular myoid cells in mice results in similar fertility despite decreased sperm output as compared to wild-type controls; and 4) the deletion of AR gene in mouse germ cells does not affect spermatogenesis and male fertility. This review tries to clarify the useful information regarding how androgen/AR functions in individual cells of the testis. The future studies of detailed molecular mechanisms in these in vivo animals with cell-specific AR knockout could possibly lead to useful insights for improvements in the treatment of male infertility, hypogonadism, and testicular dysgenesis syndrome, and in attempts to create safe as well as effective male contraceptive methods.

Chronic hypothyroidism only marginally affects adult-type Leydig cell regeneration after EDS administration

Chronic prenatally induced dietary hypothyroidism delays adult-type Leydig cell development, but does not block this process. Using a chemical model to induce hypothyroidism, it was suggested that development of a new population of Leydig cells was completely inhibited following the addition of the cytotoxic compound ethane-1,2-dimethyl sulphonate (EDS). In this study, we used a dietary approach to induce hypothyroidism and reinvestigated the regeneration of the Leydig cell population following EDS administration. Eighty-four day old euthyroid and chronically hypothyroid rats received an injection of EDS and were killed directly before or at regular intervals up to 77 days after EDS. In some control and hypothyroid animals, the first progenitor-type Leydig cells were observed at day 12 after EDS. At day 16, Leydig cell progenitors were present in all rats. The percentage of proliferating Leydig cells peaked in the euthyroid animals at day 21 after EDS. In the hypothyroid testis such a peak was not observed, although the percentage of proliferating regenerating Leydig cells was significantly higher from days 35 to 56 compared with the controls. This suggested that the wave of Leydig cell proliferation was delayed in the hypothyroid animals as compared with the euthyroid controls. On the day of EDS injection, the Leydig/Sertoli cell ratio was 37% lower in the hypothyroid rats compared with the controls. The Leydig/Sertoli cell ratio remained lower in the EDS-treated hypothyroid animals compared with the controls at all time points investigated. At day 77 after EDS, the Leydig cell population had returned to its pre-treatment size in both groups. Plasma testosterone production was reduced to below detectable levels immediately after EDS injection, and started to increase again on day 16, reaching pre-treatment values on day 21 in both groups. Taken together, severely reduced thyroid hormone levels did not block the regeneration of the adult-type Leydig cell population following EDS, as has been suggested previously.

Prenatal induced chronic dietary hypothyroidism delays but does not block adult-type Leydig cell development

Transient hypothyroidism induced by propyl-2-thiouracyl blocks postpartum Leydig cell development. In the present study, the effects of chronic hypothyroidism on the formation of this adult-type Leydig cell population were investigated, using a more physiological approach. Before mating, dams were put on a diet consisting of an iodide-poor feed supplemented with a low dose of perchlorate and, with their offspring, were kept on this diet until death. In the pups at day 12 postpartum, plasma thyroid-stimulating hormone levels were increased by 20-fold, whereas thyroxine and free tri-iodothyronine levels were severely depressed, confirming a hypothyroid condition. Adult-type progenitor Leydig cell formation and proliferation were reduced by 40-60% on days 16 and 28 postpartum. This was followed by increased Leydig cell proliferation at later ages, suggesting a possible slower developmental onset of the adult-type Leydig cell population under hypothyroid conditions. Testosterone levels were increased 2- to 10-fold in the hypothyroid animals between days 21 and 42 postpartum compared with the age-matched controls. Combined with the decreased presence of 5alpha-reductase, this implicates a lower production capacity of 5alpha-reduced androgens. In 84-day-old rats, after correction for body weight-to-testis weight ratio, plasma insulin-like factor-3 levels were 35% lower in the hypothyroid animals, suggestive of a reduced Leydig cell population. This is confirmed by a 37% reduction in the Sertoli cell-to-Leydig cell ratio in hypothyroid rats. In conclusion, we show that dietary-induced hypothyroidism delays but, unlike propyl-2-thiouracyl, does not block the development of the adult-type Leydig cell population.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Thyroid hormone receptor alpha 1-beta 1 expression in epididymal epithelium from euthyroid and hypothyroid rats

The objectives of the present work were to assess whether epithelial cells from the different segments of epididymis express TR alpha 1-beta 1 isoforms, to depict its subcellular immunolocalization and to evaluate changes in their expression in rats experimentally submitted to a hypothyroid state by injection of 131I. In euthyroid and hypothyroid groups, TR protein was expressed in epididymal epithelial cells, mainly in the cytoplasmic compartment while only a few one showed a staining in the nucleus as well. A similar TR immunostaining pattern was detected in the different segments of the epididymis. In hypothyroid rats, the number of TR-immunoreactive epithelial cells as well as the intensity of the cytoplasmic staining significantly increased in all sections analyzed. In consonance to the immunocytochemical analysis, the expression of TR alpha 1-beta 1 isoforms, assessed by Western blot revealed significantly higher levels of TR in cytosol compared to the nuclear fractions. Furthermore, TR expression of both alpha 1 and beta 1 isoforms and their mRNA levels were increased by the hypothyroid state. The immuno-electron-microscopy showed specific reaction for TR in principal cells associated with eucromatin, cytosolic matrix and mitochondria. The differences in expression levels assessed in control and thyroidectomized rats ascertain a specific function of TH on this organ.

Effect of methimazole-induced hypothyroidism on adrenal and gonadal functions in male Japanese quail (Coturnix japonica)

To investigate the effect of hypothyroidism on gonadal and adrenal functions in male Japanese quail (Coturnix japonica), hypothyroidism was induced in male adult Japanese quail by daily administration of 2-Mercapto-1-methylimidazole (methimazole) in their drinking water. Four weeks after methimazole treatment, the Japanese quail were sacrificed, and the plasma concentrations of free triiodothyronine (FT3), free thyroxine (FT4), total T3 (TT3), total T4 (TT4), corticosterone, testosterone, LH and immunoreactive (ir) inhibins were measured by radioimmunoassay, the testes and adrenal glands were removed and weighed and the thyroid glands and testes were fixed in 4% paraformaldehyde for histological observation. The results showed that the hypothyroidism induced by methimazole caused a significant decrease in body and testes weight; the plasma levels of FT3, FT4 and TT4 significantly decreased, and the hypothyroid quail possessed a greater number of small follicles and more follicular epithelial cells in the thyroid gland. In addition, hypothyroidism resulted in a significant decrease in the plasma concentrations of corticosterone, LH, testosterone and ir-inhibin. Furthermore, no spermatogenesis was found in the seminiferous tubules of the methimazole treatment groups. These results clearly demonstrate that hypothyroidism caused both gonadal and adrenal disturbances in the adult male Japanese quail.

Expression of insulin-like peptide 3 in the postnatal rat Leydig cell lineage: timing and effects of triiodothyronine-treatment

Fetal (FLC) and adult Leydig cells (ALC) secrete insulin-like peptide 3 (INSL3), which is linked to cryptorchidism in the newborn rat. Its gene regulation appears to be independent of that for most steroidogenic enzymes, and may thus be a marker for other aspects of ALC differentiation. Our study examined the following on INSL3 peptide expression in ALC lineage (i) timing, (ii) which cell stage, and (iii) effects of triiodothyronine (T3). Male Sprague-Dawley (SD) rats of postnatal days (pd) 1, 5, 7-21, 28, 40, 60, and 90 were used for the objectives (i) and (ii). For the objective (iii), control and T3-treated (daily T3 SC, 50 mug/kg bw) SD rats of pd7-16 and 21 were used. INSL3 was immunolocalized in Bouin's-fixed testes. FLC were positive and mesenchymal and Leydig progenitor cells were negative for INSL3 at tested ages. INSL3 in ALC lineage was first detected in newly formed ALC on pd16, although they were present from pd10. The intensity of INSL3 label was greater in ALC of pd40-90. ALC were present in T3-treated testes at pd9, but INSL3 first detected in them was on pd12. While INSL3 in FLC regulates testicular descent, INSL3 in ALC still has no well-defined function. However, its pattern of expression correlates temporally with the development of steroidogenic function and spermatogenesis. Thus, the delay between ALC differentiation and INSL3 expression in them implies that INSL3 in ALC is associated with maturation. The advancement of INSL3 expression in the ALC of T3-treated rats implies that this function is established earlier with T3-treatment.

Neuroendocrine regulation of Leydig cell development

During development in the mouse, two populations of Leydig cells arise sequentially. The fetal Leydig cell population arises shortly after testicular differentiation and functions primarily to produce androgens that are essential for masculinization of the fetus. The origin of the fetal Leydig stem cells remains uncertain, but it has been suggested that adrenocortical cells and fetal Leydig cells may share a common origin in an adrenogenital primordium. The fetal Leydig cells require an intact pituitary for normal development and are sensitive to both luteinizing hormone (LH) and adrenocorticotrophic hormone (ACTH). Loss of either one of these hormones does not, however, affect fetal androgen production, suggesting that both LH and ACTH may act to maintain fetal Leydig cell function in vivo in a redundant fashion. The adult Leydig cell population starts to develop soon after birth in the mouse. Initial differentiation does not appear to require gonadotropin input, but subsequent development and function are completely dependent upon LH. The adult Leydig cells do not require circulating follicle-stimulating hormone, provided that LH is present, but androgen stimulation, through the androgen receptor, is required for normal Leydig cell development in the mouse. It is likely that the effects of androgen are mediated directly in the Leydig cells or indirectly through the peritubular cells.

The Effects of Triiodothyronine on Rat Testis: A Morphometric and Immunohistochemical Study

The aim of present study was to investigate the effects of 3,3',5-triiodothyronine (T(3)) on rat testis both morphometrically and immunohistochemically with determining of insulin-like growth factor I (IGF-I) expression. Adult male Wistar-albino rats used in the study were divided into two groups; control and T(3)-treated groups. After T(3) treatment there was observed to be a decrease in testicular weights, diameters of seminiferous tubules and the number of sertoli cells, and an increase in the number of leydig cells (P<0.05). Some of the seminiferous tubule lumens of T(3) administrated rats had cellular debris. IGF-I was localized in sertoli cells, late spermatids and leydig cells of all groups. IGF-I immunoreactivity in T(3) treated rats was higher than in controls in all stages of the cycle of rat seminiferous epithelium, but the staining intensity of leydig cells were similar in both groups. In conclusion, the present results suggest that T(3) may modulate the testicular function by affecting IGF-I activity at the gonadal level.

Thiourea-induced thyroid hormone depletion impairs testicular recrudescence in the air-breathing catfish, Clarias gariepinus

We used thiourea-induced thyroid hormone depletion as a strategy to understand the influence of thyroid hormones on testicular recrudescence of the air-breathing catfish, Clarias gariepinus. Treatment with 0.03% thiourea via immersion for 21 days induced hypothyroidism (thyroid hormone depletion) as evidenced by significantly reduced serum T(3) levels. Thiourea-treated males had narrowed seminiferous lobules with fewer spermatozoa in testis, very little or no secretory fluid, reduced protein and sialic acid levels in seminal vesicles when compared to controls. The histological changes were accompanied by reduction in serum and tissue levels of testosterone (T) and 11-ketotestosterone (11-KT), a potent male specific androgen in fish. Qualitative changes in the localization of catfish gonadotropin-releasing hormone (cfGnRH) and luteinizing hormone (LH, heterologous system) revealed a reduction in the distribution of immunoreactive neuronal cells and fibers in thyroid depleted fish. Interestingly, thiourea-withdrawal group showed physiological and histological signs of recovery after 21 days such as reappearance of spermatozoa and partial restoration of 11-KT and T levels. These data demonstrate that thyroid hormones play a significant role in testicular function of catfish. The mechanism of action includes modulating sex steroids either directly or through the hypothalamo (GnRH)-hypophyseal (LH) axis.

Development and function of the adult generation of Leydig cells in mice with Sertoli cell-selective or total ablation of the androgen receptor

It is established that androgens and unidentified Sertoli cell (SC)-derived factors can influence the development of adult Leydig cells (LC) in rodents, but the mechanisms are unclear. We evaluated adult LC development and function in SC-selective androgen receptor (AR) knockout (SCARKO) and complete AR knockout (ARKO) mice. In controls, LC number increased 26-fold and LC size increased by approximately 2-fold between 12 and 140 d of age. LC number in SCARKOs was normal on d 12, but was reduced by more than 40% at later ages, although LC were larger and contained more lipid droplets and mitochondria than control LC by adulthood. ARKO LC number was reduced by up to 83% at all ages compared with controls, and LC size did not increase beyond d 12. Serum LH and testosterone levels and seminal vesicle weights were comparable in adult SCARKOs and controls, whereas LH levels were elevated 8-fold in ARKOs, although testosterone levels appeared normal. Immunohistochemistry and quantitative PCR for LC-specific markers indicated steroidogenic function per LC was probably increased in SCARKOs and reduced in ARKOs. In SCARKOs, insulin-like factor-3 and estrogen sulfotransferase (EST) mRNA expression were unchanged and increased 3-fold, respectively, compared with controls, whereas the expression of both was reduced more than 90% in ARKOs. Changes in EST expression, coupled with reduced platelet-derived growth factor-A expression, are potential causes of altered LC number and function in SCARKOs. These results show that loss of androgen action on SC has major consequences for LC development, and this could be mediated indirectly via platelet-derived growth factor-A and/or estrogens/EST.

New insights into the regulation of mammalian sex determination and male sex differentiation

In mammals, sex development is a genetically and hormonally controlled process that begins with the establishment of chromosomal or genetic sex (XY or XX) at conception. At approximately 6 to 7 weeks of human gestation or embryonic day e11.5 in the mouse, expression of the Y chromosome-linked sex determining gene called SRY (described in detail in this chapter) then initiates gonadal differentiation, which is the formation of either a testis (male) or an ovary (female). Male sex differentiation (development of internal and external reproductive organs and acquisition of male secondary sex characteristics) is then controlled by three principal hormones produced by the testis: Mullerian inhibiting substance (MIS) or anti-Mullerian hormone (AMH), testosterone, and insulin-like factor 3 (INSL3). In the absence of these critical testicular hormones, female sex differentiation ensues. This sequential, three-step process of mammalian sex development is also known as the Jost paradigm. With the advent of modern biotechnologies over the past decade, such as transgenics, array-based gene profiling, and proteomics, the field of mammalian sex determination has witnessed a remarkable boost in the understanding of the genetics and complex molecular mechanisms that regulate this fundamental biological event. Consequently, a number of excellent reviews have been devoted to this topic. The purpose of the present chapter is to provide an overview of selected aspects of mammalian sex determination and differentiation with an emphasis on studies that have marked this field of study.

Autocrine regulation of steroidogenic function of Leydig cells by transforming growth factor-alpha

We have determined the effects of LH on the expression of transforming growth factor-alpha (TGFalpha) and epidermal growth factor receptor (EGFR) system in rat Leydig cells and investigated its role in steroidogenesis. LH and TGFalpha/epidermal growth factor (EGF) significantly increased the levels of TGFalpha mRNA and protein, and the levels of EGFR protein in immature rat Leydig cells (ILC). Treatment with TGFalpha or EGF for 24h resulted in significant increase in androgen production in ILC. The increase in androgen production in response to TGFalpha was associated with increased mRNA levels of SR-BI, steroidogenic acute regulatory (StAR) and P450scc but not of 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and P450c17. TGFalpha also caused a marked increase in the levels StAR protein in ILC. EGFR inhibitor (AG1478) blocked the effects of TGFalpha while MEK-inhibitor (PD98059) potentiated TGFalpha or LH effects on steroidogenesis. A PKA inhibitor (H89) blocked both TGFalpha and LH effects on steroidogenesis. We conclude that TGFalpha plays an autocrine role in LH dependent development and function of Leydig cells.

Age-related Variation of Plasma Concentrations of Skatole, Androstenone, Testosterone, Oestradiol-17beta, Oestrone Sulphate, Dehydroepiandrosterone Sulphate, Triiodothyronine and IGF-1 in Six Entire Male Pigs

This study describes the age-related variation in boar taint compounds, skatole and androstenone, and testosterone, oestradiol-17 beta (E17 beta), oestrone sulphate (ES), dehydroepiandrosterone sulphate (DHEAS), triiodothyronine (T(3)) and insulin-like growth factor-1 (IGF-1) in six boars. Three pairs of littermates of crossbred entire male pigs (from three Yorkshire x Duroc dams and one Hampshire sire) were included. Blood samples were taken at the age of 9-15 weeks and thereafter at weekly intervals from the age of 20-32 weeks. Plasma concentrations of skatole, androstenone, testosterone, E17 beta, ES, DHEAS, T(3) and IGF-1 were measured. We found that skatole levels in boars increased at the age around puberty after an increase in the levels of testicular steroids. Levels of skatole were not associated with the levels of sex steroids, T(3) and IGF-1. However, the increased level of testicular steroids is probably the underlying factor needed for high skatole levels to occur although the specific mechanism leading to increased skatole levels remains unknown.

Stimulation of TM3 Leydig cell proliferation via GABA(A) receptors: a new role for testicular GABA

The neurotransmitter gamma-aminobutyric acid (GABA) and subtypes of GABA receptors were recently identified in adult testes. Since adult Leydig cells possess both the GABA biosynthetic enzyme glutamate decarboxylase (GAD), as well as GABA(A) and GABA(B) receptors, it is possible that GABA may act as auto-/paracrine molecule to regulate Leydig cell function. The present study was aimed to examine effects of GABA, which may include trophic action. This assumption is based on reports pinpointing GABA as regulator of proliferation and differentiation of developing neurons via GABA(A) receptors. Assuming such a role for the developing testis, we studied whether GABA synthesis and GABA receptors are already present in the postnatal testis, where fetal Leydig cells and, to a much greater extend, cells of the adult Leydig cell lineage proliferate. Immunohistochemistry, RT-PCR, Western blotting and a radioactive enzymatic GAD assay evidenced that fetal Leydig cells of five-six days old rats possess active GAD protein, and that both fetal Leydig cells and cells of the adult Leydig cell lineage possess GABA(A) receptor subunits. TM3 cells, a proliferating mouse Leydig cell line, which we showed to possess GABA(A) receptor subunits by RT-PCR, served to study effects of GABA on proliferation. Using a colorimetric proliferation assay and Western Blotting for proliferating cell nuclear antigen (PCNA) we demonstrated that GABA or the GABA(A) agonist isoguvacine significantly increased TM3 cell number and PCNA content in TM3 cells. These effects were blocked by the GABA(A) antagonist bicuculline, implying a role for GABA(A) receptors. In conclusion, GABA increases proliferation of TM3 Leydig cells via GABA(A) receptor activation and proliferating Leydig cells in the postnatal rodent testis bear a GABAergic system. Thus testicular GABA may play an as yet unrecognized role in the development of Leydig cells during the differentiation of the testicular interstitial compartment.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

Thyroid hormones: their role in testicular steroidogenesis

Thyroid hormones are important for growth and development of many tissues. Altered thyroid hormone status causes testicular abnormalities. For instance, juvenile hypothyroidism/neonatal transient hypothyroidism induces macroorchidism, increases testicular cell number (Sertoli, Leydig, and germ cells) and daily sperm production. Triiodothyronine (T3) receptors have been identified in sperm, developing germ cells, Sertoli, Leydig, and peritubular cells. T3 stimulates Sertoli cell lactate secretion as well as mRNA expression of inhibin-alpha, androgen receptor, IGF-I, and IGFBP-4. It also inhibits Sertoli cell mRNA expression of Müllerian inhibiting substance (MIS), aromatase, estradiol receptor, and androgen binding protein (ABP) and ABP secretion. T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. It stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. Although the role of T3 on sperm, germ, and peritubular cells has not yet been completely studied, it is clear that T3 directly regulates Sertoli and Leydig cell functions. Further studies are required to elucidate the direct effect of T3 on sperm, germ, and peritubular cells.

Differentiation of adult-type Leydig cells occurs in gonadotrophin-deficient mice

During mammalian testis development distinct generations of fetal and adult Leydig cells arise. Luteinising hormone (LH) is required for normal adult Leydig cell function and for the establishment of normal adult Leydig cell number but its role in the process of adult Leydig cell differentiation has remained uncertain. In this study we have examined adult Leydig cell differentiation in gonadotrophin-releasing hormone (GnRH)-null mice which are deficient in circulating gonadotrophins. Adult Leydig cell differentiation was assessed by measuring expression of mRNA species encoding four specific markers of adult Leydig cell differentiation in the mouse. Each of these markers (3beta-hydroxysteroid dehydrogenase type VI (3betaHSD VI), 17beta-hydroxysteroid dehydrogenase type III (17betaHSD III), prostaglandin D (PGD)-synthetase and oestrogen sulphotransferase (EST)) is expressed only in the adult Leydig cell lineage in the normal adult animal. Real-time PCR studies showed that all four markers are expressed in adult GnRH-null mice. Localisation of 3betaHSD VI and PGD-synthetase expression by in situ hybridisation confirmed that these genes are expressed in the interstitial tissue of the GnRH-null mouse. Treatment of animals with human chorionic gonadotrophin increased expression of 3betaHSD VI and 17betaHSD III within 12 hours further indicating that differentiated, but unstimulated cells already exist in the GnRH-null mouse. Thus, while previous studies have shown that LH is required for adult Leydig cell proliferation and activity, results from the present study show that adult Leydig cell differentiation will take place in animals deficient in LH.

Effect of neonatal treatment of rats with potent or weak (environmental) oestrogens, or with a GnRH antagonist, on Leydig cell development and function through puberty into adulthood

This study addressed whether reduced Sertoli cell number or manipulation of the neonatal hormone environment has an influence on final Leydig cell number per testis in the rat, by applying neonatal treatments known to affect these parameters, namely administration of a GnRH antagonist (GnRHa) or diethylstilboestrol (DES, in doses of 10, 1 or 0.1 microg per injection). The effect of treatment with either of two 'environmental oestrogens', bisphenol-A (Bis-A) or octylphenol (OP), was also evaluated. Leydig (3beta-hydroxysteroid dehydrogenase immunopositive) cell development and function (plasma testosterone levels) were studied through puberty into adulthood. Treatment with GnRHa impaired testis growth, Leydig cell (nuclear) volume per testis and testosterone levels during puberty, when compared with controls, but final Leydig cell volume/number in adulthood was comparable with controls. As adult testis weight was reduced by 45% in GnRHa-treated rats, the percentage Leydig cell volume per testis was approximately double (p < 0.01) that in controls, and also at day 35. Testosterone levels in adulthood in GnRHa-treated rats were lower (p < 0.01) than in controls but were within the lower end of the normal range. Treatment with DES caused largely dose-dependent suppression of testis growth, Leydig cell (nuclear) volume per testis and testosterone levels up to day 35. Although by adulthood, Leydig cell volume/number per testis was comparable with controls in DES-treated rats, testosterone levels remained grossly subnormal. Neonatal treatment with either Bis-A or OP had little consistent effect on any of the parameters studied except that both treatments significantly elevated testosterone levels on day 18, as did treatment with DES-0.1 microg. The present findings are interpreted in the context of what is known about the hormonal regulation of Leydig cell development. These lead to the conclusion that final Leydig cell number per testis is not determined by the number of Sertoli cells per testis and appears not to be influenced in any major way by gonadotrophins, androgens or oestrogens in the first 2 weeks of postnatal life. This implies that adult Leydig cell number may be determined prior to birth.

Changes in the testis interstitium of Brown Norway rats with aging and effects of luteinizing and thyroid hormones on the aged testes in enhancing the steroidogenic potential

We tested the possibility of using LH and thyroxine (T(4)) to restore the testicular steroidogenic ability in aged Brown Norway rats. Three-, 6-, 12- (n = 8 per group), and 18-mo-old (n = 32; 3M, 6M, 12M, and 18M, respectively) rats were used. The 18M rats were divided into four groups (n = 8 per group) and implanted subdermally with Alzet mini-osmotic pumps containing saline (control), LH (24 microg/day), T(4) (5 microg/day), and LH+T(4) (24+5 microg/day), respectively, for 4 wk (to 19 mo [19M] of age). Testis volume and absolute volumes of many testicular components were unchanged with advancing age and treatments, except for the blood vessels (occasional thickening), lymphatic space (increased), and Leydig cells (decreased with age but increased to the 3M level with LH and to the 12M level with both T(4) and LH+T(4), respectively). The number of Leydig and connective tissue cells per testis was unchanged with aging and treatments. The number of macrophages was significantly higher in treated rats. The average volume of a Leydig cell was significantly decreased in 12M and 19M control rats. However, LH and LH+T(4) restored it to the 3M level, and T(4) restored to the 12M level. The steroidogenic ability of Leydig cells in vitro decreased when aging from the 3M to the 19M level, LH and T(4) enhanced it to the 12M level, and LH+T(4) raised it to the 3M level. Serum LH was unchanged from 3M to 12M rats, significantly reduced in 19M control rats, and raised above the 3M values with both LH and LH+T(4) treatment and above the 19M (control) values with T(4) treatment; the latter values were lower than the 3M level. Serum T(4) and tri-iodothyronine (T(3)) were highest in 3M and 6M rats and declined in 12M and 19M control rats; the latter group had the lowest levels. In all treated groups, T(4) and T(3) levels were significantly above those of 19M control rats but were lower than those of 3M through 12M rats. Serum testosterone was unchanged from 3M to 12M rats but was reduced in 19M control rats. Both LH and T(4) significantly raised these values above the 19M control levels, but they were still lower than the 3M through 12M levels. Additionally, LH+T(4) significantly raised the serum testosterone levels to those of 12M rats, but these values were significantly lower than those of 3M and 6M rats. These findings show that with 24+5-microg dose of LH+T(4) per day for 4 wk, a 100% recovery of the average volume of a Leydig cell and its steroidogenic ability in vitro and a 73% and 300% restoration of serum testosterone levels compared to 3M and 19M control rats, respectively, could be achieved in aged Brown Norway rats. A 100% reversibility (compared to 3M rats) in serum testosterone levels appears to be possible with adjustments in the LH and T(4) doses in the LH+T(4) treatment.

Severe and mild neonatal hypothyroidism mediate opposite effects on Leydig cells of rats

The effects of neonatal hypothyroidism on the number of Leydig cells were studied in neonatal Wistar rats. Moderate or severe hypothyroidism were induced during neonatal life by giving different amounts of methimazole (MMI; 0.05% or 0.1%) in the drinking water of pregnant and lactating dams. Rats were sacrificed on day 21 of postnatal life. Severely hypothyroid rats had approximately 45-fold higher serum thyrotropin (TSH) values and demonstrated approximately a 65% decrease in testes weight (p < 0.05) and the number of Leydig cells. However, in moderately hypothyroid rats, in which serum TSH was only approximately 16-fold higher, testicular weight was normal and the number of Leydig cells almost doubled (p < 0.01). There were no significant differences between the serum-free testosterone levels of the moderately and severely hypothyroid rats versus controls. Serum levels of 3alpha-androstanediol glucuronide, although decreased to less than 10% in severely hypothyroid rats (p < 0.01), were not changed by mild hypothyroidism. The number of Sertoli cells was increased in moderately hypothyroid rats versus controls (p < 0.05) and even further increased in severely hypothyroid rats (p < 0.05). We conclude that (1) severe neonatal hypothyroidism impairs the development and function of the testes and (2) moderate neonatal hypothyroidism stimulates the proliferation of Leydig cells.