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

Microenvironment of spermatogonial stem cells: a key factor in the regulation of spermatogenesis

Spermatogonial stem cells (SSCs) play a crucial role in the male reproductive system, responsible for maintaining continuous spermatogenesis. The microenvironment or niche of SSCs is a key factor in regulating their self-renewal, differentiation and spermatogenesis. This microenvironment consists of multiple cell types, extracellular matrix, growth factors, hormones and other molecular signals that interact to form a complex regulatory network. This review aims to provide an overview of the main components of the SSCs microenvironment, explore how they regulate the fate decisions of SSCs, and discuss the potential impact of microenvironmental abnormalities on male reproductive health.

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.

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.

Action and Interaction between Retinoic Acid Signaling and Blood–Testis Barrier Function in the Spermatogenesis Cycle

Spermatogenesis is a complex process occurring in mammalian testes, and constant sperm production depends on the exact regulation of the microenvironment in the testes. Many studies have indicated the crucial role of blood–testis barrier (BTB) junctions and retinoic acid (RA) signaling in the spermatogenesis process. The BTB consists of junctions between adjacent Sertoli cells, comprised mainly of tight junctions and gap junctions. In vitamin A-deficient mice, halted spermatogenesis could be rebooted by RA or vitamin A administration, indicating that RA is absolutely required for spermatogenesis. Accordingly, this manuscript will review and discuss how RA and the BTB regulate spermatogenesis and the interaction between RA signaling and BTB function.

Connexin43 in Germ Cells Seems to Be Dispensable for Murine Spermatogenesis

Testicular Connexin43 (Cx43) connects adjacent Sertoli cells (SC) and SC to germ cells (GC) in the seminiferous epithelium and plays a crucial role in spermatogenesis. However, the distinction whether this results from impaired inter-SC communication or between GC and SC is not possible, so far. Thus, the question arises, whether a GC-specific Cx43 KO has similar effects on spermatogenesis as it is general or SC-specific KO. Using the Cre/loxP recombinase system, two conditional KO mouse lines lacking Cx43 in premeiotic (pGCCx43KO) or meiotic GC (mGCCx43KO) were generated. It was demonstrated by qRT-PCR that Cx43 mRNA was significantly decreased in adult pGCCx43KO mice, while it was also reduced in mGCCx43KO mice, yet not statistically significant. Body and testis weights, testicular histology, tubular diameter, numbers of intratubular cells and Cx43 protein synthesis and localization did not show any significant differences in semi-quantitative Western blot analysis and immunohistochemistry comparing adult male KO and WT mice of both mouse lines. Male KO mice were fertile. These results indicate that Cx43 in spermatogonia/spermatids does not seem to be essential for successful termination of spermatogenesis and fertility as it is known for Cx43 in somatic SC, but SC-GC communication might rather occur via heterotypic GJ channels.

Insights into differentiation and function of the transition region between the seminiferous tubule and rete testis

Seminiferous tubules physically connect to the rete testis through short segments called the transition region (TR). During fetal development, this specialized junction is considered the initial site where testis cords begin to form and to grow in length well beyond birth and into adulthood and form convoluted tubular cores. Mitotic activity of the Sertoli cell, the somatic cell of the epithelium, ceases before puberty, but modified Sertoli cells in the TR remain immature and capable of proliferation. This review presents what is known about this specialized region of the testis, with an emphasis on the morphological, molecular and physiological features, which support the hypothesis that this short region of epithelial transition serves as a specialized niche for undifferentiated Sertoli cells and spermatogonial stem cells. Also, the region is populated by an elevated number of immune cells, suggesting an important activity in monitoring and responding to any leakage of autoantigens, as sperm enter the rete testis. Several structure/function characteristics of the transition region are discussed and compared across species.

Sertoli cell-only syndrome: etiology and clinical management

Almost 50% of infertility cases are due to male factors, and spermatogenesis failure is one of the most severe forms of male infertility. Sertoli cell-only syndrome (SCOS) also known as germ cell aplasia is characterized by azoospermia in which the seminiferous tubules of testicular biopsy are lined only with Sertoli cells. The definitive diagnosis of SCOS is by diagnostic testicular biopsy. Although SCOS may be a result of Klinefelter syndrome, most of the SCOS men have a normal karyotype. Along with genetic aberrations, signaling pathways and endocrine processes might be major factors in the development of SCOS. Sperm retrieval and intracytoplasmic sperm injection (ICSI) are available treatments for SCOS. However, some SCOS patients do not have therapeutic options to help them having a biological child. This review aims to summarize our present knowledge about SCOS and to highlight the importance of future researches in the diagnosis and treatment of this disorder.

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.

Establishment and functional characterization of a murine primary Sertoli cell line deficient of connexin43

The Sertoli cell (SC) specific connexin43 (Cx43) knockout (SCCx43KO) mouse line is ideal to gain insight into the mechanistic gap junction formation in SC and the seminiferous epithelium. A method for developing primary SC cultures from these mice was established, validated and successfully characterized via polymerase chain reaction, immunohistochemistry, immunofluorescence (IF), and Western blots (WB). It was evident that both knockout (KO) and wild-type (WT) primary cell cultures were similar in morphology. These highly pure SC cultures were subjected to cell proliferation assays indicating no notable proliferation in cultures of both genotypes. Measurements of cell monolayer integrity indicated significant increases in transepithelial electrical resistance and consequently in tight junction expression of the KO cultures. Using semi-quantitative WB and IF, tight junction protein claudin-11 was analyzed. These results support a role for Cx43 in regulating blood-testis barrier (BTB) function, composition, and dynamics in vitro. Thus, the SC deficient Cx43 cell cultures may provide a valuable in vitro tool for a better understanding of the mechanistic role of Cx43 in spermatogenesis and BTB assembly.

Loss of Cx43 in Murine Sertoli Cells Leads to Altered Prepubertal Sertoli Cell Maturation and Impairment of the Mitosis-Meiosis Switch

Male factor infertility is a problem in today’s society but many underlying causes are still unknown. The generation of a conditional Sertoli cell (SC)-specific connexin 43 (Cx43) knockout mouse line (SCCx43KO) has provided a translational model. Expression of the gap junction protein Cx43 between adjacent SCs as well as between SCs and germ cells (GCs) is known to be essential for the initiation and maintenance of spermatogenesis in different species and men. Adult SCCx43KO males show altered spermatogenesis and are infertile. Thus, the present study aims to identify molecular mechanisms leading to testicular alterations in prepubertal SCCx43KO mice. Transcriptome analysis of 8-, 10- and 12-day-old mice was performed by next-generation sequencing (NGS). Additionally, candidate genes were examined by qRT-PCR and immunohistochemistry. NGS revealed many significantly differentially expressed genes in the SCCx43KO mice. For example, GC-specific genes were mostly downregulated and found to be involved in meiosis and spermatogonial differentiation (e.g., Dmrtb1, Sohlh1). In contrast, SC-specific genes implicated in SC maturation and proliferation were mostly upregulated (e.g., Amh, Fshr). In conclusion, Cx43 in SCs appears to be required for normal progression of the first wave of spermatogenesis, especially for the mitosis-meiosis switch, and also for the regulation of prepubertal SC maturation.

Induction of Sertoli-like cells from human fibroblasts by NR5A1 and GATA4

Sertoli cells are essential nurse cells in the testis that regulate the process of spermatogenesis and establish the immune-privileged environment of the blood-testis-barrier (BTB). Here, we report the in vitro reprogramming of fibroblasts to human induced Sertoli-like cells (hiSCs). Initially, five transcriptional factors and a gene reporter carrying the AMH promoter were utilized to obtain the hiSCs. We further reduce the number of reprogramming factors to two, NR5A1 and GATA4, and show that these hiSCs have transcriptome profiles and cellular properties that are similar to those of primary human Sertoli cells. Moreover, hiSCs can sustain the viability of spermatogonia cells harvested from mouse seminiferous tubules. hiSCs suppress the proliferation of human T lymphocytes and protect xenotransplanted human cells in mice with normal immune systems. hiSCs also allow us to determine a gene associated with Sertoli cell only syndrome (SCO), CX43, is indeed important in regulating the maturation of Sertoli cells.

Identification of a Novel Zebrafish Mutant Line that Develops Testicular Germ Cell Tumors

Testicular tumors are the most common solid malignant tumors in men 20-35 years of age. Although most of testicular tumors are curable, current treatments still fail in 15%-20% of patients. However, insufficient understanding of the molecular basis and lack of animal models limit development of more effective treatments. This study reports the identification of a novel zebrafish mutant line, ns1402, which develops testicular germ cell tumors (TGCTs). While both male and female ns1402 mutants were fertile at young age, male ns1402 mutants became infertile as early as 9 months of age. This infertility was associated with progressive loss of mature sperm. Failure of spermatogenesis was, at least in part, explained by progressive loss of mature Leydig cells, a source of testosterone that is essential for spermatogenesis. Interestingly, TGCTs in ns1402 mutants contained a large number of Sertoli cells and gene expression profiles of Sertoli cells were altered before loss of mature Leydig cells. This suggests that changes in Sertoli cell properties happened first, followed by loss of mature Leydig cells and failure of spermatogenesis. Taken together, this study emphasizes the importance of cell-cell interactions and cell signaling in the testis for spermatogenesis and tissue homeostasis.

Loss of connexin43 in murine Sertoli cells and its effect on blood-testis barrier formation and dynamics

Connexin43 (Cx43) is the predominant testicular gap junction protein and in cases of impaired spermatogenesis, Cx43 expression has been shown to be altered in several mammals. Amongst other functions, Cx43 is supposed to regulate junction formation of the blood-testis barrier (BTB). The aim of the present study was to investigate the expression pattern of different tight junction (TJ) proteins of the murine BTB using SC-specific Cx43 knockout mice (SCCx43KO). Adult homozygous male SCCx43KO mice (SCCx43KO-/-) predominantly show an arrest of spermatogenesis and SC-only tubules that might have been caused by an altered BTB assembly, composition or regulation. TJ molecules claudin-3, -5 and -11 were examined in adult wild type (WT) and SCCx43KO-/- mice using immunohistochemistry (IHC) and quantitative real-time PCR (qRT-PCR). In this context, investigation of single tubules with residual spermatogenesis in SCCx43KO-/- mice was particularly interesting to identify a potential Cx43-independent influence of germ cells (GC) on BTB composition and dynamics. In tubules without residual spermatogenesis, a diffuse cytoplasmic distribution pattern for claudin-11 protein could be demonstrated in mutant mice. Nevertheless, claudin-11 seems to form functional TJ. Claudin-3 and -5 could not be detected immunohistochemically in the seminiferous epithelium of those tubules. Correspondingly, claudin-3 and -5 mRNA expression was decreased, providing evidence of generally impaired BTB dynamics in adult KO mice. Observations of tubules with residual spermatogenesis suggested a Cx43-independent regulation of TJ proteins by GC populations. To determine initial BTB formation in peripubertal SCCx43KO-/- mice, immunohistochemical staining and qRT-PCR of claudin-11 were carried out in adolescent SCCx43KO-/- and WT mice. Additionally, BTB integrity was functionally analysed using a hypertonic glucose fixative. These analyses revealed that SCCx43KO-/- mice formed an intact BTB during puberty in the same time period as WT mice, which however seemed to be accelerated.

Establishment, maintenance and functional integrity of the blood-testis barrier in organotypic cultures of fresh and frozen/thawed prepubertal mouse testes

STUDY QUESTION

Can the spatio-temporal formation of an intact blood-testis barrier (BTB), which is essential for the progression of spermatogenesis, be reproduced in cultures of fresh or frozen/thawed prepubertal mouse testes?

SUMMARY ANSWER

Organotypic cultures allow the establishment and maintenance of major BTB components and the formation of a functional BTB in mouse testicular tissues.

WHAT IS KNOWN ALREADY

In vitro maturation of prepubertal testicular tissues is a promising approach to restore fertility in adult survivors of childhood cancer. Although gametes can be successfully obtained from prepubertal mouse testes in organotypic cultures, the spermatogenic yield remains low compared to in vivo controls.

STUDY DESIGN, SIZE, DURATION

Mouse testicular tissues were frozen using controlled slow freezing (CSF) or solid surface vitrification (SSV) procedures. A total of 158 testes (fresh n = 58, CSF n = 58 or SSV n = 42) from 6 to 7 days postpartum (dpp) mice were cultured at 34°C in basal medium (α-MEM, 10% KnockOut Serum Replacement, 5 μg/ml gentamicin) at a gas-liquid interphase (under 20% O2), with or without 10-6 M retinol, for 9, 16 and 30 days. In addition, 32 testes from 6-7, 15-16, 22-23 and 36-37 dpp mice were used as in vivo controls.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The mRNA levels of BTB genes (Claudin 3, Claudin 11, Zonula occludens 1 and Connexin-43), germ cell-specific genes (Sal-like protein 4, Kit oncogene, Stimulated by retinoic acid gene 8, Synaptonemal complex protein 3, Transition protein 1 and Protamine 2), markers of Sertoli cell immaturity/maturity (anti-Mullerian hormone, androgen receptor, cyclin-dependent kinase inhibitor 1b) and the androgen-regulated gene Reproductive homeobox 5 (Rhox5) were measured by quantitative RT-PCR (RT-qPCR). The localization of BTB proteins in seminiferous tubules was studied by immunohistochemistry and spermatogenic progression was evaluated histologically. The integrity of the BTB was assessed using a biotin tracer.

MAIN RESULTS AND THE ROLE OF CHANCE

Modest differences in Claudin 11 (Cldn11), Zonula occludens 1 (Zo-1), Connexin-43 (Cx43) transcript levels and in the localization of the corresponding proteins were found between in vitro cultures of fresh or frozen/thawed testes and in vivo controls (P < 0.05). However, a 32-77-fold decrease in Claudin 3 (Cldn3) mRNA levels and a lack of CLDN3 immunolabelling in 36-44% of seminiferous tubules were observed in 30-day organotypic cultures (P < 0.05). Although Sertoli cell maturation and the completion of a full spermatogenic cycle were achieved after 30 days of culture, meiotic and postmeiotic progression was altered in cultured testicular tissues (P < 0.05). Moreover, an increased BTB permeability and a decreased expression of Rhox5 were observed at the end of the culture period in comparison with in vivo controls (P < 0.05). Completion of spermatogenesis occurred in vitro in seminiferous tubules with an intact BTB, and in those expressing or lacking CLDN3.

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

Further studies will be needed to determine whether the expression of other BTB components is altered and to decipher the reason for lower Cldn3 and Rhox5 mRNA levels in organotypic cultures.

WIDER IMPLICATIONS OF THE FINDINGS

This work contributes to a better understanding of the molecular mechanisms occurring in in vitro matured prepubertal testes. The organotypic culture system will have to be developed further and optimized for human tissue, before potential clinical applications can be envisaged.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported by Rouen University Hospital, Ligue contre le Cancer (to L.D.), and co-supported by European Union and Région Normandie (to A.O.). Europe gets involved in Normandie with European Régional Development Fund (ERDF). The authors declare that they have no conflict of interest.

Signaling pathways regulating blood-tissue barriers - Lesson from the testis

Signaling pathways that regulate blood-tissue barriers are important for studying the biology of various blood-tissue barriers. This information, if deciphered and better understood, will provide better therapeutic management of diseases particularly in organs that are sealed by the corresponding blood-tissue barriers from systemic circulation, such as the brain and the testis. These barriers block the access of antibiotics and/or chemotherapeutical agents across the corresponding barriers. Studies in the last decade using the blood-testis barrier (BTB) in rats have demonstrated the presence of several signaling pathways that are crucial to modulate BTB function. Herein, we critically evaluate these findings and provide hypothetical models regarding the underlying mechanisms by which these signaling molecules/pathways modulate BTB dynamics. This information should be carefully evaluated to examine their applicability in other tissue barriers which shall benefit future functional studies in the field. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Cytological study on Sertoli cells and their interactions with germ cells during annual reproductive cycle in turtle

Sertoli cells (SCs) play a central role in the development of germ cells within functional testes and exhibit varying morphology during spermatogenesis. This present study investigated the seasonal morphological changes in SCs in the reproductive cycle of Pelodiscus sinensis by light microscopy, transmission electron microscopy (TEM), and immunohistochemistry. During hibernation period with the quiescent of spermatogenesis, several autophagosomes were observed inside the SCs, the processes of which retracted. In early spermatogenesis, when the germ cells started to proliferate, the SCs contained numerous lipid droplets instead of autophagosomes. In late spermatogenesis, the SCs processes became very thin and contacted several round/elongated spermatids in pockets. At this time, abundant endoplasmic reticulum and numerous mitochondria were present in the SCs. The organization of the tight junctions and the adherens junctions between the SCs and germ cells also changed during the reproductive cycle. Moreover, SCs were involved in the formation of cytoplasmic bridges, phagophores, and exosome secretions during spermatogenesis. Tubulobulbar complexes (TBC) were also developed by SCs around the nucleus of the spermatid at the time of spermiation. Strong, positive expression of vimentin was noted on the SCs during late spermatogenesis compared with the hibernation stage and the early stage of spermatogenesis. These data provide clear cytological evidence about the seasonal changes in SCs, corresponding with their different roles in germ cells within the Chinese soft‐shelled turtle Pelodiscus sinensis.

Transfection of Sertoli cells with androgen receptor alters gene expression without androgen stimulation

Background

Androgens play an important role for the development of male fertility and gained interest as growth and survival factors for certain types of cancer. Androgens act via the androgen receptor (AR/Ar), which is involved in various cell biological processes such as sex differentiation. To study the functional mechanisms of androgen action, cell culture systems and AR-transfected cell lines are needed. Transfection of AR into cell lines and subsequent gene expression analysis after androgen treatment is well established to investigate the molecular biology of target cells. However, it remains unclear how the transfection with AR itself can modulate the gene expression even without androgen stimulation. Therefore, we transfected Ar-deficient rat Sertoli cells 93RS2 by electroporation using a full length human AR.

Results

Transfection success was confirmed by Western Blotting, immunofluorescence and RT-PCR. AR transfection-related gene expression alterations were detected with microarray-based genome-wide expression profiling of transfected and non-transfected 93RS2 cells without androgen stimulation. Microarray analysis revealed 672 differentially regulated genes with 200 up- and 472 down-regulated genes. These genes could be assigned to four major biological categories (development, hormone response, immune response and metabolism). Microarray results were confirmed by quantitative RT-PCR analysis for 22 candidate genes.

Conclusion

We conclude from our data, that the transfection of Ar-deficient Sertoli cells with AR has a measurable effect on gene expression even without androgen stimulation and cause Sertoli cell damage. Studies using AR-transfected cells, subsequently stimulated, should consider alterations in AR-dependent gene expression as off-target effects of the AR transfection itself.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-015-0051-7) contains supplementary material, which is available to authorized users.

Blood-testis barrier and Sertoli cell function: lessons from SCCx43KO mice

The gap junction protein connexin43 (CX43) plays a vital role in mammalian spermatogenesis by allowing for direct cytoplasmic communication between neighbouring testicular cells. In addition, different publications suggest that CX43 in Sertoli cells (SC) might be important for blood-testis barrier (BTB) formation and BTB homeostasis. Thus, through the use of the Cre-LoxP recombination system, a transgenic mouse line was developed in which only SC are deficient of the gap junction protein, alpha 1 (Gja1) gene. Gja1 codes for the protein CX43. This transgenic mouse line has been commonly defined as the SC specific CX43 knockout (SCCx43KO) mouse line. Within the seminiferous tubule, SC aid in spermatogenesis by nurturing germ cells and help them to proliferate and mature. Owing to the absence of CX43 within the SC, homozygous KO mice are infertile, have reduced testis size, and mainly exhibit spermatogenesis arrest at the level of spermatogonia, seminiferous tubules containing only SC (SC-only syndrome) and intratubular SC-clusters. Although the SC specific KO of CX43 does not seem to have an adverse effect on BTB integrity, CX43 influences BTB composition as the expression pattern of different BTB proteins (like OCCLUDIN, β-CATENIN, N-CADHERIN, and CLAUDIN11) is altered in mutant males. The supposed roles of CX43 in dynamic BTB regulation, BTB assembly and/or disassembly and its possible interaction with other junctional proteins composing this unique barrier are discussed. Data collectively indicate that CX43 might represent an important regulator of dynamic BTB formation, composition and function.

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.

A Sertoli cell-specific connexin43 knockout leads to altered interstitial connexin expression and increased Leydig cell numbers

The Sertoli cell (SC)-specific knockout (KO) of connexin43 (Cx43) results in spermatogenic arrest at the level of spermatogonia and/or SC-only syndrome. Histology of the interstitial compartment suggests Leydig cell (LC) hyperplasia. Our aim has been to investigate possible effects of the SC-specific KO of Cx43 (SCCx43KO) on interstitial LC. We therefore counted LC via the optical dissector method (per microliter of testicular tissue and per testis) and found LC to be significantly increased in SCCx43KO(-/-) compared with wild-type mice. Semiquantitative western blot together with Cx43 and 3β-hydroxysteroid dehydrogenase immunohistochemistry showed that Cx43 protein was significantly reduced and barely detectable in LC in adult SCCx43KO(-/-) mice. This reduction of Cx43 protein was accompanied by a reduction of Cx43 mRNA as analyzed by laser-assisted microdissection of interstitial cells and subsequent quantitative real-time polymerase chain reaction (PCR). Interestingly, Cx45, another recently detected connexin in LC, was also downregulated. Preliminary qualitative data of LC differentiation markers (Thb2, Hsd3b6) and a steroidogenic marker (Hsd17b3) obtained by reverse transcription plus PCR revealed no obvious differences. Thus, the loss of Cx43 in SC also provokes the downregulation of connexins in interstitial LC at the transcriptional and translational levels. Moreover, SCCx43KO leads to alterations in LC numbers. Despite these alterations, steroidogenesis seems not to be impaired. Further studies, including ultrastructural analysis of the tissue as well as quantitative examination of additional LC markers and testosterone, and functional in vitro experiments, should provide more information about LC differentiation and function in SCCx43KO(-/-) mice.

Immunohistochemical Characterization of Spontaneous Sertoli Cell Clusters in the Seminiferous Tubules of C57BL/6J Mice

Cell clusters were observed in the seminiferous tubules of C57BL/6J mice as a spontaneous lesion in a 2-week toxicity study, and they were demonstrated to be basically composed of Sertoli cells by immunohistochemistry for claudin-11 and GATA-4 (GATA-binding protein 4), which are both Sertoli cell markers. The clusters were composed of about 5 to 50 cells, which had eosinophilic and occasionally vacuolated cytoplasm with an unclear cell boundary. The cell clusters involved some sperm. No mitotic figures were observed and no immunoreactivity for proliferating cell nuclear antigen (PCNA) was detected in the clusters. In most cases, the cell clusters were observed in seminiferous tubules that also showed degenerative changes. In rare instances, cell aggregates immunohistochemically positive for claudin-11 were observed in the lumen of the epididymis, suggesting that some of the Sertoli cell clusters were sloughed off from the seminiferous epithelium into the epididymal ducts. To our knowledge, this is the first report of Sertoli cell clusters in any animal species except for transgenic or surgically altered animals.

Connexin 43 a check-point component of cell proliferation implicated in a wide range of human testis diseases

Gap junction channels link cytoplasms of adjacent cells. Connexins, their constitutive proteins, are essential in cell homeostasis and are implicated in numerous physiological processes. Spermatogenesis is a sophisticated model of germ cell proliferation, differentiation, survival, and apoptosis, in which a connexin isotype, connexin 43, plays a crucial role as evidenced by genomic approaches based on gene deletion. The balance between cell proliferation/differentiation/apoptosis is a prerequisite for maintaining levels of spermatozoa essential for fertility and for limiting anarchic cell proliferation, a major risk of testis tumor. The present review highlights the emerging role of connexins in testis pathogenesis, focusing specifically on two intimately interconnected human testicular diseases (azoospermia with impaired spermatogenesis and testicular germ cell tumors), whose incidence increased during the last decades. This work proposes connexin 43 as a potential cancer diagnostic and prognostic marker, as well as a promising therapeutic target for testicular diseases.

Long-term vitamin A deficiency induces alteration of adult mouse spermatogenesis and spermatogonial differentiation: direct effect on spermatogonial gene expression and indirect effects via somatic cells

The objective of this study was to further understand the genetic mechanisms of vitamin A deficiency (VAD) induced arrest of spermatogonial stem-cell differentiation. Vitamin A and its derivatives (the retinoids) participate in many physiological processes including vision, cellular differentiation and reproduction. VAD affects spermatogenesis, the subject of our present study. Spermatogenesis is a highly regulated process of differentiation and complex morphologic alterations that leads to the formation of sperm in the seminiferous epithelium. VAD causes early cessation of spermatogenesis, characterized by degeneration of meiotic germ cells, leading to seminiferous tubules containing mostly type A spermatogonia and Sertoli cells. These observations led us to the hypothesis that VAD affects not only germ cells but also somatic cells. To investigate the effects of VAD on spermatogenesis in mice we used adult Balb/C mice fed with Control or VAD diet for an extended period of time (6-28 weeks). We first observed the chronology, then the extent of the effects of VAD on the testes. Using microarray analysis of isolated pure populations of spermatogonia, Leydig and Sertoli cells from control and VAD 18- and 25-week mice, we examined the effects of VAD on gene expression and identified target genes involved in the arrest of spermatogonial differentiation and spermatogenesis. Our results provide a more precise definition of the chronology and magnitude of the consequences of VAD on mouse testes than the previously available literature and highlight direct and indirect (via somatic cells) effects of VAD on germ cell differentiation.

Thyroid hormone limits postnatal Sertoli cell proliferation in vivo by activation of its alpha1 isoform receptor (TRalpha1) present in these cells and by regulation of Cdk4/JunD/c-myc mRNA levels in mice

Hypo- and hyperthyroidism alter testicular functions in the young. Among T3 receptors, TRalpha1 is ubiquitous, and its previously described knockout leads to an increase in testis weight and sperm production. We tested, for the first time, the hypothesis that TRalpha1-dependent regulation of Sertoli cell (SC) proliferation was directly regulated by TRalpha1 present in these cells. Thus, after crossing with the AMH-Cre line, we generated and analyzed a new line that expressed a dominant-negative TRalpha1 isoform (TRalpha(AMI)) in SCs only. So-called TRalpha(AMI)-SC (TRalpha(AMI/+) Cre(+)) mice exhibited similar phenotypic features to the knockout line: heavier testicular weight and higher sperm reserve, in comparison with their adequate controls (TRalpha(AMI/+) Cre(-)). SC density increased significantly as a result of a higher proliferative index at ages Postnatal Day (P) 0 and P3. When explants of control testes were cultured (at age P3), a significant decrease in the proliferation of SCs was observed in response to an excess of T3. This response was not observed in the TRalpha(AMI)-SC and knockout lines. Finally, when TRalpha(AMI) is present in SCs, the phenotype observed is similar to that of the knockout line. This study demonstrates that T3 limits postnatal SC proliferation by activation of TRalpha1 present in these cells. Moreover, quantitative RT-PCR provided evidence that regulation of the Cdk4/JunD/c-myc pathway was involved in this negative control.

Sertoli-cell-specific knockout of connexin 43 leads to multiple alterations in testicular gene expression in prepubertal mice

A significant decline in human male reproductive function has been reported for the past 20 years but the molecular mechanisms remain poorly understood. However, recent studies showed that the gap junction protein connexin-43 (CX43; also known as GJA1) might be involved. CX43 is the predominant testicular connexin (CX) in most species, including in humans. Alterations of its expression are associated with different forms of spermatogenic disorders and infertility. Men with impaired spermatogenesis often exhibit a reduction or loss of CX43 expression in germ cells (GCs) and Sertoli cells (SCs). Adult male transgenic mice with a conditional knockout (KO) of the Gja1 gene [referred to here as connexin-43 (Cx43)] in SCs (SCCx43KO) show a comparable testicular phenotype to humans and are infertile. To detect possible signaling pathways and molecular mechanisms leading to the testicular phenotype in adult SCCx43KO mice and to their failure to initiate spermatogenesis, the testicular gene expression of 8-day-old SCCx43KO and wild-type (WT) mice was compared. Microarray analysis revealed that 658 genes were significantly regulated in testes of SCCx43KO mice. Of these genes, 135 were upregulated, whereas 523 genes were downregulated. For selected genes the results of the microarray analysis were confirmed using quantitative real-time PCR and immunostaining. The majority of the downregulated genes are GC-specific and are essential for mitotic and meiotic progression of spermatogenesis, including Stra8, Dazl and members of the DM (dsx and map-3) gene family. Other altered genes can be associated with transcription, metabolism, cell migration and cytoskeleton organization. Our data show that deletion of Cx43 in SCs leads to multiple alterations of gene expression in prepubertal mice and primarily affects GCs. The candidate genes could represent helpful markers for investigators exploring human testicular biopsies from patients showing corresponding spermatogenic deficiencies and for studying the molecular mechanisms of human male sterility.

The apical ectoplasmic specialization-blood-testis barrier functional axis is a novel target for male contraception

The blood-testis barrier (BTB), similar to other blood-tissue barriers, such as the blood-brain barrier and the blood-retinal barrier, is used to protect the corresponding organ from harmful substances (e.g., xenobiotics) including drugs and foreign compounds. More importantly, the BTB allows postmeiotic spermatid development to take place in an immune privileged site at the adluminal (or apical) compartment to avoid the production of antibodies against spermatid-specific antigens, many of which express transiently during spermiogenesis and spermiation. The BTB, however, also poses an obstacle in developing nonhormonal-based male contraceptives by sequestering drugs (e.g., adjudin) that exert their effects on germ cells in the adluminal compartment. The effects of these drugs include disruption of germ cell cycle progression and development, apoptosis, cell adhesion, metabolism and others. Recent studies have demonstrated that there is a functional axis that operates locally in the seminiferous epithelium to co-ordinate different cellular events across the Sertoli cell epithelium, such as spermiation and BTB restructuring during the seminiferous epithelial cycle of spermatogenesis. Components of this functional axis, such as the apical ectoplasmic specialization (apical ES, a testis-specific atypical anchoring junction type) and the BTB, in particular their constituent protein complexes, such as alpha6beta1-integrin and occludin at the apical ES and the BTB, respectively, can be the target of male contraception. In this chapter, we highlight recent advances regarding the likely mechanism of action of adjudin in this functional axis with emphasis on the use of molecular modeling technique to facilitate the design of better compounds in male contraceptive development.

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.