A potential novel mechanism involving connexin 43 gap junction for control of sertoli cell proliferation by thyroid hormones

Effects of a Sertoli cell-specific knockout of Connexin43 on maturation and proliferation of postnatal Sertoli cells

Adult male Sertoli cell-specific Connexin43 knockout mice (SCCx43KO) exhibit higher Sertoli cell (SC) numbers per seminiferous tubule compared to their wild type (WT) littermates. Thus, deletion of this testicular gap junction protein seems to affect the proliferative potential and differentiation of "younger" SC. Although SC have so far mostly been characterised as postmitotic cells that cease to divide and become an adult, terminally differentiated cell population at around puberty, there is rising evidence that there exist exceptions from this for a very long time accepted paradigm. Aim of this study was to investigate postnatal SC development and to figure out underlying causes for observed higher SC numbers in adult KO mice. Therefore, the amount of SC mitotic figures was compared, resulting in slightly more and prolonged detection of SC mitotic figures in KO mice compared to WT. SC counting per tubular cross section revealed significantly different time curves, and comparing proliferation rates using Bromodesoxyuridine and Sox9 showed higher proliferation rates in 8-day old KO mice. SC proliferation was further investigated by Ki67 immunohistochemistry. SC in KO mice displayed a delayed initiation of cell-cycle-inhibitor p27Kip1 synthesis and prolonged synthesis of the phosphorylated tumour suppressor pRb and proliferation marker Ki67. Thus, the higher SC numbers in adult male SCCx43KO mice may arise due to two different reasons: Firstly, in prepubertal KO mice, the proliferation rate of SC was higher. Secondly, there were differences in their ability to cease proliferation as shown by the delayed initiation of p27Kip1 synthesis and the prolonged production of phosphorylated pRb and Ki67. Immunohistochemical results indicating a prolonged period of SC proliferation in SCCx43KO were confirmed by detection of proliferating SC in 17-days-old KO mice. In conclusion, deletion of the testicular gap junction protein Cx43 might prevent normal SC maturation and might even alter also the proliferation potential of adult SC.

Transcriptomic profiling identified altered expression of genes associated with testicular functions in adult F1 rats exposed to carbimazole during fetal period

The central goal of this study was to investigate the alterations in transcriptome of testis in F1 generation adult rats exposed to carbimazole prenatally. At post-natal day 100, the testis of rats delivered to carbimazole exposed (time-mated pregnant rats orally administered with carbimazole from gestation day 9 to 21) and control (untreated pregnant rats) groups were subjected to transcriptomic analysis using NGS platform. A total of 187 differentially expressed (up regulated: 49 genes; down regulated: 138) genes were identified in carbimazole exposed rats over controls and the major processes associated with these altered testicular transcripts were examined. Functional clustering analysis suggest that the involvement of identified DEGs were linked to intrinsic and extrinsic apoptotic pathways, mitochondrial solute carriers slc25a members, nuclear receptors/zinc family members, steroidogenic pathway and cholesterol synthesis, and growth factors and protein kinases and thus represent potential mediators of the developmental toxic effects of carbimazole in F1 generation rats. Based on the findings, it can be concluded that prenatal exposure to carbimazole prominently affects expression of multiple transcripts implicating key regulatory events associated with testicular functions, spermatogenesis and steroidogenesis in rats at their adulthood. These results support our earlier findings and hypothesis. This background information obtained at the testicular transcriptome during gestational hypothyroidism might be helpful for future studies and experiments to gain additional in-depth analysis and to develop strategies to protect F1 generation male reproductive health. SIGNIFICANCE: The rationale for the paper described thyroid gland changes in the off springs. Antithyroid drugs are widely used to control thyroid disorders and used to control thyroid hormone levels during surgeries. Carbimazole is one of the antithyroid drugs and is a parent molecule of methimazole. Both the drugs can able to cross placenta. During fetal period, the development of thyroid gland is not completely formed and hence, the fetus entirely depends on the maternal thyroid hormones. Therefore, it is conceivable that the disturbances at the level of maternal thyroid hormones could interfere with the development of vital organs such as testis and glands including thyroid gland (Kala et al., 2012). To address this notion, the present study was designed and executed.

Potassium perchlorate effects on primordial germ cells of developing medaka larvae

Perchlorate is a chemical compound commonly used in military artillery and equipment. It has been detected in drinking water, air, soil, and breast milk. Exposure of humans to perchlorate can occur in the theater of war and areas adjacent to military training grounds. A high concentration of perchlorate has been found to affect reproduction in vertebrates, including fish. However, whether environmental concentrations of perchlorate can affect primordial germ cells (PGCs), the founders of sperm and eggs, is not clearly understood. In the present study, we examined the effects of 0, 10, 100, and 1000 μg/L potassium perchlorate exposure on the embryonic development of medaka and their PGCs. Perchlorate exposure delayed hatching time, reduced heartbeat, inhibited migration of PGCs, and increased developmental deformities in the larvae. The 10 and 20 mg/L concentrations of perchlorate were lethal to embryos, whereas vitamin C co-treatment (1 mg/L) completely blocked perchlorate-induced mortality. RNA-seq analysis of isolated PGCs showed a non-linear pattern in expression profiles of differentially altered genes. Significantly upregulated genes were found in PGCs from the 10 and 1000 μg/L groups, whereas the 100 μg/L groups showed the highest number of significantly downregulated genes. Gene ontology analysis predicted differentially expressed genes to be involved in proteolysis, metabolic processes, peptides activity, hydrolase activity, and hormone activity. Among the cellular components, extracellular, intracellular, sarcoplasmic, and 6-phosphofructokinase and membrane-bounded processes were affected. Ingenuity Pathway Analysis of PGC transcriptomes revealed thyroid hormone signaling to be affected by all concentrations of perchlorate. The present results suggested that perchlorate affected the development of medaka larvae and vitamin C was able to ameliorate perchlorate-induced embryo mortality. Additionally, perchlorate altered the global transcriptional network in PGCs in a non-linear fashion suggesting its potential effects on developing germ cells and fertility.

Connexin43 represents an important regulator for Sertoli cell morphology, Sertoli cell nuclear ultrastructure, and Sertoli cell maturation

The Sertoli cell (SC)-specific knockout (KO) of connexin43 (Cx43) was shown to be an effector of multiple histological changes in tubular morphology, resulting in germ cell loss through to a Sertoli-cell-only (SCO) phenotype and vacuolated seminiferous tubules containing SC-clusters. Our present study focused on the effects of Cx43 loss on SC ultrastructure. Using serial block-face scanning electron microscopy (SBF-SEM), we could confirm previous results. Ultrastructural analysis of Sertoli cell nuclei (SCN) revealed that these appear in clusters with a phenotype resembling immature/proliferating SCs in KO mice. Surprisingly, SCs of fertile wild type (WT) mice contained SCN with a predominantly smooth surface instead of deep indentations of the nuclear envelope, suggesting that these indentations do not correlate with germ cell support or spermatogenesis. SBF-SEM facilitated the precise examination of clustered SCs. Even if the exact maturation state of mutant SCs remained unclear, our study could detect indications of cellular senescence as well as immaturity, emphasising that Cx43 affects SC maturation. Moreover, Sudan III staining and transmission electron microscopy (TEM) demonstrated an altered lipid metabolism in SCs of Cx43 deficient mice.

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.

Multi Species Analyses Reveal Testicular T3 Metabolism and Signalling as a Target of Environmental Pesticides

Thyroid hormones (THs) regulate many biological processes in vertebrates, including reproduction. Testicular somatic and germ cells are equipped with the arrays of enzymes (deiodinases), transporters, and receptors necessary to locally maintain the optimal level of THs and their signalling, needed for their functions and spermatogenesis. Pesticides, as chlorpyrifos (CPF) and ethylene thiourea (ETU), impair the function of thyroid and testis, affecting male fertility. However, their ability to disarrange testicular T3 (t-T3) metabolism and signalling is poorly considered. Here, a multi-species analysis involving zebrafish and mouse suggests the damage of t-T3 metabolism and signalling as a mechanism of gonadic toxicity of low-doses CPF and ETU. Indeed, the developmental exposure to both compounds reduces Dio2 transcript in both models, as well as in ex-vivo cultures of murine seminiferous tubules, and it is linked to alteration of steroidogenesis and germ cell differentiation. A major impact on spermatogonia was confirmed molecularly by the expression of their markers and morphologically evidenced in zebrafish. The results reveal that in the adopted models, exposure to both pesticides alters the t-T3 metabolism and signalling, affecting the reproductive capability. Our data, together with previous reports suggest zebrafish as an evaluable model in assessing the action of compounds impairing locally T3 signalling.

The Molecular Mechanism of Sex Hormones on Sertoli Cell Development and Proliferation

Sustaining and maintaining the intricate process of spermatogenesis is liable upon hormones and growth factors acting through endocrine and paracrine pathways. The Sertoli cells (SCs) are the major somatic cells present in the seminiferous tubules and are considered to be the main regulators of spermatogenesis. As each Sertoli cell supports a specific number of germ cells, thus, the final number of Sertoli cells determines the sperm production capacity. Similarly, sex hormones are also major regulators of spermatogenesis and they can determine the proliferation of Sertoli cells. In the present review, we have critically and comprehensively discussed the role of sex hormones and some other factors that are involved in Sertoli cell proliferation, differentiation and maturation. Furthermore, we have also presented a model of Sertoli cell development based upon the recent advancement in the field of reproduction. Hence, our review article provides a general overview regarding the sex hormonal pathways governing Sertoli cell proliferation and development.

Role of non-classical effects of testosterone and epitestosterone on AMH balance and testicular development parameters

Testosterone (T) and its 17-α epimer, epitestosterone (EpiT), are described as having non-classical effects in addition to their classical androgen actions via the intracellular androgen receptor (iAR). The actions of these androgens play an essential role in triggering factors that shift Sertoli cells from the proliferation phase to the maturation phase. This process is essential for successful spermatogenesis and normal fertility. The aim of this work was to investigate the difference between T and EpiT effects in normal and in chemically castrated Wistar rats. We also tested the effects of these hormones when the iAR-dependent pathways were inhibited by the antiandrogen flutamide. Rats were chemically castrated on postnatal day (pnd) 5 using EDS, a cytotoxic agent that promotes apoptosis of Leydig cells, reducing androgen levels. Then, animals received replacement with T or EpiT and were treated or not with flutamide from pnd 6 to pnd 13 or 20 and were euthanized on pnd 14 and 21. Animals treated with EpiT and flutamide had lower body weight overall. Epididymis weight was also reduced in animals treated with EpiT and flutamide. Flutamide per se reduced epididymis weight at both ages (pnd 14 and 21). Testicular weight and the testicular/body weight ratio were reduced in EDS animals, and flutamide further reduced this weight in animals which received T replacement. EDS administration reduced mRNA levels of both AMH (anti-Müllerian hormone) and its receptor, AMHR2, at pnd 14. In the testes of flutamide-treated animals, EpiT reduced AMH, and both T and EpiT replacement diminished AMHR2 mRNA expression also on pnd 14. EDS decreased iAR expression, and androgen replacement did not change this effect on pnd 21. In rats receiving flutamide, only those also receiving T and EpiT replacement exhibited decreased iAR expression. An increase in connexin 43 expression was observed in animals treated with EpiT without flutamide, whereas in rats treated with flutamide, both hormones were ineffective to increase connexin 43 expression reduced by EDS. Our results suggest that EpiT has an antiandrogen effect on androgen-sensitive tissues such as the epididymis. Nonetheless, the effects of T and EpiT on testicular development parameters are similar. Both hormones may act through their iAR-independent non-classical pathway, regulating AMH and AMHR2, as well as iAR expression.

3, 3', 5-Triiodo-L-thyronine affects polarity proteins of bovine Sertoli cells via WT1/non-canonical Wnt signaling pathway

To determine the role of 3, 3', 5-triiodo-L thyroxine (T3) in the differentiation of Sertoli cells (SCs) and the factors influencing maturity via the Wilms' tumor 1 (WT1)/non-canonical Wnt signaling pathway, high purity SCs were isolated from newborn calves' testes and cultured in vitro. The SCs were stimulated with T3, and co-treated with short interference (si) RNA to knockdown endogenous WT1 and non-canonical Wnt signalling inhibitor Wnt-c59. Our results suggested that the addition of different concentrations (0, 25, 50, and 100 nM) of T3 in the culture medium changed the expression of KRT-18 (SCs immature marker) and accelerated the differentiation of SCs. T3 (100 nM) treatment induced up-regulated expression of WT1 over time (p < 0.05), while the expression of polarity proteins (Par3, Par6b, and E-cadherin) and Wnt4 were affected to varying degrees (p < 0.05). SCs were treated simultaneously with T3 + Wnt-c59 and T3 + WT1 siRNA, and the results showed that T3 could affect the expression of polarity proteins via WT1/non-canonical Wnt signaling pathway. These data put together indicate that T3 plays a dependent role in the induction of bovine SCs differentiation via WT1/non-canonical Wnt signaling pathway in vitro. This study proposes for the first time that WT1 is a major target for T3.

Wilms' tumour 1 (WT1) negatively regulates the expression of connexin 43 via a non-canonical Wnt signalling pathway in cultured bovine Sertoli cells

The gap junction protein connexin (Cx) 43 between adjacent Sertoli cells (SCs) is the main testicular factor regulating the growth and development of SCs, and plays a vital role in controlling cell differentiation and maturation. However, the endogenous testicular factors that regulate Cx43 and the downstream signalling pathways that mediate Cx43-dependent SC differentiation are unclear. In this study, high-purity SCs were isolated from newborn calves' testes by differential adherence. The SCs were then cultured invitro and treated with short interference RNA to knockdown endogenous Wilms' tumour 1 (WT1). In WT1-knockdown SCs, Cx43 expression was upregulated. To elucidate the intracellular signalling mechanism of Cx43 in the differentiation and maturation of immature SCs, SCs were treated simultaneously with non-canonical Wnt signalling inhibitors CCG-1423 and GO-6983; in these SCs, Cx43 expression was upregulated. Together, these data indicate that WT1 negatively regulates the expression of Cx43 produced from SCs via a non-canonical Wnt signalling pathway in cultured bovine SCs.

Arecoline inhibits pineal-testis function in experimentally induced hypothyroid rats

Arecoline is known to cause endocrine dysfunction. In the current article role of arecoline on pineal-testis activity was investigated in hypothyroid rats induced by propylthiouracil (PTU). PTU treatment caused thyroid dysfunction ultrastructurally with a fall in T₃ and T₄ levels followed by a rise of thyroid stimulating hormone (TSH) level. Pineal activity was impaired by PTU treatment, as evident from degenerated synaptic ribbons and mitochondria of the pinealocytes with depletion of pineal and serum N-acetyl serotonin and melatonin levels. Leydig cell function was suppressed, evident from reduced smooth endoplasmic reticulum and depletion of testosterone level. Sex accessories function was impaired by showing scanty rough endoplasmic reticulum with depletion of fructose and sialic acid levels. Arecoline treatment that caused pineal dysfunction and testicular stimulation in control rats, suppressed both pineal and testis functions after PTU treatment. The findings suggest that arecoline inhibits pineal-testis function in experimentally induced hypothyroid rats.

The impact of thyroid diseases starting from birth on reproductive function

The aim of this review is to provide relevant information regarding the impact of thyroid disease, starting from birth and mainly concerning hyperthyroidism and hypothyroidism, on reproduction. Hyperthyroidism occurs much less commonly in children than hypothyroidism, with Graves' disease (GD) being the most common cause of thyrotoxicosis in children. Children born with neonatal GD have no defects in the reproductive system that could be related to hyperthyroidism. Current treatment options include antithyroid drugs (ATD), surgery, and radioactive iodine (RAI). In males, normal thyroid function seems important, at least in some parameters, for maintenance of semen quality via genomic or non-genomic mechanisms, either by locally acting on Sertoli cells, Leydig cells, or germ cells, or by affecting crosstalk between the HPT axis and the HPG axis. Sexual behavior may also be affected in thyroxic men, although many of these patients may have normal free testosterone levels. In women, menstrual irregularities are the most common reproduction-related symptoms in thyrotoxicosis, while this disorder is also associated with reduced fertility, although most women remain ovulatory. An increase in sex hormone-binding globulin (SHBG) and androgens, thyroid autoimmunity, and an impact on uterine oxidative stress are the main pathophysiological mechanisms which may influence female fertility. Thyroid hormones are responsible for normal growth and development during pre- and postnatal life, congenital hypothyroidism (CH) being the most common cause of neonatal thyroid disorders, affecting about one newborn infant in 3500. The reproductive tract appears to develop normally in cretins. Today, CH-screening programs allow for early identification and treatment, and, as a result, affected children now achieve normal or near-normal development. Hypothyroidism in males is associated with decreased libido or impotence. Although little is currently known about the effects of hypothyroidism on spermatogenesis and fertility, it has been established that sperm morphology and motility are mainly affected. In women of reproductive age, hypothyroidism results in changes in cycle length and amount of bleeding. Moreover, a negative effect on fertility and higher miscarriage rates has also been described.

In vitro formation of the blood-testis barrier during long-term organotypic culture of human prepubertal tissue: comparison with a large cohort of pre/peripubertal boys

STUDY QUESTION

How does the formation of the blood-testis barrier (BTB), as reflected by the expression of connexin 43 and claudin 11 proteins during the pubertal transition period, take place in vitro compared to samples from a large cohort of pre/peripubertal boys?

SUMMARY ANSWER

The BTB connexin 43 and claudin 11 expression patterns appeared to be partially achieved in organotypic culture when compared to that in samples from 71 pre/peripubertal patients.

WHAT IS KNOWN ALREADY

Although alterations in the protein expression patterns of the BTB, whose main components are connexin 43 and claudin 11, are known to be associated with impaired spermatogenesis in mice and adult men, there is a lack of knowledge on its formation in pre-peripubertal human tissue both in vitro and in vivo. Moreover, despite Sertoli cell (SC) maturation during long-term organotypic culture of immature testicular tissue (ITT), initiation of spermatogenesis has not yet been achieved.

STUDY DESIGN, SIZE, DURATION

Histological sections from 71 pre-peripubertal patients were evaluated for the formation of the BTB acting as in vivo controls according to age, SC maturation, clinical signs of puberty and germ cell differentiation. Testicular tissue fragments retrieved from three prepubertal boys were cultured in a long-term organotypic system to analyze the BTB formation and expression pattern in correlation with SC maturation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Testicular histological sections from 71 patients aged 0-16 years who underwent a biopsy between 2005 and 2014 to preserve their fertility before gonadotoxic treatment were examined. Immunohistochemistry (IHC) results for connexin 43 and claudin 11 as BTB markers, using a semi-quantitative score for their expression, and for Anti-Mullerian hormone (AMH), as SC maturation marker, were analyzed. Germ cell differentiation was evaluated on Hematoxylin-Eosin sections. Tanner stages at the time of biopsy were recorded from medical files. A longitudinal analysis of connexin 43, claudin 11 and AMH expressions on immunohistological sections of organotypic cultured testicular tissue from three prepubertal boys who underwent a biopsy for fertility preservation was performed. Immunostaining was evaluated at culture Days 0, 1, 3, 10, 16, 27, 32, 53, 64 and 139 for two different types of culture media.

MAIN RESULTS AND THE ROLE OF CHANCE

Immunohistochemical control sections showed progressive maturation of SCs, as shown by the decrease in AMH expression, with increasing age (P ≤ 0.01) and the AMH expression was negatively correlated with the expression of connexin 43 and claudin 11 (P ≤ 0.01 for both proteins). Androgen receptor (AR) expression increased with age (P ≤ 0.01) and was significantly correlated with the expression of connexin 43 (P = 0.002) and claudin 11 (P = 0.03). A statistical correlation was also found between the reduction of AMH expression and both the advancement of Tanner stages (P ≤ 0.01) and the differentiation of germ cells (P ≤ 0.01). Furthermore, positive correlations between BTB formation (using connexin 43 and claudin 11 expression) and age (P ≤ 0.01 for both the proteins), higher Tanner stages (P ≤ 0.001 and P ≤ 0.01 for connexin 43 and claudin 11, respectively), and presence of more advanced germ cells (P ≤ 0.001 for both proteins) were observed. In the subanalysis on organotypic cultured ITT, where a significant decrease in AMH expression as a marker of SC maturation was already reported, we showed the onset of expression of connexin 43 at Day 16 (P ≤ 0.001) and a constant expression of claudin 11 from Days 0 to 139, for all three patients, without differences between the two types of culture media.

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

Accessibility of prepubertal human testicular tissue is a major limiting factor to the analysis of cultured tissue samples from a wide number of patients, as would be needed to assess the in vitro development of the BTB according to the age. The impossibility of performing longitudinal studies on in vivo BTB formation in the same patient prevents a comparison of the time needed to achieve effective BTB formation and protein expression patterns in vivo and in vitro.

WIDER IMPLICATIONS OF THE FINDINGS

To the best of our knowledge, this is the first report describing the expression of two BTB proteins in samples from a cohort of prepubertal and peripubertal boys, for the in vivo pattern, and in cultured ITT from a few prepubertal boys, for the in vitro evaluation. Since the formation of this barrier is essential for spermatogenesis and because little is known about its protein expression patterns and development in humans, a deeper understanding of the testicular microenvironment is essential to improve ITT in vitro culture conditions. The final aim is to restore fertility by acheiving in vitro differentiation of spermatogonial stem cells, using cryopreserved ITT collected before gonadotoxic therapies.

STUDY FUNDING AND COMPETING INTEREST(S)

Funding was received from Fonds National de la Recherche Scientifique de Belgique (Grant Télevie Nos. 7.4554.14F and 7.6511.16) and Fondation Salus Sanguinis. No conflict of interest has to be disclosed.

Molecular Mechanisms and Signaling Pathways Involved in Sertoli Cell Proliferation

Sertoli cells are somatic cells present in seminiferous tubules which have essential roles in regulating spermatogenesis. Considering that each Sertoli cell is able to support a limited number of germ cells, the final number of Sertoli cells reached during the proliferative period determines sperm production capacity. Only immature Sertoli cells, which have not established the blood-testis barrier, proliferate. A number of hormonal cues regulate Sertoli cell proliferation. Among them, FSH, the insulin family of growth factors, activin, and cytokines action must be highlighted. It has been demonstrated that cAMP/PKA, ERK1/2, PI3K/Akt, and mTORC1/p70SK6 pathways are the main signal transduction pathways involved in Sertoli cell proliferation. Additionally, c-Myc and hypoxia inducible factor are transcription factors which participate in the induction by FSH of various genes of relevance in cell cycle progression. Cessation of proliferation is a pre-requisite to Sertoli cell maturation accompanied by the establishment of the blood-testis barrier. With respect to this barrier, the participation of androgens, estrogens, thyroid hormones, retinoic acid and opioids has been reported. Additionally, two central enzymes that are involved in sensing cell energy status have been associated with the suppression of Sertoli cell proliferation, namely AMPK and Sirtuin 1 (SIRT1). Among the molecular mechanisms involved in the cessation of proliferation and in the maturation of Sertoli cells, it is worth mentioning the up-regulation of the cell cycle inhibitors p21Cip1, p27Kip, and p19INK4, and of the gap junction protein connexin 43. A decrease in Sertoli cell proliferation due to administration of certain therapeutic drugs and exposure to xenobiotic agents before puberty has been experimentally demonstrated. This review focuses on the hormones, locally produced factors, signal transduction pathways, and molecular mechanisms controlling Sertoli cell proliferation and maturation. The comprehension of how the final number of Sertoli cells in adulthood is established constitutes a pre-requisite to understand the underlying causes responsible for the progressive decrease in sperm production that has been observed during the last 50 years in humans.

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 affects testicular glucose homeostasis through increased oxidative stress in prepubertal mice: effects on GLUT3, GLUT8 and Cx43

Thyroid hormones (THs) play an important role in maintaining the link between metabolism and reproduction and the altered THs status is associated with induction of oxidative stress in various organs like brain, heart, liver and testis. Further, reactive oxygen species play a pivotal role in regulation of glucose homeostasis in several organs, and glucose utilization by Leydig cells is essential for testosterone biosynthesis and thus is largely dependent on glucose transporter 8 (GLUT8). Glucose uptake by Sertoli cells is mediated through glucose transporter 3 (GLUT3) under the influence of THs to meet energy requirement of developing germ cells. THs also modulate level of gap junctional protein such as connexin 43 (Cx43), a potential regulator of cell proliferation and apoptosis in the seminiferous epithelium. Although the role of transient neonatal hypothyroidism in adult testis in terms of testosterone production is well documented, the effect of THs deficiency in early developmental period and its role in testicular glucose homeostasis and oxidative stress with reference to Cx43 in immature mice remain unknown. Therefore, the present study was conducted to evaluate the effect of neonatal hypothyroidism on testicular glucose homeostasis and oxidative stress at postnatal days (PND) 21 and 28 in relation to GLUT3, GLUT8 and Cx43. Hypothyroidism induced by 6-propyl-2-thiouracil (PTU) markedly decreased testicular glucose level with considerable reduction in expression level of GLUT3 and GLUT8. Likewise, lactate dehydrogenase (LDH) activity and intratesticular concentration of lactate were also decreased in hypothyroid mice. There was also a rise in germ cell apoptosis with increased expression of caspase-3 in PTU-treated mice. Further, neonatal hypothyroidism affected germ cell proliferation with decreased expression of proliferating cell nuclear antigen (PCNA) and Cx43. In conclusion, our results suggest that neonatal hypothyroidism alters testicular glucose homeostasis via increased oxidative stress in prepubertal mice, thereby affecting germ cell survival and proliferation.

The Type 3 Deiodinase Is a Critical Determinant of Appropriate Thyroid Hormone Action in the Developing Testis

Timely and appropriate levels of thyroid hormone (TH) signaling are necessary to ensure normal developmental outcomes in many tissues. Studies using pharmacological models of altered TH status have revealed an influence of these hormones on testis development and size, but little is known about the role of endogenous determinants of TH action in the developing male gonads. Using a genetic approach, we demonstrate that the type 3 deiodinase (D3), which inactivates TH and protects developing tissues from undue TH action, is a key factor. D3 is highly expressed in the developing testis, and D3-deficient (D3KO) mice exhibit thyrotoxicosis and cell proliferation arrest in the neonatal testis, resulting in an approximately 75% reduction in testis size. This is accompanied by larger seminiferous tubules, impaired spermatogenesis, and a hormonal profile indicative of primary hypogonadism. A deficiency in the TH receptor-α fully normalizes testis size and adult testis gene expression in D3KO mice, indicating that the effects of D3 deficiency are mediated through this type of receptor. Similarly, genetic deficiencies in the D2 or in the monocarboxylate transporter 8 partially rescue the abnormalities in testis size and gonadal axis gene expression featured in the D3KO mice. Our study highlights the testis as an important tissue in which determinants of TH action coordinately converge to ensure normal development and identifies D3 as a critical factor in testis development and in testicular protection from thyrotoxicosis.

Physiological roles of connexins and pannexins in reproductive organs

Reproductive organs are complex and well-structured tissues essential to perpetuate the species. In mammals, the male and female reproductive organs vary on their organization, morphology and function. Connectivity between cells in such tissues plays pivotal roles in organogenesis and tissue functions through the regulation of cellular proliferation, migration, differentiation and apoptosis. Connexins and pannexins can be seen as major regulators of these physiological processes. In the present review, we assembled several lines of evidence demonstrating that these two families of proteins are essential for male and female reproduction.

Lens ion homeostasis relies on the assembly and/or stability of large connexin 46 gap junction plaques on the broad sides of differentiating fiber cells

The eye lens consists of layers of tightly packed fiber cells, forming a transparent and avascular organ that is important for focusing light onto the retina. A microcirculation system, facilitated by a network of gap junction channels composed of connexins 46 and 50 (Cx46 and Cx50), is hypothesized to maintain and nourish lens fiber cells. We measured lens impedance in mice lacking tropomodulin 1 (Tmod1, an actin pointed-end capping protein), CP49 (a lens-specific intermediate filament protein), or both Tmod1 and CP49. We were surprised to find that simultaneous loss of Tmod1 and CP49, which disrupts cytoskeletal networks in lens fiber cells, results in increased gap junction coupling resistance, hydrostatic pressure, and sodium concentration. Protein levels of Cx46 and Cx50 in Tmod1(-/-);CP49(-/-) double-knockout (DKO) lenses were unchanged, and electron microscopy revealed normal gap junctions. However, immunostaining and quantitative analysis of three-dimensional confocal images showed that Cx46 gap junction plaques are smaller and more dispersed in DKO differentiating fiber cells. The localization and sizes of Cx50 gap junction plaques in DKO fibers were unaffected, suggesting that Cx46 and Cx50 form homomeric channels. We also demonstrate that gap junction plaques rest in lacunae of the membrane-associated actin-spectrin network, suggesting that disruption of the actin-spectrin network in DKO fibers may interfere with gap junction plaque accretion into micrometer-sized domains or alter the stability of large plaques. This is the first work to reveal that normal gap junction plaque localization and size are associated with normal lens coupling conductance.

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.

Thyroid hormone inhibits the proliferation of piglet Sertoli cell via PI3K signaling pathway

Accumulating researches show that thyroid hormone (TH) inhibits Sertoli cells (SCs) proliferation and stimulates their functional maturation in prepubertal rat testis, confirming that TH plays a key role in testicular development. However, the mechanism under the T3 regulation of piglet SC proliferation remains unclear. In the present study, in order to investigate the possible mechanism of T3 on the suppression of SC proliferation, the expression pattern of TRα1 and cell cycle-related molecules, effect of T3 on SC proliferation, and the role of phosphoinositide 3-kinase (PI3K)/Akt signaling pathway on the T3-mediated SC proliferation in piglet testis were explored. Our results demonstrated that TRα1 was expressed in all tested stages of SCs and decreased along with the ages. T3 inhibited the proliferation of SCs in a time- and dose-dependent manner, and T3 treatment downregulated the expressions of cell cycling molecules, such as cyclinA2, cyclinD1, cyclinE1, PCNA, and Skp2, but upregulated the p27 expression in SCs. Most importantly, the suppressive effects of T3 on SC proliferation seemed dependent on the inhibition of PI3K/Akt signaling pathway, and pre-stimulation of PI3K could enhance such suppressive effects. Together, our findings demonstrate that TH inhibits the proliferation of piglet SCs via the suppression of PI3K/Akt signaling pathway.

Receptors and signaling pathways involved in proliferation and differentiation of Sertoli cells

The identification of the hormones and other factors regulating Sertoli cell survival, proliferation, and maturation in neonatal, peripubertal, and pubertal life remains one of the most critical questions in testicular biology. The regulation of Sertoli cell proliferation and differentiation is thought to be controlled by cell-cell junctions and a set of circulating and local hormones and growth factors. In this review, we will focus on receptors and intracellular signaling pathways activated by androgen, follicle-stimulating hormone, thyroid hormone, activin, retinoids, insulin, insulin-like growth factor, relaxin, and estrogen, with special emphasis on estrogen receptors. Estrogen receptors activate intracellular signaling pathways that converge on cell cycle and transcription factors and play a role in the regulation of Sertoli cell proliferation and differentiation.

Thyroid hormone and leptin in the testis

Leptin is primarily expressed in white adipose tissue; however, it is expressed in the hypothalamus and reproductive tissues as well. Leptin acts by activating the leptin receptors (Ob-Rs). Additionally, the regulation of several neuroendocrine and reproductive functions, including the inhibition of glucocorticoids and enhancement of thyroxine and sex hormone concentrations in human beings and mice are leptin functions. It has been suggested that thyroid hormones (TH) could directly regulate leptin expression. Additionally, hypothyroidism compromises the intracellular integration of leptin signaling specifically in the arcuate nucleus. Two TH receptor isoforms are expressed in the testis, TRa and TRb, with TRa being the predominant one that is present in all stages of development. The effects of TH involve the proliferation and differentiation of Sertoli and Leydig cells during development, spermatogenesis, and steroidogenesis. In this context, TH disorders are associated with sexual dysfunction. An endocrine and/or direct paracrine effect of leptin on the gonads inhibits testosterone production in Leydig cells. Further studies are necessary to clarify the effects of both hormones in the testis during hypothyroidism. The goal of this review is to highlight the current knowledge regarding leptin and TH in the testis.

Thyroid and male reproduction

Male reproduction is governed by the classical hypothalamo-hypophyseal testicular axis: Hypothalamic gonadotropin releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) and the gonadal steroid, principally, testosterone. Thyroid hormones have been shown to exert a modulatory influence on this axis and consequently the sexual and spermatogenic function of man. This review will examine the modulatory influence of thyroid hormones on male reproduction.

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.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Thyroid hormone stimulates the proliferation of Sertoli cells and single type A spermatogonia in adult zebrafish (Danio rerio) testis

Thyroid hormones participate in regulating growth and homeostatic processes in vertebrates, including development and adult functioning of the reproductive system. Here we report a new stimulatory role of thyroid hormone on the proliferation of Sertoli cells (SCs) and single, type A undifferentiated spermatogonia (A(und)) in adult zebrafish testes. A role for T3 in zebrafish testis is suggested by in situ hybridization studies, which localized thyroid receptor α (thrα) in SCs and the β (thrβ) mRNA in Sertoli and Leydig cells. Using a primary zebrafish testis tissue culture system, the effect of T3 on steroid release, spermatogenesis, and the expression of selected genes was evaluated. Basal steroid release and Leydig cell gene expression did not change in response to T3. However, in the presence of FSH, T3 potentiated gonadotropin-stimulated androgen release as well as androgen receptor (ar) and 17α-hydroxylase/17,20 lyase (cyp17a1) gene expression. Moreover, T3 alone stimulated the proliferation of both SCs and A(und), potentially resulting in newly formed spermatogonial cysts. Additional tissue culture studies demonstrated that Igf3, a new, gonad-specific member of the IGF family, mediated the stimulatory effect of T3 on the proliferation of A(und) and SCs. Finally, T3 induced changes in connexin 43 mRNA levels in the testis, a known T3-responsive gene. Taken together, our studies suggest that T3 expands the population of SCs and A(und) involving Igf signaling and potentiates gonadotropin-stimulated testicular androgen production as well as androgen sensitivity.

Maturational changes in connexin 43 expression in the seminiferous tubules may depend on thyroid hormone action

INTRODUCTION

Connexin 43 (Cx43) mediates the effect of thyroid hormone on Sertoli cell maturation in vitro. We investigated the influence of triiodothyronine (T3) administration on Cx43 expression in relation to the progress in seminiferous tubule maturation.

MATERIAL AND METHODS

Male rats were daily injected with 100 µg T3/kg body weight from birth until postnatal day (pnd) 5 (transient treatment - tT3) or until pnd 15 (continuous treatment - cT3) or solvent - control (C). On pnd 16 serum hormone levels, body and testes weight, seminiferous tubule morphometry, Cx43 immunostaining and germ cell degeneration were investigated. Cx43 expression was also assessed in six 50-day-old adult untreated rats.

RESULT

tT3 increased 2.6-fold serum level of T3, testes weight, and seminiferous tubule diameter, and induced maturation-like dislocation of Cx43 expression from the apical to the peripheral region of Sertoli cell cytoplasm. In addition, incidence of Cx43-positive tubules declined from 86% in C to 46% after tT3, being similar to the adult value (30% of tubules Cx43-positive). In turn, cT3 increased serum T3 level 12-fold, and decreased body weight. Seminiferous tubules became shortened and distended, Sertoli cell cytoplasm vacuolated, Cx43 expression had minimal intensity and germ cell degeneration increased.

CONCLUSIONS

Cx43 might intermediate a short and transient stimulatory effect of T3 on seminiferous tubule maturation that disappeared together with exposure to the toxic effect of a continuously high level of the hormone.

Minireview: regulation of gap junction dynamics by nuclear hormone receptors and their ligands

Gap junctions are plasma membrane channels comprising connexin proteins that mediate intercellular permeability and communication. The presence, composition, and function of gap junctions can be regulated by diverse sets of physiological signals. Evidence from many hormone-responsive tissues has shown that connexin expression, modification, stability, and localization can be targeted by nuclear hormone receptors and their ligands through both transcriptional and nontranscriptional mechanisms. The focus of this review is to discuss molecular, cellular, and physiological studies that directly link receptor- and ligand-triggered signaling pathways to the regulation of gap junction dynamics.

Epidermal growth factor regulates connexin 43 in the human epididymis: role of gap junctions in azoospermia

BACKGROUND

Gap junctions (GJs) allow for direct communication between adjacent cells. They are composed of connexons consisting of transmembrane proteins, connexins (Cxs). The objectives of this study were to determine if GJ proteins GJA1 (Cx43), GJB1 (Cx32) and GJB2 (Cx26) are present in the epididymis of men with a normal epididymis, to assess whether or not Cx expression and localization are altered in azoospermic patients, and to determine if epidermal growth factor (EGF) regulates GJA1 expression.

METHODS

Epididymides were obtained from men with localized testis cancer with active spermatogenesis and histologically normal epididymal tubule (group 1), men with non-obstructive azoospermia secondary to Sertoli-cell only syndrome (group 2) and from azoospermic men with normal spermatogenesis and epididymal obstruction (group 3). Epididymides were subdivided into three segments: caput, corpus and cauda. Quantitative real-time RT-PCR was performed to assess GJA1, GJB1, GJB2 and EGF receptor (EGFR) mRNA levels in epididymides from patients from each group (all n=3, except n=1 for caput blockage). A human caput epididymal cell line was then used to determine the role of EGFR signaling on the regulation of human epididymal GJA1.

RESULTS

Real-time RT-PCR analysis revealed that GJA1, GJB1, GJB2 and EGFR were expressed along the human epididymis. In the cauda epididymidis of group 2 and 3 men, we observed a significant decrease in GJA1 (P=0.0456 and P=0.0465, respectively) and GJB1 (P=0.0450 and P=0.0497, respectively) mRNA levels when compared with group 1 men. We also observed a decrease in EGFR mRNA levels (P=0.0358) in the cauda epididymidis of group 3 men when compared with group 1. Immunocytochemistry revealed that in the epididymis, GJA1 and EGFR were localized between basal and principal cells and between adjacent principal cells. In group 2 and 3 patients, however, we noted a dramatic increase in cytosolic immunostaining for both GJA1 and EGFR in both principal and basal cells. Using a human caput epididymal cell line derived from fertile men, we demonstrated that changes in GJA1 phosphorylation could be regulated by EGF (P=0.015) and the extracellular regulated kinase 1/2 signaling pathway (P=0.03). Furthermore, while the phosphoinositide-3-kinase (PI3K)/AKT signaling pathway did not alter GJA1 phosphorylation, treatment with PI3K/AKT inhibitor LY294002 significantly (P=0.024) inhibited the EGF-stimulated increase in GJA1 total protein levels at 24 h. Immunolocalization indicated that loss of PI3K/AKT signaling was associated with increased cytosolic localization of Cx43 in this cell line.

CONCLUSIONS

Together, these data suggest that in azoospermic men decreased expression of EGFR may be responsible for decreasing GJA1 levels and increasing its cytosolic localization via the PI3K/AKT signaling pathway.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

Is the adult Sertoli cell terminally differentiated?

New data have challenged the convention that the adult Sertoli cell population is fixed and unmodifiable. The Sertoli cell has two distinct functions: 1) formation of the seminiferous cords and 2) provision of nutritional and structural support to developing germ cells. For these to occur successfully, Sertoli cells must undergo many maturational changes between fetal and adult life, the main switches occurring around puberty, including the loss of proliferative activity and the formation of the blood-testis barrier. Follicle-stimulating hormone plays a key role in promoting Sertoli cell proliferation, while thyroid hormone inhibits proliferative activity in early postnatal life. Together these regulate the Sertoli-germ cell complement and sperm output in adulthood. By puberty, the Sertoli cell population is considered to be stable and unmodifiable by hormones. But there is mounting evidence that the size of the adult Sertoli cell population and its maturational status is modifiable by hormones and that Sertoli cells can gain proliferative ability in the spermatogenically disrupted hamster and human model. This new information demonstrates that the adult Sertoli cell population, at least in the settings of testicular regression in the hamster and impaired fertility in humans in vivo and from mice and men in vitro, is not a terminally differentiated population. Data from the hamster now show that the adult Sertoli cell population size is regulated by hormones. This creates exciting prospects for basic and clinical research in testis biology. The potential to replenish an adult Sertoli-germ cell complement to normal in a setting of infertility may now be realized.

Molecular connexin partner remodeling orchestrates connexin traffic: from physiology to pathophysiology

Connexins, through gap junctional intercellular communication, are known to regulate many physiological functions involved in developmental processes such as cell proliferation, differentiation, migration and apoptosis. Strikingly, alterations of connexin expression and trafficking are often, if not always, associated with human developmental diseases and carcinogenesis. In this respect, disrupted trafficking dynamics and aberrant intracytoplasmic localization of connexins are considered as typical features of functionality failure leading to the pathological state. Recent findings demonstrate that interactions of connexins with numerous protein partners, which take place throughout connexin trafficking, are essential for gap junction formation, membranous stabilization and degradation. In the present study, we give an overview of the physiological molecular machinery and of the specific interactions between connexins and their partners, which are involved in connexin trafficking, and we highlight their changes in pathological situations.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

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.

Testicular connexin 43, a precocious molecular target for the effect of environmental toxicants on male fertility

Many recent epidemiological, clinical and experimental findings support the hypothesis that environmental toxicants are responsible for the increasing male reproductive disorders (congenital malformations, declining sperm counts and testicular cancer) over the past 20 years. It has also been reported that exposure to these toxicants, during critical periods of development (fetal and neonatal), represents a more considerable risk for animals and humans than exposure during adulthood. However, the molecular targets for these chemicals have not been clearly identified. Recent studies showed that a family of transmembranous proteins, named connexins, regulates numerous physiological processes involved in testicular development and function, such as Sertoli and germ cell proliferation, differentiation, germ cell migration and apoptosis. In the testis, knockout strategy revealed that connexin 43, the predominant connexin in this organ, is essential for spermatogenesis. In addition, there is evidence that many environmental toxicants could alter testicular connexin 43 by dysregulation of numerous mechanisms controlling its function. In the present work, we propose first to give an overview of connexin expression and intercellular gap junction coupling in the developing fetal and neonatal testes. Second, we underline the impact of maternally chemical exposure on connexin 43 expression in the perinatal developing testis. Lastly, we attempt to link this precocious effect to male offspring fertility.

The blood-testis barrier: the junctional permeability, the proteins and the lipids

The elucidation of how individual components of the Sertoli cell junctional complexes form and are dismantled to allow not only individual cells but whole syncytia of germinal cells to migrate from the basal to the lumenal compartment of the seminiferous epithelium without causing a permeability leak in the blood-testis barrier is amongst the most enigmatic yet, challenging and timely questions in testicular physiology. The intriguing key event in this process is how the barrier modulates its permeability during the periods of formation and dismantling of individual Sertoli cell junctions. The purpose of this review is therefore to first provide a reliable account on the normal formation, maintenance and dismantling process of the Sertoli cells junctions, then to assess the influence of the expression of their individual proteins, of the cytoskeleton associated with the junctions, and of the lipid content in the seminiferous tubules on the regulation of the their permeability barrier function. To help focus on the formation and dismantling of the Sertoli cell junctions, several considerations are based on data gleaned not only from rodents but from seasonal breeders as well because these animal models are characterized by exhaustive periods of junction assembly during development and the onset of the seasonal re-initiation of spermatogenesis as well as by an extensive junction dismantling period at the beginning of testicular regression, something unavailable in normal physiological conditions in continual breeders. Thus, the modulation of the permeability barrier function of the Sertoli cell junctions is analyzed in the physiological context of the blood-epidydimis barrier and in particular of the blood-testis barrier rather than in the context of a detailed account of the molecular composition and signalisation pathways of cell junctions. Moreover, the considerations discussed in this review are based on measurements performed on seminiferous tubule-enriched fractions gleaned at regular time intervals during development and the annual reproductive cycle.

Altered differentiation and clustering of Sertoli cells in transgenic mice showing a Sertoli cell specific knockout of the connexin 43 gene

Histological analysis revealed that Sertoli cell specific knockout of the predominant testicular gap junction protein connexin 43 results in a spermatogenic arrest at the level of spermatogonia or Sertoli cell-only syndrome, intratubular cell clusters and still proliferating adult Sertoli cells, implying an important role for connexin 43 in the Sertoli and germ cell development. This study aimed to determine the (1) Sertoli cell maturation state, (2) time of occurrence and (3) composition, differentiation and fate of clustered cells in knockout mice. Using immunohistochemistry connexin 43 deficient Sertoli cells showed an accurate start of the mature markers androgen receptor and GATA-1 during puberty and a vimentin expression from neonatal to adult. Expression of anti-Muellerian hormone, as a marker of Sertoli cell immaturity, was finally down-regulated during puberty, but its disappearance was delayed. This observed extended anti-Müllerian hormone synthesis during puberty was confirmed by western blot and Real-Time PCR and suggests a partial alteration in the Sertoli cell differentiation program. Additionally, Sertoli cells of adult knockouts showed a permanent and uniform expression of GATA-1 at protein and mRNA level, maybe caused by the lack of maturing germ cells and missing negative feedback signals. At ultrastructural level, basally located adult Sertoli cells obtained their mature appearance, demonstrated by the tripartite nucleolus as a typical feature of differentiated Sertoli cells. Intratubular clustered cells were mainly formed by abnormal Sertoli cells and single attached apoptotic germ cells, verified by immunohistochemistry, TUNEL staining and transmission electron microscopy. Clusters first appeared during puberty and became more numerous in adulthood with increasing cell numbers per cluster suggesting an age-related process. In conclusion, adult connexin 43 deficient Sertoli cells seem to proliferate while maintaining expression of mature markers and their adult morphology, indicating a unique and abnormal intermediate phenotype with characteristics common to both undifferentiated and differentiated Sertoli cells.

Connexines testiculaires: marqueurs physiopathologiques et cibles potentielles aux toxiques environnementaux

Les jonctions communicantes et leurs protéines constitutives, les connexines (Cxs), sont des constituants nécessaires à la cohésion tissulaire et reconnus comme suppresseurs de tumeurs. Le but de la présente revue est de faire le point sur l’organisation et le rôle des Cxs au sein du testicule et d’analyser leur expression en physiopathologie testiculaire. Organisées en structures hexamèriques formant un canal reliant directement les cytoplasmes des cellules adjacentes, les Cxs sont impliquées dans de nombreux processus physiologiques tels que la prolifération et la différenciation cellulaires. Le maintien d’une balance entre prolifération, différenciation et apoptose est un équilibre primordial évitant une prolifération cellulaire anarchique, risque de cancer. La spermatogenèse est un modèle sophistiqué de prolifération et de différenciation des cellules germinales dans lequel les Cxs jouent un rôle essentiel. Il est acquis qu’une altération de l’expression membranaire des Cxs est l’un des signes avant-coureurs de la cinétique tumorale germinale, et il a été suggéré que les toxiques environnementaux qui, dans leur grande majorité, affectent l’expression de ces protéines, puissent être impliqués dans le développement de cette pathologie. La recherche de molécules capables de freiner les effets délétères de toxiques carcinogènes sur les Cxs semble être à l’heure actuelle une voie intéressante ouvrant de nouvelles perspectives en santé humaine.

Regulation of the connexin 43 promoter in the brook trout testis: role of the thyroid hormones and cAMP

Gap junctions are critical for spermatogenesis. They are composed of integral proteins, the connexins. In mammals, a loss of Cx43 expression results in the inhibition of spermatogenesis. We have shown that Cx43 is expressed in the Sertoli cells of rainbow trout and that cAMP and triiodothyronine (T(3)) regulate testicular Cx43 expression in brook trout testis. The objective of this study was to determine if cAMP and T(3) act at the level of the cx43 promoter to regulate its expression. A 607 bp 5' flanking sequence of the cx43 promoter was obtained by Genome Walking. A TATA box was predicted to be located between positions -36 and -30 relative to the transcriptional initiation site. 5'-Rapid amplification of cDNA ends indicated a single transcriptional start site. Single C/EBP (-164 to -156) and tr-beta (-112 to -107) response elements were identified and electrophoretic mobility shift assays indicated the presence of competitive protein binding sites at each region. Immortalized rainbow trout gonadal cell line (RTG-2) which express cx43 and tr-beta transcripts were transfected with a vector containing the Cx43 promoter inserted into a luciferase expression vector. Transactivation of the reporter genes was stimulated by either cAMP or T(3). Sequential deletion and point mutations in either the C/EBP or tr-beta response element indicated that T(3) but not cAMP directly induced luciferase transactivation of the luciferase gene by acting on different sites of the Cx43 promoter. Together, these data indicate that T(3) stimulates cx43 expression via direct regulation of gene transcription.