Disruption of gap junctional intercellular communication by lindane is associated with aberrant localization of connexin43 and zonula occludens-1 in 42GPA9 Sertoli cells

Prenatal exposure to Di(2-ethylhexyl) phthalate and high-fat diet synergistically disrupts gonadal function in male mice†

Prenatal exposure to Di (2-ethylhexyl) phthalate (DEHP) impairs the reproductive system and causes fertility defects in male offspring. Additionally, high-fat (HF) diet is a risk factor for reproductive disorders in males. In this study, we tested the hypothesis that prenatal exposure to a physiologically relevant dose of DEHP in conjunction with HF diet synergistically impacts reproductive function and fertility in male offspring. Female mice were fed a control or HF diet 7 days prior to mating and until their litters were weaned on postnatal day 21. Pregnant dams were exposed to DEHP or vehicle from gestational day 10.5 until birth. The male offspring's gross phenotype, sperm quality, serum hormonal levels, testicular histopathology, and testicular gene expression pattern were analyzed. Male mice born to dams exposed to DEHP + HF had smaller testes, epididymides, and shorter anogenital distance compared with those exposed to HF or DEHP alone. DEHP + HF mice had lower sperm concentration and motility compared with DEHP mice. Moreover, DEHP + HF mice had more apoptotic germ cells, fewer Leydig cells, and lower serum testosterone levels than DEHP mice. Furthermore, testicular mRNA expression of Dnmt1 and Dnmt3a was two to eight-fold higher than in DEHP mice by qPCR, suggesting that maternal HF diet and prenatal DEHP exposure additively impact gonadal function by altering the degree of DNA methylation in the testis. These results suggest that the combined exposure to DEHP and high-fat synergistically impairs reproductive function in male offspring, greater than exposure to DEHP or HF diet alone.

Endocytic trafficking of connexins in cancer pathogenesis

Gap junctions are specialized regions of the plasma membrane containing clusters of channels that provide for the diffusion of ions and small molecules between adjacent cells. A fundamental role of gap junctions is to coordinate the functions of cells in tissues. Cancer pathogenesis is usually associated with loss of intercellular communication mediated by gap junctions, which may affect tumor growth and the response to radio- and chemotherapy. Gap junction channels consist of integral membrane proteins termed connexins. In addition to their canonical roles in cell-cell communication, connexins modulate a range of signal transduction pathways via interactions with proteins such as β-catenin, c-Src, and PTEN. Consequently, connexins can regulate cellular processes such as cell growth, migration, and differentiation through both channel-dependent and independent mechanisms. Gap junctions are dynamic plasma membrane entities, and by modulating the rate at which connexins undergo endocytosis and sorting to lysosomes for degradation, cells rapidly adjust the level of gap junctions in response to alterations in the intracellular or extracellular milieu. Current experimental evidence indicates that aberrant trafficking of connexins in the endocytic system is intrinsically involved in mediating the loss of gap junctions during carcinogenesis. This review highlights the role played by the endocytic system in controlling connexin degradation, and consequently gap junction levels, and discusses how dysregulation of these processes contributes to the loss of gap junctions during cancer development. We also discuss the therapeutic implications of aberrant endocytic trafficking of connexins in cancer cells.

Endocrine-disrupting chemicals affect sertoli TM4 cell functionality through dysregulation of gap junctional intercellular communication in vitro

The frequencies of adverse outcomes associated with male reproductive health, including infertility and testicular cancer, are increasing. These adverse trends are partially attributed to increased exposure to environmental agents such as endocrine-disrupting chemicals (EDCs). This study addresses effects on EDCs on adjacent prepubertal Sertoli TM4 cells, specifically on 1) testicular gap junctional intercellular communication (GJIC), one of the hallmarks of non-genotoxic carcinogenicity, 2) GJIC building blocks connexins (Cx), and 3) mitogen-activated protein kinases MAPKs. We selected eight representatives of EDCs: bisphenol A and organochlorine chemicals such as pesticides dichlorodiphenyltrichloroethane, lindane, methoxychlor, and vinclozolin, industrial chemical 2,2',4,4',5,5'-hexachlorobiphenyl, and components of personal care products, triclocarban and triclosan. EDCs rapidly dysregulated GJIC in Sertoli TM4 cells mainly via MAPK p38 and/or Erk1/2/pathways by the intermediate hyper- or de-phosphorylation of Cx43 (Ser368, Ser282) and translocalization of Cx43 from the plasma membrane, suggesting disturbed intracellular trafficking of Cx43 protein. Surprisingly, EDCs did not rapidly activate MAPK Erk1/2 or p38; on the contrary, TCC and TCS decreased their activity (phosphorylation). Our results indicate that EDCs might disrupt testicular homeostasis and development via testicular GJIC, junctional and non-junctional functions of Cx43 and MAPK-signalling pathways in Sertoli cells.

Applicability of Scrape Loading-Dye Transfer Assay for Non-Genotoxic Carcinogen Testing

Dysregulation of gap junction intercellular communication (GJIC) is recognized as one of the key hallmarks for identifying non-genotoxic carcinogens (NGTxC). Currently, there is a demand for in vitro assays addressing the gap junction hallmark, which would have the potential to eventually become an integral part of an integrated approach to the testing and assessment (IATA) of NGTxC. The scrape loading-dye transfer (SL-DT) technique is a simple assay for the functional evaluation of GJIC in various in vitro cultured mammalian cells and represents an interesting candidate assay. Out of the various techniques for evaluating GJIC, the SL-DT assay has been used frequently to assess the effects of various chemicals on GJIC in toxicological and tumor promotion research. In this review, we systematically searched the existing literature to gather papers assessing GJIC using the SL-DT assay in a rat liver epithelial cell line, WB-F344, after treating with chemicals, especially environmental and food toxicants, drugs, reproductive-, cardio- and neuro-toxicants and chemical tumor promoters. We discuss findings derived from the SL-DT assay with the known knowledge about the tumor-promoting activity and carcinogenicity of the assessed chemicals to evaluate the predictive capacity of the SL-DT assay in terms of its sensitivity, specificity and accuracy for identifying carcinogens. These data represent important information with respect to the applicability of the SL-DT assay for the testing of NGTxC within the IATA framework.

Decabromodiphenyl ether (BDE-209) exposure to lactating mice perturbs steroidogenesis and spermatogenesis in adult male offspring

Decabromodiphenyl ether (BDE-209) is widely used as a flame retardant in many products like electronic equipments, plastics, furniture and textiles. BDE-209, a thyroid hormones (THs)-disrupting chemical, affects male reproductive health through altered THs status in mouse model. The present study was designed in continuation to our earlier work to elucidate whether early life exposure to BDE-209 has a long term potential risk to male reproductive health. This study, therefore, aimed to evaluate the effect of maternal BDE-209 exposure during lactation and to elucidate possible mechanism(s) of its action on male reproduction in adult Parkes mice offspring. Lactating female Parkes mice were orally gavaged with 500, and 700 mg/kg body weight of BDE-209 in corn oil from postnatal day (PND) 1 to PND 28 along with 6-propyl-2-thiouracil (PTU)-treated positive controls and vehicle-treated controls. Male pups of lactating dams were euthanized at PND 75. Maternal BDE-209 exposure during lactation markedly affected histoarchitecture of testis and testosterone production with concomitant down-regulation in the expression of various steroidogenic markers in adult offspring. Maternal exposure to BDE-209 during lactation also interfered with germ cell dynamics and oxidative status in testes of adult mice offspring. A decreased expression of connexin 43 and androgen receptor was also evident in testes of these mice offspring; further, number, motility and viability of spermatozoa were also adversely affected in these mice. The results thus provide evidences that maternal exposure to BDE-209 during lactation causes reproductive toxicity in adult mice offspring.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

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.

Endocrine-disrupting chemicals rapidly affect intercellular signaling in Leydig cells

A decline in male fertility possibly caused by environmental contaminants, namely endocrine-disrupting chemicals (EDCs), is a topic of public concern and scientific interest. This study addresses a specific role of testicular gap junctional intercellular communication (GJIC) between adjacent prepubertal Leydig cells in endocrine disruption and male reproductive toxicity. Organochlorine pesticides (lindane, methoxychlor, DDT), industrial chemicals (PCB153, bisphenol A, nonylphenol and octylphenol) as well as personal care product components (triclosan, triclocarban) rapidly dysregulated GJIC in murine Leydig TM3 cells. The selected GJIC-inhibiting EDCs (methoxychlor, triclosan, triclocarban, lindane, DDT) caused the immediate GJIC disruption by the relocation of gap junctional protein connexin 43 (Cx43) from the plasma membrane and the alternation of Cx43 phosphorylation pattern (Ser368, Ser279, Ser282) of its full-length and two N-truncated isoforms. After more prolonged exposure (24 h), EDCs decreased steady-state levels of full-length Cx43 protein and its two N-truncated isoforms, and eventually (triclosan, triclocarban) also tight junction protein Tjp-1. The disturbance of GJIC was accompanied by altered activity of mitogen-activated protein kinases MAPK-Erk1/2 and MAPK-p38, and a decrease in stimulated progesterone production. Our results indicate that EDCs might disrupt testicular homeostasis and development via disruption of testicular GJIC, a dysregulation of junctional and non-junctional functions of Cx43, activation of MAPKs, and disruption of an early stage of steroidogenesis in prepubertal Leydig cells. These critical disturbances of Leydig cell development and functions during a prepubertal period might be contributing to impaired male reproduction health later on.

Effects of Chemotherapy and Radiotherapy on Spermatogenesis: The Role of Testicular Immunology

Substantial improvements in cancer treatment have resulted in longer survival and increased quality of life in cancer survivors with minimized long-term toxicity. However, infertility and gonadal dysfunction continue to be recognized as adverse effects of anticancer therapy. In particular, alkylating agents and irradiation induce testicular damage that results in prolonged azoospermia. Although damage to and recovery of spermatogenesis after cancer treatment have been extensively studied, there is little information regarding the role of differences in testicular immunology in cancer treatment-induced male infertility. In this review, we briefly summarize available rodent and human data on immunological differences in chemotherapy or radiotherapy.

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.

The role of gap junctions in inflammatory and neoplastic disorders (Review)

Gap junctions are intercellular channels made of connexin proteins, mediating both electrical and biochemical signals between cells. The ability of gap junction proteins to regulate immune responses, cell proliferation, migration, apoptosis and carcinogenesis makes them attractive therapeutic targets for treating inflammatory and neoplastic disorders in different organ systems. Alterations in gap junction profile and expression levels are observed in hyperproliferative skin disorders, lymphatic vessel diseases, inflammatory lung diseases, liver injury and neoplastic disorders. It is now recognized that the therapeutic effects mediated by traditional pharmacological agents are dependent upon gap junction communication and may even act by influencing gap junction expression or function. Novel strategies for modulating the function or expression of connexins, such as the use of synthetic mimetic peptides and siRNA technology are considered.

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.

Similar causes of various reproductive disorders in early life

During the past few decades, scientific evidence has been accumulated concerning the possible adverse effects of the exposure to environmental chemicals on the well-being of wildlife and human populations. One large and growing group of such compounds of anthropogenic or natural origin is referred to as endocrine-disrupting chemicals (EDCs), due to their deleterious action on the endocrine system. This concern was first focused on the control of reproductive function particularly in males, but has later been expanded to include all possible endocrine functions. The present review describes the underlying physiology behind the cascade of developmental events that occur during sexual differentiation of males and the specific role of androgen in the masculinization process and proper organogenesis of the external male genitalia. The impact of the genetic background, environmental exposures and lifestyle factors in the etiology of hypospadias, cryptorchidism and testicular cancer are reviewed and the possible role of EDCs in the development of these reproductive disorders is discussed critically. Finally, the possible direct and programming effects of exposures in utero to widely use therapeutic compounds, environmental estrogens and other chemicals on the incidence of reproductive abnormalities and poor semen quality in humans are also highlighted.

Connexins: mechanisms regulating protein levels and intercellular communication

Intercellular communication can occur through gap junction channels, which are comprised of connexin proteins. Therefore, levels of connexins can directly correlate with gap junctional intercellular communication. Because gap junctions have a critical role in maintaining cellular homeostasis, the regulation of connexin protein levels is important. In the connexin life cycle, connexin protein levels can be modified through differential gene transcription or altered through trafficking and degradation mechanisms. More recently, significant attention has been directed to the pathways that cells utilize to increase or decrease connexin levels and thus indirectly, gap junctional communication. Here, we review the studies revealing the mechanisms that affect connexin protein levels and gap junctional intercellular communication.

The dynamics of connexin expression, degradation and localisation are regulated by gonadotropins during the early stages of in vitro maturation of swine oocytes

Gap junctional communication (GJC) plays a primordial role in oocyte maturation and meiotic resumption in mammals by directing the transfer of numerous molecules between cumulus cells and the oocyte. Gap junctions are made of connexins (Cx), proteins that regulate GJC in numerous ways. Understanding the dynamic regulation of connexin arrangements during in vitro maturation (IVM) could provide a powerful tool for controlling meiotic resumption and consequently in vitro development of fully competent oocytes. However, physiological events happening during the early hours of IVM may still be elucidated. The present study reports the dynamic regulation of connexin expression, degradation and localization during this stage. Cx43, Cx45 and Cx60 were identified as the main connexins expressed in swine COC. Cx43 and Cx45 transcripts were judged too static to be a regulator of GJC, while Cx43 protein expression was highly responsive to gonadotropins, suggesting that it might be the principal regulator of GJC. In addition, the degradation of Cx43 expressed after 4.5 h of IVM in response to equine chorionic gonadotropin appeared to involve the proteasomal complex. Cx43 localisation appeared to be associated with GJC. Taken together, these results show for the first time that gonadotropins regulate Cx43 protein expression, degradation and localisation in porcine COC during the first several hours of IVM. Regulation of Cx43 may in turn, via GJC, participate in the development of fully competent oocytes.

FSH induces the development of circadian clockwork in rat granulosa cells via a gap junction protein Cx43-dependent pathway

The present study was designed to assess the relationship between gap junctions and the maturation of a clock system in rat granulosa cells stimulated by follicle-stimulating hormone (FSH). Immature and mature granulosa cells were prepared by puncturing the ovaries of diethylstilbestrol- and equine chorionic gonadotropin (eCG)-treated mouse Period2 (Per2)-dLuc reporter gene transgenic rats, respectively. Mature granulosa cells exposed to dexamethasone (DXM) synchronization displayed several Per2-dLuc oscillations and a rhythmic expression of clock genes. Intriguingly, we observed clear evidence that the FSH stimulation significantly increased the amplitude of Per2 oscillations in the granulosa cells, which was confirmed by the elevation of the Per2 and Rev-erbα (Nr1d1) mRNA levels. FSH also induced a major phase-advance shift of Per2 oscillations. The mature granulosa cells cultured for 2 days with FSH expressed higher mRNA levels of Per2, Rev-erbα, Bmal1 (Arnt1), Lhcgr, and connexin (Cx) 43 (Gja1) compared with the immature granulosa cells. Consistently, our immunofluorescence results revealed abundant Cx43 protein in antral follicles stimulated with eCG and weak or no fluorescence signal of Cx43 in primary and preantral follicles. Similar results were confirmed by Western blotting analysis. Two gap junction blockers, lindane and carbenoxolone (CBX), significantly decreased the amplitude of Per2 oscillations, which further adhered significant decreases in Per2 and Rev-erbα transcript levels. In addition, both lindane and CBX induced a clear phase-delay shift of Per2 oscillations. These findings suggest that FSH induces the development of the clock system by increasing the expression of Cx43.

Electrophysiology of single and aggregate Cx43 hemichannels

Connexin43 (Cx43) is the most ubiquitous gap junction protein in the human body and is essential for cell-to-cell communication in a variety of organs and organ systems. As a result, Cx43 is responsible for mediating both electrical and chemical signals, passing dissolved solutes and small signaling molecules between cells in a coordinated fashion. Here, we explore the electrophysiological properties of hemichannels formed from Cx43 and Cx43 fused to eGFP (Cx43eGFP) and their interactions in a planar lipid membrane (BLM). Unlike in vivo patch clamp experiments, Cx43 was purified and isolated from other membrane constituents allowing elucidation of individual protein responses to various electrical and chemical stimuli. Using this system, we examined hemichannel electrophysiology and the roles of several well-known gap junction blockers, namely: lanthanum, heptanol, carbenoxalone and lindane. We also observed a critical number of hemichannels required for an accelerated conductance increase, an emergent electrical signature indicative of plaque formation.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Autophagy modulates dynamics of connexins at the plasma membrane in a ubiquitin-dependent manner

Connexins modulate intercellular communication when assembled in gap junctions. Compromised macroautophagy increases cellular communication due to failure to degrade connexins at gap junctions. Nedd4-mediated ubiquitinylation of the connexin molecule is required to trigger its autophagy-dependent internalization and degradation.

Different pathways contribute to the turnover of connexins, the main structural components of gap junctions (GJs). The cellular pool of connexins targeted to each pathway and the functional consequences of degradation through these degradative pathways are unknown. In this work, we focused on the contribution of macroautophagy to connexin degradation. Using pharmacological and genetic blockage of macroautophagy both in vitro and in vivo, we found that the cellular pool targeted by this autophagic system is primarily the one organized into GJs. Interruption of connexins' macroautophagy resulted in their retention at the plasma membrane in the form of functional GJs and subsequent increased GJ-mediated intercellular diffusion. Up-regulation of macroautophagy alone is not sufficient to induce connexin internalization and degradation. To better understand what factors determine the autophagic degradation of GJ connexins, we analyzed the changes undergone by the fraction of plasma membrane connexin 43 targeted for macroautophagy and the sequence of events that trigger this process. We found that Nedd4-mediated ubiquitinylation of the connexin molecule is required to recruit the adaptor protein Eps15 to the GJ and to initiate the autophagy-dependent internalization and degradation of connexin 43. This study reveals a novel regulatory role for macroautophagy in GJ function that is directly dependent on the ubiquitinylation of plasma membrane connexins.

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.

Connexin 43 gap junction plaque endocytosis implies molecular remodelling of ZO-1 and c-Src partners

Gap junctions, through their constitutive proteins, connexins (Cx), are involved in several processes including regulation of cellular proliferation, tissue differentiation, homeostasis and neoplasic transformation. Internalization of the gap junction plaque to form annular gap junction is a dynamic process, which present similarities with endocytosis, and participates in the control of gap junction coupling. Cx43 exhibits dynamic trafficking that needs sequential implication of a large number of protein partners. We have recently shown that ZO-1 localized in both sides of the gap junction plaque was restricted to one side during internalization. The dissociation between ZO-1 and Cx43 particularly occurred on the face where c-Src specifically associated with Cx43 and was abnormally accelerated in response to a carcinogen. In this addendum we summarize and further discuss these results.

The effect of environmental contaminants on testicular function

Male reproductive health has deteriorated considerably in the last few decades. Nutritional, socioeconomic, lifestyle and environmental factors (among others) have been attributed to compromising male reproductive health. In recent years, a large volume of evidence has accumulated that suggests that the trend of decreasing male fertility (in terms of sperm count, quality and other changes in male reproductive health) might be due to exposure to environmental toxicants. These environmental contaminants can mimic natural oestrogens and target testicular spermatogenesis, steroidogenesis, and the function of both Sertoli and Leydig cells. Most environmental toxicants have been shown to induce reactive oxygen species, thereby causing a state of oxidative stress in various compartments of the testes. However, the molecular mechanism(s) of action of the environmental toxicants on the testis have yet to be elucidated. This review discusses the effects of some of the more commonly used environmental contaminants on testicular function through the induction of oxidative stress and apoptosis.

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

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

The cyclophosphamide metabolite, acrolein, induces cytoskeletal changes and oxidative stress in Sertoli cells

The aim of this study is to explore the mechanism by which acrolein (ACR), a metabolite of cyclophosphamide (CP), induces immature Sertoli cell cytoskeletal changes. Sertoli cells obtained from rats were cultivated and treated with 50 and 100 μM ACR. XTT assays were performed to detect cell viability. Activities of superoxide dismutase (SOD), glutathione peroxidases (GSH-Px), and catalase (CAT), as well as total anti-oxidation competence (T-AOC) were examined. Superoxide anion levels were detected by a fluorescent probe. Cell ultrastructure changes were observed by transmission fluorescent microscope. Actin filament (F-actin) distribution was detected by immunofluorescence, and ERK and p38MAPK expression were detected by western blot analysis. ACR significantly decreased the viability of Sertoli cells in a dose- and time-dependent manner. T-AOC and the antioxidant activity of SOD, CAT and GSH-Px, were decreased in ACR-treated groups compared with the control group. The levels of reactive oxygen species (ROS) in ACR-treated Sertoli cells were increased. In addition, characteristics of cell apoptosis such as mitochondrial swelling, aggregated chromatin, condensed cytoplasm, nuclei splitting, and nuclei vacuolization were observed in ACR-treated cells. Furthermore, ACR-treatment also induced microfilament aggregation, marginalization and regionalization. The expression levels of ERK and p38MAPK were also increased in ACR-treated cells in a dose- and time-dependent manner. ACR, a major CP metabolite, impairs the cytoskeleton which is likely caused by induction of the oxidative stress response through up-regulation of ERK and p38MAPK expression.

Connexines testiculaires: marqueurs physiopathologiques et cibles potentielles aux toxiques environnementaux

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

Biochemical effects induced by the hexachlorocyclohexanes

The hexachlorocyclohexanes (HCHs) are synthetic compounds that have been widely used for the control of pests. The most common HCH isomers are the α-, β-, δ-, and γ-HCH. Although the have the same chlorine substitution pattern, the spatial orientation of chlorine atoms is different on each one of them, resulting in unique structures that have distinct molecular properties. Humans are exposed to individual HCH isomers through various routes, including ingestion of contaminated water or food, absorbed through the skin or by inhalation, and because of their liposolubility, these chemicals are mostly stored in fat-containing tissues. The isomer-specific spectrum of biochemical actions for these compounds has been wee characterized for different endpoints such as enzyme activation, calcium homeostasis, gap junctional intercellular communication, endocrine disruption, and cancer, among others. The interaction with the GABA reception has been one of the most extensively studied properties of the HCHs. For instance, γ-HCH acts as a GABAA channel blocker, whereas α- and δ-HCH potentiate currents , all working as allosteric modulators of the receptor. The changes in calcium homeostasis elicited by HCHs are both isomer and cell type specific. For example, in neurons, both the δ- and γ-isomers of HCH stimulate Ca²+ influx through different voltage-gated Ca²+ channels. In human neutrophils, α-,δ-, and γ-HCH, but not β-HCH, increase intracellular Ca²+ concentrations. This isomer-dependent behavior is also similar to that observed for phospholipase A2 activation and also correlates with oxidative stress generation. On the other hand, there are several lines of evidence suggesting that HCHs alter genomic integrity, and, therefore, these compounds have been classified as possibly carcinogenic to humans . Finally, HCHs have been reported to be endocrine disrupters. In fact, γ- and β-HCH have been shown to have weak estrogenic activity, and together with the α- and the δ-isomer, also interfere with steroidogenesis. In short, the HCH isomers are good examples of structurally related chemicals, for which the geometrical patterns present in each one of the different conformers create structures that possess specific mechanisms of action and toxicological properties.

Unexpected role for the human Cx37 C1019T polymorphism in tumour cell proliferation

Connexins are a large family of proteins that form gap junction channels allowing exchange of ions and small metabolites between neighboring cells. They have been implicated in pathological processes such as tumourigenesis in which they may act as tumour suppressors. A polymorphism in the human connexin37 (Cx37) gene (C1019T), resulting in a non-conservative amino acid change in the regulatory C-terminus (CT) of the Cx37 protein (P319S) has been suggested to be implicated in predisposition to angiosarcomas. In this study, we have used communication-deficient HeLa and SK-HEP-1 cells transfected with Cx37-319S, Cx37-319P or empty vector. We showed that the expression of Cx37-319P limited proliferation of HeLa and SK-HEP-1 cells, whereas Cx37-319S expression was without effect. Using an in vitro kinase assay, we demonstrated phosphorylation of Cx37 CT by glycogen synthase kinase-3 (GSK-3), a kinase known to be implicated in cell proliferation and cancer. GSK-3-induced phosphorylation was associated with reduced gap junctional intercellular communication (GJIC) as measured by microinjection of the tracer neurobiotin. Inhibition of GSK-3 by LiCl or SB415286 reduced phosphorylation of Cx37-319P and increased GJIC. This latter effect on GJIC involved the beta and not the alpha isoform of GSK-3. In contrast, GSK-3 inhibitors were without effect on HeLa cells expressing Cx37-319S. In conclusion, our data indicate functional effects of the Cx37 C1019T polymorphism on GJIC that might contribute to tumour cell growth.

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.

Cell surface sialic acid inhibits Cx43 gap junction functions in constructed Hela cancer cells involving in sialylated N-cadherin

Numerous studies have shown that changes in the glycan structures of cells correlate with tumorigenesis, however, a casual link between the altered glycan structures and the abnormal GJIC in cancer cells is rarely studied. In this paper, we investigated the effects of sialic acid on the Cx43 gap junction functions, and clarified its potential mechanisms thereby. Sialidase significantly increased Cx43 gap junction functions in constructed Cx43-Hela cells along with down-regulation of cell surface sialic acid, which is dramatically reversed by sialidase inhibitor NeuAc2en. Further study indicated that sialidase failed to affect Cx43 at either protein or phosphorylation level, instead, it induced a considerable fraction of Triton X-100 insoluble, as compared with the untreated cells. We also found that sialidase treatment reduced the N-cadherin glycosylation and enhanced both Cx43-ZO-1 interaction and N-cadherin-ZO-1 association. Moreover, sialidase promoted the cell-cell adhesion with elevating N-cadherin binding to β-catenin, accompanied by increasing colocalization of Cx43 with microtubules at the cell periphery. Based on live cell microscopy, with the FARP technology in the Cx43-EGFP-Hela cells, we found that Cx43 in the plague recovered more quickly in sialidase treatment group, indicating that sialidase could promote the Cx43 traffic to the plague. Overall, these studies indicate cell surface sialic acid on cancer cells may suppress Cx43 gap junction functions via inhibiting Cx43 traffic to the plague involving in sialylated N-cadherin, a process that likely underlies the intimate association between abnormal GJIC and glycosylation on cancer development.

Major involvement of connexin 43 in seminiferous epithelial junction dynamics and male fertility

In different epithelia, cell membranes contacting one another form intercellular junctional complexes including tight, adherens and gap junctions, which could mutually influence the expression of each other. We have here investigated the role of Cx43 in the control of adherens and tight junction proteins (N-cadherin, beta-catenin, occludin and ZO-1) by using conditional Sertoli cell knockout Cx43 (SCCx43KO(-/-)) transgenic mice and specific anti-Cx43 siRNA. Gap junction coupling and Cx43 levels were reduced in SCCx43KO(-/-) as compared to Wild-type testes. Ultrastructural analysis revealed disappearance of gap junctions, the presence of tight and adherens junctions and persistent integrity of the blood-testis barrier in SCCx43KO(-/-) testis. Occludin, N-cadherin and beta-catenin levels were enhanced in SCCx43KO(-/-) mice as compared to Wild-type animals whereas ZO-1 levels were reduced. Cx43 siRNA blocked gap junction functionality in Sertoli cells and altered tight and adherens protein levels. The Cx43 control of tight and adherens junctions appeared channel-dependent since gap junction blockers (glycyrrhetinic acid and oleamide) led to similar results. These data suggest that the control of spermatogenesis by Cx43 may be mediated through Sertoli cell Cx43 channels, which are required, not only in cell/cell communication between Sertoli and germ cells, but also in the regulation of other junctional proteins essential for the blood-testis barrier.

ZO-1 is involved in trophoblastic cell differentiation in human placenta

Trophoblastic cell-cell fusion is an essential event required during human placental development. Several membrane proteins have been described to be directly involved in this process, including connexin 43 (Cx43), syncytin 1 (Herv-W env), and syncytin 2 (Herv-FRD env glycoprotein). Recently, zona occludens (ZO) proteins (peripheral membrane proteins associated with tight junctions, adherens junctions, and gap junctions) were shown to be involved in mouse placental development. Moreover, zona occludens 1 (ZO-1) was localized mainly at the intercellular boundaries between human trophoblastic cells. Therefore the role of ZO-1 in the dynamic process of human trophoblastic cell-cell fusion was investigated using primary trophoblastic cells in culture. In vitro as in situ, ZO-1 was localized mainly at the intercellular boundaries between trophoblastic cells where its expression substantially decreased during differentiation and during fusion. At the same time, Cx43 was localized at the interface of trophoblastic cells and its expression increased during differentiation. To determine a functional role for ZO-1 during trophoblast differentiation, small interfering RNA (siRNA) was used to knock down ZO-1 expression. Cytotrophoblasts treated with ZO-1 siRNA fused poorly, but interestingly, decreased Cx43 expression without altering the functionality of trophoblastic cell-cell communication as measured by relative permeability time constant determined using gap-FRAP experiments. Because kinetics of Cx43 and ZO-1 proteins show a mirror image, a potential association of these two proteins was investigated. By using coimmunoprecipitation experiments, a physical interaction between ZO-1 and Cx43 was demonstrated. These results demonstrate that a decrease in ZO-1 expression reduces human trophoblast cell-cell fusion and differentiation.

Up-regulation of Cx43 expression and GJIC function in acute leukemia bone marrow stromal cells post-chemotherapy

Gap junction intercellular communication (GJIC) among bone marrow stromal cells (BMSCs) most frequently occurs through a channel composed of connexin43 (Cx43). Dysregulation of connexin expression is believed to have a role in carcinogenesis. In earlier work, we found that in acute leukemia BMSCs, expression of Cx43 and functioning GJIC declined. However, there has been no evaluation of whether GJIC in BMSCs in complete remission (CR) post-chemotherapy is different from GJIC pre-chemotherapy. We studied Cx43 expression and tested GJIC function in human bone marrow cultures under different physiological and pathological conditions. To assay Cx43 expression we used immunocytochemistry, laser scan confocal microscopy (LSCM), flow cytometry and RT-PCR. The results showed that the expression level of Cx43 and its mRNA in acute leukemia BMSCs post-chemotherapy was significantly higher and similar to normal levels than in primary acute leukemia BMSCs (p<0.01). Functional tests in cultures using dye transfer and fluorescence recovery after photobleaching (FRAP) assays showed that the function of GJIC in acute leukemia BMSCs was significantly improved following effective chemotherapy. Our findings suggest Cx43 and GJIC might be involved in the courses of occurrence, development and termination of acute leukemia, and effective chemotherapy could improve Cx43 expression and GJIC function that were dysfunctional prior to treatment.

Disruption of gap junctional intercellular communication by antibiotic gentamicin is associated with aberrant localization of occludin, N-cadherin, connexin 43, and vimentin in SerW3 Sertoli cells in vitro

Spermatogenesis is a very complex process by which male germ cells differentiate into mature spermatozoa. The sophisticated communication network that controls spermatogenesis can be derailed so that dysfunction of one cell type propagates to all types as a cascade. This accounts for the particular vulnerability of the testis to environmental factors such as drugs and xenobiotics. Sertoli cells play an important role in protecting developing germ cells by forming a physiological barrier, limiting exposure to potentially toxic substrates, or conversely, facilitating uptake of xenobiotics within the testis. In this study, cells from the rat Sertoli line (SerW3) were incubated for 3, 6 and 9 subsequent days in serum free DMEM (SFDM) composed of DMEM supplemented with three different concentrations of antibiotic gentamicin (10, 30, and 100 μg). The effect of the three different concentrations of this antibiotic was determined on Sertoli cell-cell interaction through impaired expression of their constitutive tight junction proteins as early targets for different toxicants in vitro by immunochemistry analysis. The Sertoli SerW3 cell line illustrated the cytotoxicity of GS, as the intercellular junction proteins such as occludin, N-cadherin, connexin 43, and vimentin were delocalized from the membrane to the cytoplasmic compartment during exposure to the antibiotic. This study underlines the potential deleterious effects of the routine use of antibiotics during continuous cell culture.

Connexin 33 impairs gap junction functionality by accelerating connexin 43 gap junction plaque endocytosis

Connexin 33 (Cx33) is a testis-specific gap junction protein. We previously reported that Cx33 exerts dominant-negative effect on gap junction intercellular communication by sequestering Cx43 within early endosomes in Sertoli cells. However, the molecular mechanisms that drive this process are unknown. The present study analyzed: (i) the trafficking of Cx33 and Cx43 in wild-type Sertoli cells transfected with Cx33-DsRed2 and Cx43-green fluorescent protein vectors; (ii) the formation of heteromeric Cx33/Cx43 hemi-channels and their incorporation into gap junction plaques. Fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer and videomicroscopy studies demonstrated that Cx33 and Cx43 associated to form heteromeric oligomers that trafficked along microtubules to the plasma membrane. However, the plaques containing Cx33 were not functional. Immunoprecipitation experiments revealed that zonula occludens-1 (ZO-1), a scaffold protein proposed to secure Cx in gap junction plaques at the cell-cell boundary, associated with Cx33 in testis extracts. In cells expressing Cx33, Cx33 and ZO-1 specifically interacted with P(1) phosphorylated and P(0) unphosphorylated isoforms of Cx43, and the ZO-1 membranous signal level was reduced. It is suggested that alteration of Cx43/ZO-1 association by Cx33 could be one mechanism by which Cx33 exerts its dominant-negative effect on gap junction plaque.

ERp29 restricts Connexin43 oligomerization in the endoplasmic reticulum

Connexin43 (Cx43) is a gap junction protein that forms multimeric channels that enable intercellular communication through the direct transfer of signals and metabolites. Although most multimeric protein complexes form in the endoplasmic reticulum (ER), Cx43 seems to exit from the ER as monomers and subsequently oligomerizes in the Golgi complex. This suggests that one or more protein chaperones inhibit premature Cx43 oligomerization in the ER. Here, we provide evidence that an ER-localized, 29-kDa thioredoxin-family protein (ERp29) regulates Cx43 trafficking and function. Interfering with ERp29 function destabilized monomeric Cx43 oligomerization in the ER, caused increased Cx43 accumulation in the Golgi apparatus, reduced transport of Cx43 to the plasma membrane, and inhibited gap junctional communication. ERp29 also formed a specific complex with monomeric Cx43. Together, this supports a new role for ERp29 as a chaperone that helps stabilize monomeric Cx43 to enable oligomerization to occur in the Golgi apparatus.