Expression of connexin43 in mouse Leydig, Sertoli, and germinal cells at different stages of postnatal development

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.

Exposure to manganese during sertoli cell formation and proliferation disturbs early testicular development in rats

Manganese (Mn) is a metal and important micronutrient. However, exposure to supraphysiological levels of Mn, which occur through fungicides, atmospheric emissions, drainages, and spills, has been related to health risks, including morphometric changes in the male reproductive organs and impairment on gametogenesis and sperm quality, impacting the fertile ability of adult animals. Despite the relevance of the fetal/perinatal period for toxicological studies on Mn, previous data only deal with the physical and neurological development of the offspring, without mentioning their reproductive development. The present study investigated whether exposure to Mn during fetal/perinatal phase, specifically during the period of formation and proliferation of Sertoli cells, impairs the reproductive development of male offspring in early postnatal life. Therefore, pregnant Wistar rats were randomly distributed into 3 experimental groups: Ctl (received saline solution), Mn-9 (received 9 mg/kg of MnCl2), and Mn-90 (received 90 mg/kg of MnCl2). The female rats received the experimental treatment by gavage from gestational day 13 to lactational day 15, i.e., postnatal day (PND) 15 of the pups. Oxidative damage to the genetic material of germ and Sertoli cells, together with a decrease in connexin 43 immunolabeling were observed in the testis of male pups evaluated at PND 15. In addition, an increase in the seminiferous tubules presenting slight epithelium vacuolization and cells with eosinophilic cytoplasm were observed, without apparent epididymal changes. In conclusion, it was demonstrated that Mn perturbed the initial testicular development by altering Sertoli cell integrity through oxidative insult, which may compromise the spermatogenesis in the long-term.

The AP-1 family of transcription factors are important regulators of gene expression within Leydig cells

PURPOSE

Members of the AP-1 family of transcription factors are immediate early genes being modulated by different extracellular signals. The aim of this review is to highlight the important roles of AP-1 members in transcriptional regulation of genes important for testicular Leydig cell function and male testosterone production.

METHODS

A search of the relevant literature was performed in Google Scholar and NCBI Pubmed for AP-1 members and Leydig cells. Additional information was accessed from references of relevant articles. Only primary data from original peer-reviewed articles was considered for this review.

RESULTS

Different signaling pathways important for Leydig cells' functions are involved in the regulation of the activity of AP-1 members. These transcription factors participate in the regulation of genes related to different biological processes important for Leydig cells.

CONCLUSIONS

We conclude that members of the AP-1 family of transcription factors play critical roles in the regulation of Leydig cell proliferation, steroidogenesis, and cell-to-cell communication.

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.

The Effect of Lead Acetate and Quercetin on the Tight and Gap Junctions in the Mouse Testis

Environmental pollutant effects on fertility sometime are irretrievable. The aim of this study was to investigate the effect of lead acetate and quercetin on tight (claudin 11 and occludin) and gap junctional (connexin 43) proteins and the integrity of the blood-testis barrier status. Experimental groups, including the lead acetate (Pb), quercetin (QE), lead acetate with quercetin (Pb + QE), and control mice, were treated at least one spermatogenic cycle. Gene expression of claudin 11 and occludin decreased in Pb + QE, Pb, and QE compared with the control group. Connexin 43 (Cx43) expression in the control and Pb groups was lower than in Pb + QE and QE. The immunohistochemical data were generally in line with these findings. In conclusion, the results showed that Pb exposure led to disorders in cellular interactions that affect testicular function; however, simultaneous treatment with quercetin did not alleviate these effects. Graphical Abstract.

The transcription factors SF-1 and SOX8 cooperate to upregulate Cx43 expression in mouse TM4 sertoli cells

Gap junctions made by connexins within the adult testis are essential for communication between Sertoli cells and for spermatogenesis. Sertoli cells play an important role in supporting germ cells differentiation and maturation into spermatozoa. Connexin43 (Cx43) is the most abundant and important connexin of the testis. We have shown previously that the expression of Cx43 is being regulated by SOX and AP-1 transcription factors in Sertoli cells. However, additional regulatory elements being able to recruit orphan nuclear receptors may be involved. Since SOX and SF-1 transcription factors have been shown to cooperate to regulate gene expression in Sertoli cells, we wondered if such mechanism could be involved in the activation of Cx43 expression. Thus, the activity of the Cx43 promoter was measured by co-transfections of luciferase reporter plasmid constructs with different expression vectors for transcription factors in the TM4 Sertoli cell line. The recruitment of SF-1 to the proximal region of the Cx43 promoter was evaluated by chromatin immunoprecipitation. Our results indicate that SOX8 and SF-1, as well as SOX9 and Nur77, cooperate to activate the expression of Cx43 and that SF-1 is being recruited to the −132 to −26 bp region of the Cx43 promoter. These results allow us to have a better understanding of the mechanisms regulating Cx43 expression and could explain some disturbances in communication between Sertoli cells responsible for impaired fertility.

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SF-1 and SOX8 cooperate to activate Cx43 expression in TM4 Sertoli cells.

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SF-1 is being recruited to the proximal region of the Cx43 promoter.

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LRH-1 and Nur77 also cooperate with SOX factors to activate Cx43 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.

Connexin 43 expression in the testes during postnatal development of finasteride-treated male rat offspring

INTRODUCTION

Hormone-dependent events that occur throughout the first wave of spermatogenesis, such as cellular communication within seminiferous epithelium during early postnatal testis maturation, are important for adult male fertility. Any changes in the T/DHT ratio in male progeny born from females fertilized by finasteride-treated male rats can result in impairment of testicular physiology. The aim of the study was to verify whether finasteride has a transgenerational effect on the expression of connexin 43 (Cx43), a gap junction protein in testes of the F1 generation.

MATERIAL AND METHODS

The subjects of the study were 7, 14, 21/22, 28, and 90-day-old Wistar male rats born by females fertilized by finasteride-treated rats (F1:Fin). The offspring born by untreated rats were used as controls (F1:Control). Connexin 43 was evaluated in the seminiferous epithelium by immunohistochemistry, and in the testis homogenates by Western blot and qRT-PCR. The Cx43 mRNA and protein expression was correlated with intratesticular levels of T and DHT by Spearman's rank correlation coefficient.

RESULTS

We observed a difference in the Cx43 expression in the testis of male rats born by female rats fertilized by finasteride-treated male rats, as compared to the control on following PND (7, 22 and 28 PND, p < 0.001; 14 PND, p < 0.01); and a strong, positive correlation between Cx43 with DHT was only in the F1:Fin group (mRNA: rs = +0.51, p = 0.004; protein: rs = +0.54, p = 0.002).

CONCLUSIONS

Finasteride treatment of male adult rats may cause changes in the communication between the testicular cells of their offspring, leading to a defective course of spermatogenesis.

Loss of connexin 43 in Sertoli cells provokes postnatal spermatogonial arrest, reduced germ cell numbers and impaired spermatogenesis

For the reason that adult Sertoli cell specific connexin 43 knockout (SCCx43KO) mice show arrested spermatogenesis at spermatogonial level or Sertoli cell only tubules and significantly reduced germ cell (GC) numbers, the aims of the present study were (1) to characterize the remaining GC population and (2) to elucidate possible mechanisms of their fading. Apoptosis was analyzed in both, KO and wild type (WT) male littermates during postnatal development and in adulthood using TUNEL. Although GC numbers were significantly reduced in KO at 2 and 8 days postpartum (dpp) when compared to WT, no differences were found concerning apoptotic incidence between genotypes. From 10 dpp, the substantial GC deficiency became more obvious. However, significantly higher apoptotic GC numbers were seen in WT during this period, possibly related to the first wave of spermatogenesis, a known phenomenon in normal pubertal testes associated with increased apoptosis. Characterization of residual spermatogonia in postnatal to adult KO and WT mice was performed by immunohistochemical reaction against VASA (marker of GCs in general), Lin28 and Fox01 (markers for undifferentiated spermatogonia) and Stra8 (marker for differentiating spermatogonia and early spermatocytes). During puberty, the GC component in SCCx43KO mice consisted likely of undifferentiated spermatogonia, few differentiating spermatogonia and very few early spermatocytes, which seemed to be rapidly cleared by apoptosis. In adult KOs, spermatogenesis was arrested at the level of undifferentiated spermatogonia. Overall, our data indicate that Cx43 gap junctions in SCs influence male GC development and differentiation rather than their survival.

Gja1 expression is regulated by cooperation between SOX8/SOX9 and cJUN transcription factors in TM4 and 15P-1 Sertoli cell lines

Within the seminiferous tubules of the testis, Gja1-encoded connexin43 plays a critical role in intercellular communication between Sertoli cells. These cells nurture, protect and stimulate the developing germ cells and spermatids. SOX transcription factors are known to play an important role in male fertility and sex determination; however, their physiological function and the identity of their target genes in postnatal Sertoli cells remain to be defined. Members of the activating protein-1 (AP-1) family have been shown to regulate Gja1 expression in myometrial and testicular cells and to physically interact with SOX members, suggesting that these transcription factors may regulate its expression within the testis. Hence, we performed co-transfections of expression plasmids encoding SOX4, SOX8, SOX9 and cJUN with different mouse Gja1 promoter/luciferase reporter constructs within TM4 and 15P-1 Sertoli cells. We showed that a functional cooperation between cJUN and SOX8 or SOX9 regulates Gja1 expression and may involve DNA regulatory elements located between -132 and -26 bp. Such synergy relies on the recruitment of cJUN to the -47 base pair (bp) AP-1 DNA regulatory element of the mouse Gja1 promoter. Hence, SOX and AP-1 members cooperate to regulate Gja1 within testicular Sertoli cells.

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.

Roles of connexins in testis development and spermatogenesis

The development and differentiation of cells involved in spermatogenesis requires highly regulated and coordinated interactions between cells. Intercellular communication, particularly via connexin43 (Cx43) gap junctions, plays a critical role in the development of germ cells during fetal development and during spermatogenesis in the adult. Loss of Cx43 in the fetus results in a decreased number of germ cells, while the loss of Cx43 in the adult Sertoli cells results in complete inhibition of spermatogenesis. Connexins 26, 32, 33, 36, 45, 46 and 50 have also been localized to specific compartments of the testis in various mammals. Loss of Cx46 is associated with an increase in germ cell apoptosis and loss of the integrity of the blood-testis barrier, while loss of other connexins appears to have more subtle effects within the seminiferous tubule. Outside the seminiferous tubule, the interstitial Leydig cells express connexins 36 and 45 along with Cx43; deletion of the latter connexin did not reveal it to be crucial for steroidogenesis or for the development and differentiation of Leydig cells. In contrast, loss of Cx43 from Sertoli cells results in Leydig cell hyperplasia, suggesting important cross-talk between Sertoli and Leydig cells. In the epididymis connexins 26, 30.3, Cx31.1, 32, and 43 have been identified and differentiation of the epithelium is associated with dramatic changes in their expression. Decreased expression of Cx43 results in decreased sperm motility, a function acquired by spermatozoa during epididymal transit. Clearly, intercellular gap junctional communication within the testis and epididymis represents a critical aspect of male reproductive function and fertility. The implications of this mode of intercellular communication for male fertility remains a poorly understood but important facet of male 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.

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.

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.

Effects of a murine germ cell-specific knockout of Connexin 43 on Connexin expression in testis and fertility

Connexin 43 (Cx 43)--expressed by germ cells (GC), Sertoli cells (SC) and Leydig cells--is one of at least eleven Cx in the murine testis. A general knockout (KO) of Cx 43 in mice results in perinatal death and a SC-specific KO of Cx 43 (SCCx43KO) causes infertility of male mice by preventing the initiation of spermatogenesis. To further elucidate the role of Cx 43 in the testis, a new mouse model with a GC-specific KO of Cx 43 (GCCx43KO) was created by using the Cre/loxP recombination system. A transgenic mouse line expressing the Cre gene under the tissue non-specific alkaline phosphatase promoter and a transgenic floxed Cx 43-LacZ mouse line were mated. The resulting F1-generation was backcrossed with homozygous Cx 43 floxed mice, and offspring was genotyped. Immunohistochemical analysis of testes of different aged homozygous mice revealed normal spermatogenesis and reduced Cx 43 immunoreactions. RT-qPCR and Western blots showed a downregulation of Cx 43 mRNA and protein, and a nearly unchanged mRNA expression of Cx 26, Cx 33 and Cx 45 in pubertal and adult KO mice. Western blots revealed considerable immunoreactive bands for Cx 26 and Cx 45. Male and female homozygous GCCx43KO mice were viable and fertile. Our data suggest, in contrast to inter SC and inter SC-GC cross talk in SCCx43KO mice which depends selectively on Cx 43 expression, that Cx 43 in GC seems not to be essential in GC-SC communication, when other Cx persist to be expressed.

Acute slices of mice testis seminiferous tubules unveil spontaneous and synchronous Ca2+ oscillations in germ cell clusters

Spermatogenic cell differentiation involves changes in the concentration of cytoplasmic Ca(2+) ([Ca(2+)]i); however, very few studies exist on [Ca(2+)]i dynamics in these cells. Other tissues display Ca(2+) oscillations involving multicellular functional arrangements. These phenomena have been studied in acute slice preparations that preserve tissue architecture and intercellular communications. Here we report the implementation of intracellular Ca(2+) imaging in a sliced seminiferous tubule (SST) preparation to visualize [Ca(2+)]i changes of living germ cells in situ within the SST preparation. Ca(2+) imaging revealed that a subpopulation of male germ cells display spontaneous [Ca(2+)]i fluctuations resulting from Ca(2+) entry possibly throughout Ca(V)3 channels. These [Ca(2+)]i fluctuation patterns are also present in single acutely dissociated germ cells, but they differ from those recorded from germ cells in the SST preparation. Often, spontaneous Ca(2+) fluctuations of spermatogenic cells in the SST occur synchronously, so that clusters of cells can display Ca(2+) oscillations for at least 10 min. Synchronous Ca(2+) oscillations could be mediated by intercellular communication via gap junctions, although intercellular bridges could also be involved. We also observed an increase in [Ca(2+)]i after testosterone application, suggesting the presence of functional Sertoli cells in the SST. In summary, we believe that the SST preparation is suitable to explore the physiology of spermatogenic cells in their natural environment, within the seminiferous tubules, in particular Ca(2+) signaling phenomena, functional cell-cell communication, and multicellular functional arrangements.

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 effects of 2-aminoethoxydiphenyl borate and diphenylboronic anhydride on gap junctions composed of Connexin43 in TM₄ sertoli cells

2-Aminoethoxydiphenyl borate (2-APB) has recently been demonstrated to inhibit gap junction (GJ) channels, whereas the underlying mechanisms are still unknown. Using mouse TM₄ Sertoli cell which expresses connexin43 (Cx43), we explored the effects of 2-APB and its analogues on dye-coupling through junctional channels formed by Cx43 and on expression of Cx43. Exposure of the cells to 2-APB (1-50 µM) and one of its analogues diphenylboronic anhydride (DPBA) (1-30 µM) for 4 h leads to a significant decrease in dye coupling of GJ in a concentration-dependent manner. The inhibitory effects of 2-APB and DPBA are reversible since decreased GJ coupling resumes after the two compounds are washed out. The disfunction of GJ induced by 2-APB and DPBA is associated with a decrease in total amount of Cx43 protein and number of GJs on the cell membrane. 2-APB and DPBA do not alter Cx43 phosphorylation state and the level of Cx43 mRNA expression. The loss of Cx43 protein is prevented by either lysosomal or proteasomal inhibitor, suggesting that the decrease in Cx43 results from a 2-APB or DPBA-enhanced degradation of Cx43. The present results indicate that 2-APB and DPBA inhibit GJ communication through decreasing Cx43 expression in TM₄ cells.

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.

Physiological and physiopathological aspects of connexins and communicating gap junctions in spermatogenesis

Spermatogenesis is a highly regulated process of germ cell proliferation and differentiation, starting from spermatogonia to spermatocytes and giving rise to spermatids, the future spermatozoa. In addition to endocrine regulation, testicular cell-cell interactions are essential for spermatogenesis. This precise control is mediated through paracrine/autocrine pathways, direct intercellular contacts and through intercellular communication channels, consisting of gap junctions and their constitutive proteins, the connexins. Gap junctions are localized between adjacent Leydig cells, between Sertoli cells and between Sertoli cells and specific germ cells. This review focuses on the distribution of connexins within the seminiferous epithelium, their participation in gap junction channel formation, the control of their expression and the physiological relevance of these junctions in both the Sertoli-Sertoli cell functional synchronization and the Sertoli-germ cell dialogue. In this review, we also discuss the potential implication of disrupted connexin in testis cancer, since impaired expression of connexin has been described as a typical feature of tumoral proliferation.

Effects of pre- and postnatal exposure to flutamide on connexin 43 expression in testes and ovaries of prepubertal pigs

The aim of this study was to show whether the connexin43 (Cx43) expression in gonads is affected by an anti-androgen action. To test this, pigs were prenatally (on gestational days 20-28 and 80-88; GD20, GD80), and postnatally (on days 2-10 after birth; PD2) exposed to flutamide that was given in five doses, every second day and its effect was observed in prepubertal gilts and boars. Morphology and expression of Cx43 was investigated in testes and ovaries by means of routine histology, immunohistochemistry, Western blotting, and RT-PCR, respectively. Qualitative analysis of immunohistochemical staining for Cx43 was confirmed by quantitative image analysis in which the staining intensity was expressed as relative optical density of diaminobenzidine deposits. There were statistically significant differences in Cx43 signal intensity between interstitial tissue of control and GD20 pigs (p less than 0.01), between seminiferous tubules of control and PD2 boars (p less than 0.01), between granulosa cells of preantral follicles of control and GD20 and PD2 pigs (p less than 0.01 and p less than 0.05, respectively), and between theca cells of control and GD80 and PD2 gilts (p less than 0.01). In Western blotting Cx43 appeared as a band of 43 kDa, whereas the size of the PCR-amplified product was 232 bp in all gonad tissue samples. Since we demonstrated changes in gonad morphology and in the expression of Cx43 at the level of protein of prepubertal boars and gilts, it seems possible that flutamide through blocking androgen action, causes delayed gonadal maturation in later postnatal life and, among other factors, may be involved in the regulation of Cx43 gene expression in pig gonads.

Expression of connexin 43 in normal canine testes and canine testicular tumors

In human testis, gap junctions containing connexin(Cx)43 are located within the seminiferous epithelium between Sertoli cells and between Sertoli and germ cells. Cx43 is known to play a role in the differentiation and proliferation of these cell types. It can further be associated with human seminoma development. The dog has been proposed as a model for studies of the male reproductive system, because of the frequent occurrence of testicular neoplasms. Thus, we investigated Cx43-mRNA and -protein expression in testes of normal prepubertal dogs, adult dogs, and in canine testicular tumors. Sertoli cells in prepubertal cords express Cx43 mRNA, but do synthesize only less Cx43 protein. Within the seminiferous tubules, Cx43 mRNA was detected in Sertoli cells, spermatogonia, and spermatocytes. Cx43 protein was mainly present in the basal compartment. In canine testicular tumors Cx43 mRNA was detectable in both seminoma and neoplastic Sertoli cells, whereas Cx43 protein was only found in neoplastic Sertoli cells. Our data indicate that Cx43 is regulated differentially in testicular tumors and that alterations of Cx43 expression may be involved in the pathogenesis of canine testicular malignancies. This study represents the first morphological work on the spatiotemporal expression pattern of Cx43 in normal and neoplastic canine testis.

Leptin inhibits basal but not gonadotrophin-stimulated testosterone production in the immature mouse and sheep testis

The mechanisms whereby leptin regulates testosterone secretion are complex and are likely to involve actions at different levels of the hypothalamus–pituitary–gonadal axis. In the present study, the effect of leptin on testicular steroidogenesis at different developmental stages in mice and sheep was investigated. Testosterone data from testicular slice and Leydig cells of immature and adult mice testes demonstrated that the action of leptin in the regulation of steroidogenesis appears to be dependent on the developmental stage of the testis. Leptin biphasically modulates basal testosterone production in immature testicular slice cultures: at relatively low concentrations (6.25–12.5 ng mL–1) leptin exerts a significant inhibitory effect, but has less of an effect at very low (1.25 ng mL–1) or high concentrations (25 ng mL–1). However, leptin failed to modulate basal testosterone levels in Leydig cell preparations. In contrast with immature testes, leptin was unable to regulate either basal or human chorionic gonadotrophin (10 IU mL–1)-stimulated testosterone production in adult testicular slices or Leydig cell cultures. The age- and concentration-dependent regulation pattern was confirmed using sheep testicular slice culture. Leptin (1.56–25 ng mL–1) significantly inhibited basal testosterone production in the testis from birth to Day 21, but had no effect on Day 27 or older testes. However, the plasma and testicular concentrations of leptin and testosterone data in the ram indicate that such a regulatory effect of leptin on testis steroidogenesis in vitro is unable to efficiently influence testosterone concentrations in vivo. This does not exclude the possibility of a non-competitive mechanism of interaction between leptin and luteinising hormone to regulate testosterone production. Thus, we hypothesise that leptin is not an important independent regulator of testosterone concentration in the normal physiological state. The physiological significance and mechanism of leptin regulation of basal testosterone production are not known; further studies are required to elucidate these important issues.

A sertoli cell-specific knockout of connexin43 prevents initiation of spermatogenesis

The predominant testicular gap junctional protein connexin43 (cx43) is located between neighboring Sertoli cells (SCs) and between SCs and germ cells. It is assumed to be involved in testicular development, cell differentiation, initiation, and maintenance of spermatogenesis with alterations of its expression being correlated with various testicular disorders. Because total disruption of the cx43 gene leads to perinatal death, we generated a conditional cx43 knockout (KO) mouse using the Cre/loxP recombination system, which lacks the cx43 gene solely in SCs (SCCx43KO), to evaluate the SC-specific functions of cx43 on spermatogenesis in vivo. Adult SCCx43KO(-/-) mice showed normal testis descent and development of the urogenital tract, but testis size and weight were drastically lower compared with heterozygous and wild-type littermates. Histological analysis and quantitation of mRNA expression of germ cell-specific marker genes revealed a significant reduction in the number of spermatogonia but increased SC numbers/tubule with only a few tubules left showing normal spermatogenesis. Thus, SC-specific deletion of cx43 mostly resulted in an arrest of spermatogenesis at the level of spermatogonia or SC-only syndrome and in intratubular SC clusters. Our data demonstrate for the first time that cx43 expression in SCs is an absolute requirement for normal testicular development and spermatogenesis.

The Expression of Multiple Connexins Throughout Spermatogenesis in the Rainbow Trout Testis Suggests a Role for Complex Intercellular Communication1

Certain fish, such as rainbow trout (Oncorhynchus mykiss), are seasonal breeders. Spermatogenesis in rainbow trout is synchronous; therefore, at any time point during this process, germ cells are predominantly at the same stage of development. As such, rainbow trout represent an excellent model in which to study spermatogenesis. Gap junctions are composed of connexons, which are themselves formed by six transmembrane proteins termed connexins (Cxs). The objectives of this study were to assess which Cxs are expressed in the rainbow trout testis, and if their expression was stage specific during gonadal maturation. Rainbow trout were killed at various stages of maturation, and total cellular RNA was isolated from the testes. RT-PCR using degenerate primers recognizing all vertebrate Cxs indicates that there are several different Cxs in trout testes. Amplicons were cloned and sequenced. Homology comparisons indicate that these were cx43, cx43.4, cx31, and cx30. Immunolocalization of these Cxs indicate that Cx43 was localized primarily to Sertoli cells, while Cx43.4 was localized along the lateral plasma membranes between adjacent spermatocytes. Cx30 was localized to the interstitial Leydig cells, and Cx31 was localized primarily to the endothelium of interstitial blood vessels. The expression of each Cx varied as a function of the stage of spermatogenesis, suggesting that the expression of these proteins is highly regulated. Together, these results indicate that intercellular communication in the testis is complex, involves several different Cxs, and is a highly regulated process.

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

There is strong evidence that thyroid hormones through triiodothyronine (T3) regulate Sertoli cell proliferation and differentiation in the neonatal testis. However, the mechanism(s) by which they are able to control Sertoli cell proliferation is unclear. In the present study in vivo approaches (PTU-induced neonatal hypothyroidism known to affect Sertoli cell proliferation) associated with in vitro experiments on a Sertoli cell line were developed to investigate this question. We demonstrated that the inhibitory effect of T3 on Sertoli cell growth, analyzed by evaluating DNA-incorporated [3H] thymidine, was associated with a time and dose-dependent increase in the levels of Cx43, a constitutive protein of gap junctions, known to participate in the control of cell proliferation and the most predominant Cx in the testis. These Cx43 changes were associated with increased gap junction communication measured by gap FRAP. Consistent with these results two specific inhibitors of gap junction coupling, AGA and oleamide, were able to significantly reverse the T3 inhibitory effect on Sertoli cell proliferation. The present data also revealed a nongenomic effect of T3 on Cx43 Sertoli cells that was evidenced by a rapid up-regulation of gap junction plaque number as identified in Cx43-GFP transfected cells exposed to the hormone. This process appears mediated through actin cytoskeleton since incubation of the cells with cytochalasin D totally reversed the T3 stimulatory effect on Cx43-GFP gap junction plaques. Based on these data, we propose a working hypothesis in which Cx43 could be an intermediate target for T3 inhibition of neonatal Sertoli cell proliferation.

Leydig cell function in mice lacking connexin43

Connexin43 (Cx43) is the most abundantly expressed member of the connexin (gap junction protein) family and the only one so far identified in mouse Leydig cell gap junctions. Mice lacking Cx43 were used to investigate its role in testicular androgen production and regulation. Testes from term fetuses were grafted under the kidney capsules of castrated adult males. After 3 weeks, serum from host mice was analyzed for androgens. In order to test their response to stimulation, the grafted testes were incubatedin vitrowith varying concentrations of LH and their androgen end products analyzed. Incubation with radiolabeled progesterone was followed by high performance liquid chromatography to quantify the androgen-intermediate metabolites. Radiolabeled testosterone in the presence of NADPH was used to determine the activity of testosterone-metabolizing enzymes 17β-hydroxysteroid dehydrogenase (17βHSD), 5α-reductase (5αR), and 3α-hydroxysteroid dehydrogenase (3α HSD). Serum androgen levels did not differ between hosts carrying wild-type versus null mutant grafts although Cx43-deficient testes had more 17βHSD and 5αR activity than wild-type controls. Furthermore, the genotype of grafted testes did not influence LH-stimulated androgen productionin vitro. These results indicate that the steroidogenic function of Leydig cells is not compromised by the absence of Cx43, perhaps because other gap junction proteins are present. Dye transfer experiments demonstrated that Cx43-deficient Leydig cells retain intercellular coupling, indicating that Cx43 is not the only protein contributing to their gap junctions. Thus, despite their prominence in Leydig cells, Cx43 gap junctions are not essential for androgen production.

Opposite regulation of connexin33 and connexin43 by LPS and IL-1α in spermatogenesis

The gap junction proteins, connexins (Cxs), are present in the testis, and among them, Cx43 play an essential role in spermatogenesis. In the present study, we investigated the testicular expression and regulation of another Cx, Cx33, previously described as a negative regulator of gap junction communication. Cx33 mRNA was present in testis and undetectable in heart, liver, ovary, and uterus. In the mature testis, Cx33 was specifically immunolocalized in the basal compartment of the seminiferous tubules, whereas Cx43 was present in both seminiferous tubule and interstitial compartments. During stages IX and X of spermatogenesis, characterized by Sertoli cell phagocytosis of residual bodies, Cx43 was poorly expressed within seminiferous tubules, while Cx33 signal was strong. To evaluate the role of phagocytosis in the control of Cx33 and Cx43 expression, the effect of LPS was analyzed in the Sertoli cell line 42GPA9. We show herein that phagocytosis activation by LPS concomitantly stimulated Cx33 and inhibited Cx43 mRNA levels. These effects appear to have been mediated through IL-1α, because the exposure of Sertoli cells to the IL-1 receptor antagonist partly reversed these effects. IL-1α enhanced and reduced, respectively, the levels of Cx33 and Cx43 mRNA in a time- and dose-dependent manner. These data reveal that Cx33 and Cx43 genes are controlled differently within the testis and suggest that these two Cxs may exert opposite and complementary effects on spermatogenesis.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

The role of connexins in the differentiation of NT2 cells in Sertoli-NT2 cell tissue constructs grown in the rotating wall bioreactor

Neural transplantation is developing as a successful treatment for neurodegenerative diseases such as Parkinson's disease. The human Ntera-2/D1 (NT2) cell line is an attractive alternative to the use of human fetal neurons as a cell source for transplantation. We have explored combining NT2 cells, as a neuronal source, and Sertoli cells, which may act as a graft facilitator to enhance neuronal survival and differentiation, and ameliorate the host immune response, into a tissue construct for use in cell replacement therapy for neurodegenerative disease. This Sertoli-NT2-aggregated cell (SNAC) tissue construct is formed in the high aspect ratio vessel (HARV) bioreactor. NT2 cells differentiate to dopaminergic NT2N neurons within the SNAC tissue construct without retinoic acid. We report here that the gap junction protein connexin 43 is decreased among differentiated NT2N neurons. Inhibition of connexin 43 with 18beta glycyrrhetinic acid and carbenoxolone, a glycyrrhetinic acid derivative, during formation of the SNAC tissue constructs disrupts the differentiation of NT2 cells. Therefore, connexin 43 is important in the differentiation of NT2 cells in the SNAC tissue construct.

Role of connexin-based gap junction channels in testis

Spermatogenesis is a highly controlled process that allows proliferation and differentiation of male germ cells. This is under classical endocrine and paracrine controls. There is also evidence that gap junctions between Leydig cells, between Sertoli cells and between Sertoli and germ cells participate in the local regulation of spermatogenesis. Recent studies reveal that connexin 43 (Cx43), the predominant gap junction protein in the testis, is essential for the initiation and maintenance of spermatogenesis. In this review, we focus on the identification, distribution and control of connexins in the mammalian testis. The implication of connexin-based gap junctions in testicular physiology and in pathological disorders of spermatogenesis (spermatogenic arrest and testis cancer) is also discussed.

Sequestration of connexin43 in the early endosomes: An early event of Leydig cell tumor progression

Connexins form gap junction channels that allow intercellular communication between neighboring cells. Compelling evidence has revealed that Cx are tumor-suppressor genes and reduced Cx expression has been related with uncontrolled cell growth in tumors and transformed cells. In the present study, we addressed Cx transcriptional and posttranscriptional regulations during the earlier stage of testicular tumors confined to Leydig cells in a transgenic mice model. In situ hybridization indicated that connexin43 (Cx43) mRNA was highly expressed either at early tumorogenesis (3 m) characterized by intense proliferation of Leydig cells, or at advanced tumorogenesis (6-7 m) when tumor cells completely invaded the testis. In contrast, Cx43 protein analyzed by Western blotting or classic immunohistochemical analyses was present at the beginning of tumor progression, but was dramatically reduced as tumor advanced. Application of high-resolution deconvolution microscopy to testis sections demonstrates that cells that proliferate exhibited an aberrant cytoplasmic Cx43 localization, in contrast to the expected plasma membrane Cx43 localization in normal Leydig cells. Dual immunofluorescence labeling with specific markers of cellular compartments shows that cytoplasmic Cx43 signal was mainly sequestered within early endosomes. Altogether, this study provides the first evidence that impaired Cx43 trafficking in endosomes is an early event associated with uncontrolled cell proliferation that could serve as a neoplastic marker.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Connexin43 is expressed in mouse fetal ovary

Developmental studies have shown that connexin43 (Cx43) is expressed in the ovary from the first day of life and throughout the rest of postnatal development. In both mouse embryonic ovaries and testes, target-directed deletion of Cx43 gene induces a significant decrease in germinal cells, but the exact mechanism determining this reduction remains unknown. Moreover, recently we found that Cx43 is abundantly expressed in mouse testes from the earliest stages of its fetal development. In the present work we investigate whether Cx43 transcript and protein are expressed in mouse embryonic ovaries. Total RNA was analyzed with specific Cx43 oligonucleotides in RT-PCR studies. A Cx43 PCR product was detected in ovaries at 16.5 and 18.5 days postcoitum (dpc). Bands of 43-45 kDa, characteristic of Cx43, were detected in immunoblots of total homogenates of ovaries at 14.5 and 18.5 dpc. Cell type-specific expression of Cx43 was investigated using double-labeled sections incubated with specific antibodies against Cx43 and the enzyme 3beta-hydroxysteroid dehydrogenase (3betaHSD) or a germ cell nuclear antigen (GCNA1), which are cell markers of steroidogenic and germinal cells, respectively. At 18.5 dpc, Cx43 was found in conglomerates of 3betaHSD-positive cells. Cx43 was also localized at homocellular junctions between parenchyma pregranulosa cells, and at heterocellular junctions between pregranulosa and germinal cells. At these two latter localizations, Cx43 was traced back to 12.5 dpc. In conclusion, this study demonstrates for the first time that from the earliest stages of embryonic ovary development, Cx43 is expressed in principal cell types involved in control of female fertility. These data suggest that the gap junctions formed with Cx43 between somatic and germinal cells may be necessary for prenatal expansion of germinal cells at initial stages of fetal gonadal development.

Invited Review: pulmonary alveoli: formation, the "call for oxygen," and other regulators

The lung's only known essential function is to provide sufficient alveolar surface to meet the organism's need for oxygen and elimination of CO(2). The importance of the magnitude of alveolar surface area (Sa) to O(2) uptake (VO(2)) is supported by the presence among mammals of a direct linear relationship between Sa and VO(2). This match has been achieved, despite the higher body mass-specific VO(2) of small organisms compared with large, by a greater subdivision of alveolar surface, not by a larger relative lung volume in small organisms. This highly conserved relationship between alveolar architecture and VO(2) suggests the presence of similarly conserved mechanisms that control the onset, rate, and cessation of alveolus formation and alveolar size, which are also influenced by retinoids and thyroid and corticosteroid hormones. Furthermore, the "call for oxygen" is met at a breathing rate and tidal volume at which the work of breathing is lowest. Thus there is a complex, fascinating, but poorly understood, signaling relationship among VO(2), the neural regulation of breathing, and lung architecture, composition, and mechanics.

Developmental regulation of connexin 43 expression in fetal mouse testicular cells

Multiple connexins have been identified in testicular cells. Several lines of evidences indicate that, among them, connexin 43 (Cx43) may be unique for control of gonad development and spermatogenesis. To date, however, it is not known whether Cx43 is expressed in the fetal testis and what possible types of cellular interactions mediated by this connexin are critical to male fertility. In the present work, expression of Cx43 was investigated at various developmental ages in cryosections from mouse testis by using specific antibodies against Cx43. In serial or double-labeled sections, Cx43 localization was compared with immunocytochemical distribution of steroidogenic enzyme, 3beta-hydroxysteroid dehydrogenase (3betaHSD), Mullerian inhibitory hormone (MIH), and germinal nuclear cell antigen (GCNA1), which are specific markers, respectively, of interstitial Leydig, Sertoli, and germinal cells. Sections were analyzed by fluorescence microscopy. We found that Cx43 immunofluorescence (IF) was uniformly distributed in the undifferentiated gonad at 11.5 days post coitus (dpc) and in cells of the mesonephric tubules. In the undifferentiated gonad, Cx43 was localized between primordial germ cells and somatic cells. At 12.5 dpc, when the gonad has undergone sexual differentiation, in the interstitium Cx43 was localized in Leydig cells and in the seminiferous cord it was localized between adjacent Sertoli cells. In Leydig and Sertoli cells, Cx43 labeling increased at 14.5, 16.5, and 18.5 dpc. From day 12.5 up to 18.5 dpc, Cx43 was also localized in cell borders between germinal and Sertoli cells. In conclusion, this study demonstrates that from the earliest stages of gonadal development, Cx43 is expressed in the principal cell types that participate in the control of male fertility. It also shows that Cx43 expression in Leydig and Sertoli cells increase during fetal life. Finally, it provides evidence that, throughout embryonic life, Cx43 forms gap junctions between Sertoli and germinal cells.