omega-Conotoxin-sensitive Ca2+ voltage-gated channels modulate protein secretion in cultured rat Sertoli cells

The endoplasmic reticulum, calcium signaling and junction turnover in Sertoli cells

The endoplasmic reticulum (ER) forms a continuous network throughout morphologically differentiated Sertoli cells. It is an integral component of intercellular adhesion junctions in this cell type, as well as forming membrane contact sites with the plasma membrane and intracellular organelles. One of the major functions of the ER in cells generally is maintaining calcium homeostasis and generating calcium signals. In this review, we discuss what is currently known about the overall pattern of distribution of the ER in Sertoli cells and the location of calcium regulatory machinery in the various subdomains of the organelle. Current data are consistent with the hypothesis that calcium signaling by the ER of Sertoli cells may play a significant role in events related to junction remodeling that occur in the seminiferous epithelium during spermatogenesis.

Obesogens and male fertility

In the last decades, several studies evidenced a decrease in male fertility in developed countries. Although the aetiology of this trend in male reproductive health remains a matter of debate, environmental compounds that predispose to weight gain, namely obesogens, are appointed as contributors because of their action as endocrine disruptors. Obesogens favour adipogenesis by an imbalance of metabolic processes and can be found virtually everywhere. These compounds easily accumulate in tissues with high lipid content. Obesogens change the functioning of male reproductive axis, and, consequently, the testicular physiology and metabolism that are pivotal for spermatogenesis. The disruption of these tightly regulated metabolic pathways leads to adverse reproductive outcomes. Notably, adverse effects of obesogens may also promote disturbances in the metabolic performance of the following generations, through epigenetic modifications passed by male gametes. Thus, unveiling the molecular pathways by which obesogens induce toxicity that may end up in epigenetic modifications is imperative. Otherwise, a transgenerational susceptibility to metabolic diseases may be favoured. We present an up-to-date overview of the impact of obesogens on testicular physiology, with a particular focus on testicular metabolism. We also address the effects of obesogens on male reproductive parameters and the subsequent consequences for male fertility.

Rapid responses to reverse T₃ hormone in immature rat Sertoli cells: calcium uptake and exocytosis mediated by integrin

There is increasing experimental evidence of the nongenomic action of thyroid hormones mediated by receptors located in the plasma membrane or inside cells. The aim of this work was to characterize the reverse T₃ (rT₃) action on calcium uptake and its involvement in immature rat Sertoli cell secretion. The results presented herein show that very low concentrations of rT₃ are able to increase calcium uptake after 1 min of exposure. The implication of T-type voltage-dependent calcium channels and chloride channels in the effect of rT₃ was evidenced using flunarizine and 9-anthracene, respectively. Also, the rT₃-induced calcium uptake was blocked in the presence of the RGD peptide (an inhibitor of integrin-ligand interactions). Therefore, our findings suggest that calcium uptake stimulated by rT₃ may be mediated by integrin αvβ₃. In addition, it was demonstrated that calcium uptake stimulated by rT₃ is PKC and ERK-dependent. Furthermore, the outcomes indicate that rT₃ also stimulates cellular secretion since the cells manifested a loss of fluorescence after 4 min incubation, indicating an exocytic quinacrine release that seems to be mediated by the integrin receptor. These findings indicate that rT₃ modulates the calcium entry and cellular secretion, which might play a role in the regulation of a plethora of intracellular processes involved in male reproductive physiology.

Effects of L- and T-type Ca²(+) channel blockers on spermatogenesis and steroidogenesis in the prepubertal mouse testis

PURPOSE

To assess the involvement of L-type and T-type Ca²(+) channel blockers in inducing male infertility.

METHODS

Prepubertal male mice were fed Ca²(+) channel blockers nifedipine and ethosuximide for 20 days at dosages below maximum tolerated dose (MTD) and assayed for gross morphological changes in the testis such as body weight, testis size and weight. Sperm and Leydig cell counting were conducted concomitantly with serum testosterone level measurement by radioimmunoassay (RIA) and StAR protein mRNA measurement by reverse transcription and polymerase chain reaction (RT-PCR).

RESULTS

A chronic exposure to nifedipine or ethosuximide caused a significant reduction in body weight, testis size/weight and sperm production in a dose-dependent fashion associated with a spermatogenic arrest largely at the elongating spermatid stage. The number of Leydig cells, the serum testosterone level but not the luteinizing hormone level, and the content of StAR protein mRNA were also drastically reduced relative to the controls.

CONCLUSIONS

Both T- and L-type Ca²(+) channel blockers play an adverse role in normal spermatogenesis and steroidogenesis partly by blocking postmeiotic germ cell maturation and/or by abrogating StAR protein expression, contributing to male sterility. Therefore, any therapeutic application of Ca²(+) channel blockers must be used with caution due to its potential adverse side effects on male infertility.

Chapter 4: Regulation of Clusterin activity by calcium

In this chapter, the attention is put on Ca(2+) effect on Clusterin (CLU) activity. We showed that two CLU forms (secreted and nuclear) are differently regulated by Ca(2+) and that Ca(2+) fluxes affect CLU gene expression. A secretory form (sCLU) protects cell viability whereas nuclear form (nCLU) is proapoptotic. Based on available data we suggest, that different CLU forms play opposite roles, depending on intracellular Ca(2+) concentration, time-course of Ca(2+) current, intracellular Ca(2+) compartmentalization, and final Ca(2+) targets. Discussion will be motivated on how CLU acts on cell in response to Ca(2+) waves. The impact of Ca(2+) on CLU gene activity and transcription, posttranscriptional modifications, translation of CLU mRNA, and posttranslational changes as well as biological effects of CLU will be discussed. We will also examine how Ca(2+) signal and Ca(2+)-dependent proteins are attributable to changes in CLU characteristics. Some elucidation of CLU gene activity, CLU protein formation, maturation, secretion, and intracellular translocations in response to Ca(2+) is presented. In response to cell stress (i.e., DNA damage) CLU gene is activated. We assume that commonly upregulated mRNA for nCLU versus sCLU and vice versa are dependent on Ca(2+) accessibility and its intracellular distribution. It looks as if at low intracellular Ca(2+) the delay in cell cycle allows more time for DNA repair; otherwise, cells undergo nCLU-dependent apoptosis. If cells are about to survive, intrinsic apoptosis is abrogated by sCLU interacting with activated Bax. In conclusion, a narrow range of intracellular Ca(2+) concentrations is responsible for the decision whether nCLU is mobilized (apoptosis) or sCLU is appointed to improve survival. Since the discovery of CLU, a huge research progress has been done. Nonetheless we feel that much work is left ahead before remaining uncertainties related to Ca(2+) signal and the respective roles of CLU proteins are unraveled.

Follicle-stimulating hormone-induced Galphah/phospholipase C-delta1 signaling mediating a noncapacitative Ca2+ influx through T-type Ca2+ channels in rat sertoli cells

Our previous study demonstrated that FSH-induced immediate Ca(2+) influx in rat Sertoli cells (SCs) is mediated by the Galphah/phospholipase C-delta1 (PLC-delta1) signaling pathway. As to which Ca(2+) channel is responsible for such Ca(2+) influx was not understood. In this study, thapsigargin triggered an in-store calcium release and evoked a 1.5-fold elevation of intracellular Ca(2+) in Ca(2+)-free media, whereas FSH exhibited no effect. The readdition of CaCl(2) (2.5 mm) to FSH-pretreated or thapsigargin-sensitized SCs in Ca(2+)-free media immediately elicited a rapid Ca(2+) influx or a 2-fold increase of second intracellular Ca(2+) elevation, respectively. The addition of Ca(2+) chelator EGTA (0.2 mm) reduced the FSH-induced elevation of intracellular Ca(2+) in SCs incubated with CaCl(2). However, pretreatment with dantrolene (25 microM), which inhibits in-store calcium release, did not affect the FSH-induced elevation of intracellular Ca(2+). NiCl(2) (10 microM), a T-type calcium channel blocker, abolished the FSH-induced SC Ca(2+) influx. Furthermore, mibefradil (10 and 100 microm), another specific blocker for T-type Ca(2+) channels, dose-dependently suppressed the FSH-induced Ca(2+) influx. In contrast, nifedipine (10 and 50 microm) or omega-conotoxin GVIA (100 and 500 nm), blocker of L- or N-type Ca(2+) channels, respectively, did not affect the FSH-induced SC Ca(2+) influx. On the other hand, FSH-induced Ca(2+) influx was significantly reduced by pretreatment of SCs with myristoylated synthetic peptide (0.1 and 1 microm) of PLC-delta1 fragment TIPWNSLKQGYRHVHLL but not affected by 2',5'-dideoxyadenosine (3 and 15 microm), a selective inhibitor of adenylate cyclase. In conclusion, the FSH-induced Galphah/PLC-delta1 pathway-dependent Ca(2+) influx of rat SCs is mediated by T-type Ca(2+) channels and independent of in-store calcium release.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Transient receptor potential vanilloid 1 (TRPV1) channels in cultured rat Sertoli cells regulate an acid sensing chloride channel

Sertoli cells provide a controlled microenvironment for regulation and maintenance of spermatogenesis for which an acidic milieu is crucial for male fertility. Sertoli cells also contribute to protection of spermatogenetic cells. Here, we showed that TRPV1 is expressed in rat Sertoli cells and regulates an acid sensing Cl(-) channel (ASCC). The expression of TRPV1 in rat Sertoli cells was demonstrated by RT-PCR, immunostaining and calcium measurement experiments. ASCC activity was inhibited by capsaicin (IC(50)=214.3+/-1.6 nM), olvanil (IC(50)=400+/-1.7 pM) and resiniferatoxin (IC(50)=9.3+/-1.5 nM) but potentiated by capsazepine (EC(50)=5.3+/-1.3 microM) and ruthenium red (EC(50)=2.3+/-1.5 microM). In the human airway epithelial cell line Calu-3 in which ASCC can be detected but not TRPV1, capsaicin and capsazepine were without any effect. Finally the application of the non-steroidal anti-inflammatory drug ibuprofen prevented the control of ASCC by TRPV1. Our study provides the first evidence for a regulation by TRPV1 of an acid sensing chloride channel in rat Sertoli cells. TRPV1 and ASCC may thus be considered as new potential physiological regulators of spermatogenesis and targets for pharmacological treatments of reproductive disorders as cryptorchidism, Sertoli cell tumors or torsion of the spermatic cord.

Effects of calcium channel blockers on the spermatogenesis and gene expression in peripubertal mouse testis

Treatment of Ca(2+) channel blockers (CCB) to relieve hypertension causes reversible male infertility, suggesting deregulation of Ca(2+) homeostasis in testis is closely related with male infertility. To investigate the possible toxicity of therapeutic application of CCB in childhood, the effect of nifedipine and ethosuximide, an L-type and T-type CCB, respectively, on the spermatogenesis and testicular gene expression was examined. Following the intraperitoneal injection of either drug for 7 days to 18 days on old mice, the paired testes weights were significantly lower in mice treated with nifedipine (> or = 10 mg/kg/day) or ethosuximide (100 mg/kg/day) than vehicle controls. In mice given high drug dosing (100 mg/kg), seminiferous tubules showed immaturity with spermatogenic arrest at elongating spermatid stage and poorly developed lumen. Unexpectedly, the expression of activator isoform of transcription factor cAMP-responsive element modulator (CREM) mRNA increased together with transition protein 2 and protamine 2 mRNA in drug-treated mice testes, suggesting that CCB may deregulate expression of activator isoform of CREM in male germ cells and that spermatogenic defect following CCB treatment may attribute to ectopic expression of CREM-dependent gene battery in testis. Therapeutic application of CCB in childhood should be cautious because of their potential to cause spermatogenic defect and altered gene expression in testis.

Pharmacological profile of inhibition of the chloride channels activated by extracellular acid in cultured rat Sertoli cells

Sertoli cells from mammalian testis are key cells involved in the development and maintenance of stem cell spermatogonia as well as in the secretion of a Cl(-) and K(+)-rich fluid into the lumen of seminiferous tubules. The pharmacology and contribution of Cl(-) channels to the physiology of Sertoli cells were investigated using whole-cell patch clamp and iodide efflux experiments applied to cultured rat Sertoli cells. We characterized an outwardly rectifying Cl(-) current stimulated by various acid species including the physiologically relevant lactic acid. Using the iodide efflux technique, the pharmacological properties of this Cl(-) current, noted ICl(acid), revealed Ca(2+)-independent inhibition by DIDS (IC(50) = 27 microM), glibenclamide (IC(50) = 31 microM) and DPC (IC(50) = 86 microM). ICl(acid) was neither affected by calix[4]arene nor by 9-AC. The order of potency for inhibition of ICl(acid) is DIDS approximately glibenclamide > DPC >> calix[4]arene, 9-AC. For comparison, the inhibitory profile of the swelling- and ATP-activated Cl(-) currents in Sertoli cells is DPC = DIDS >> glibenclamide = 9-AC for ICl(swell) and DPC = 9-AC = DIDS >> glibenclamide for ICl(ATP). This description provides new insights into the physiology and pharmacology of the endogenous Cl(-) channels expressed and potentially involved in fluid secretion in Sertoli cells.

Effects of PFOS and PFOA on L-type Ca2+ currents in guinea-pig ventricular myocytes

Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) are amphiphiles found ubiquitously in the environment, including wildlife and humans, and are known to have toxic effects on physiological functions of various tissues. We investigated the effects of PFOS and PFOA on action potentials and L-type Ca(2+) currents, I(CaL), in isolated guinea-pig ventricular myocytes using whole-cell patch-clamp recording. In current-clamp experiments, PFOS significantly decreased the rate of spike, action potential duration, and peak potential at doses over 10 microM. In voltage-clamp experiments, PFOS increased the voltage-activated peak amplitude of I(CaL), and shifted the half-activation and inactivation voltages of I(CaL) to hyperpolarization. PFOA had similar effects PFOS, but showed significantly lower potency. These findings are consistent with previous observations for anionic n-alkyl surfactants, suggesting that PFOS and PFOA may change membrane surface potential, thereby eliciting general effects on calcium channels. These findings provide further insights into the mechanisms of PFOA and PFOS toxicities.

Modulation of MAA-induced apoptosis in male germ cells: role of Sertoli cell P/Q-type calcium channels

Spontaneous germ cell death by apoptosis occurs during normal spermatogenesis in mammals and is thought to play a role in the physiological mechanism limiting the clonal expansion of such cell population in the male gonad. In the prepubertal rat testis, the most conspicuous dying cells are pachytene spermatocytes, which are also the primary target of the apoptosis experimentally induced by the methoxyacetic acid (MAA). Since we have recently reported that Sertoli cells, the somatic component of the seminiferous epithelium, regulate not only germ cell viability and differentiation but also their death, we have further investigated the mechanism involved in such a control. In this paper we have used the protein clusterin, produced by Sertoli cells and associated with tissue damage or injury, as indicator of germ cell apoptosis in rat seminiferous tubules treated with MAA in the presence or in the absence of omega-agatoxin, a specific inhibitor of P/Q type voltage-operated calcium channels (VOCC's). We performed both a qualitative analysis of clusterin content and germ cell apoptosis by immunofluorescence experiments and a quantitative analysis by in situ end labelling of apoptotic germ cells followed by flow cytometry. The results obtained demonstrate that Sertoli cells modulate germ cell apoptosis induced by methoxyacetic acid also throughout the P/Q-type VOCC's.

Sertoli cell modulates MAA‐induced apoptosis of germ cells throughout voltage‐operated calcium channels

Spontaneous cell death by apoptosis--occurring during normal spermatogenesis in mammals--is a prominent event, which results in the loss of up to 75% of the potential number of mature spermatozoa. In the rat testis, the most conspicuous dying cells are pachytene spermatocytes, which are also the primary target of the apoptosis experimentally induced by methoxyacetic acid (MAA). In this paper, we have used clusterin expression as an indicator of germ cell apoptosis in rat seminiferous tubules treated with MAA in the presence or in the absence of voltage-operated calcium channels (VOCCs) inhibitors. We performed both a qualitative analysis of clusterin expression by immunofluorescence experiments and a quantitative analysis of apoptosis by in situ end labeling of apoptotic germ cells followed by flow cytometry. The results obtained demonstrate that Sertoli cells, the somatic component of the seminiferous epithelium, which control male germ cell differentiation, also modulate MAA-induced apoptosis of germ cells throughout voltage-operated calcium channels.

A Novel voltage-dependent chloride current activated by extracellular acidic pH in cultured rat Sertoli cells

Sertoli cells from mammalian testis are key cells involved in development and maintenance of stem cell spermatogonia as well as secretion of a chloride- and potassium-rich fluid into the lumen of seminiferous tubules. Using whole-cell patch clamp experiments, a novel chloride current was identified. It is activated only in the presence of an extracellular acidic pH, with an estimated half-maximal activation at pH 5.5. The current is strongly outwardly rectifying, activated with a fast time-dependent onset of activation but a slow time-dependent kinetic at depolarization pulses. The pH-activated chloride current was not detected at physiological or basic pH and is not sensitive to intracellular or extracellular Ca2+ variation. Diphenylamine-2-carboxylic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid blocked the induced currents, and its anionic selectivity sequence was Cl- > Br- > I-> gluconate. We have performed a reverse transcription-PCR analysis to search for voltage-dependent chloride rClC channels in cultured rat Sertoli cells. Among the nine members of the family only rClC-2, rClC-3, rClC-6, and rClC-7 have been identified. The inwardly rectifying rClC-2 chloride current was activated by hyperpolarization but not by pH variation. A different depolarization-activated outwardly rectifying chloride current was activated only by hypotonic challenge and may correspond either to rClC-3 or rClC-6. Immunolocalization experiments demonstrate that rClC-7 resides in the intracellular compartment of Sertoli cells. This study provides the first functional identification of a native acid-activated chloride current. Based on our molecular analysis of rClC proteins, this new chloride current does not correspond to rClC-2, rClC-3, rClC-6, or rClC-7 channels. The potential physiological role of this native current in an epithelial cell from the reproductive system is discussed.

Multiple purinergic receptors lead to intracellular calcium increases in cultured rat Sertoli cells

Our previous study has demonstrated the activation of calcium-dependent Cl(-) secretion through P2Y(2) receptors by extracellular nucleotides. To evaluate the contribution and involvement of other receptor subtypes to this physiological response, the purpose of this study was to investigate the regulation of intracellular calcium concentration ([Ca(2+)](i)) by different purinergic agonists in cultured rat Sertoli cells. Cultured epithelia of Sertoli cells from immature rats were grown on glass coverslips and the purity was confirmed by immunohistochemistry. Intracellular calcium concentration was monitored by microspectrofluorimetric technique using calcium sensitive fluorescent dye Fura-2. Results showed that UTP and ATP consistently increased [Ca(2+)](i). Adenosine, AMP and 2-methylthio-ADP (2-MeSADP) failed to elicit a discernible response. Cross desensitization experiments suggested that the responses to UTP and ATP were mediated by a common receptor population. This is consistent with the presence of receptors belonging to the P2Y(2) subclass. Sertoli cells also responded to 2-methylthio-ATP (2-MeSATP) and ADP but not 2-methylthio-ADP (2-MeSADP) in a concentration-dependent manner. Moreover, the ATP-or UTP-pretreated epithelia continued to respond to ADP, indicating that these cells express at least one additional receptor population that allowed nucleotides to increase [Ca(2+)](i). Apart from the P2Y receptor agonists, the P2X(4) and P2X(7) agonist, 2' and 3'-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), also evoked [Ca(2+)](i) increases in rat Sertoli cells. Messenger RNA transcript corresponding to P2Y(1), P2Y(2), P2X(4) and P2X(7) receptors genes were detected using RT-PCR. Taken together, these data indicate that both metabotropic receptors and ionotropic receptors are present in rat Sertoli cells through which extracellular nucleotides can act.

Regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression by follicle-stimulating hormone, cAMP increase and calcium influx during rat Sertoli cell development

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell- to-cell signaling and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate proteoglycans (HSPG). Glypican-1, syndecans-1 and -4 were identified using a molecular approach. Their differential regulation was demonstrated in immature rat Sertoli cells. Follicle-stimulating hormone (FSH) is the main regulator of Sertoli cell function. Signal transduction triggered by FSH involves both an increased intracellular cAMP synthesis and a calcium influx. This study demonstrates that FSH, through its second messengers (increase in intracellular cAMP and intracellular calcium), downregulated the glypican-1 mRNA expression in Sertoli cells from 20-day-old rats. On the other hand, syndecan-1 mRNA expression is not modulated by FSH as it would result from the antagonistic effects of increased intracellular cAMP and intracellular calcium levels. Finally, syndecan-4 mRNA expression is not regulated by this pathway. The present study was extended during Sertoli cell development. Indeed, Sertoli cells undergo extensive changes during the postnatal period both in structure and function. These important transformations are critical for the establishment of spermatogenesis and development of the adult pattern of testicular function. Our data indicated that the regulation of HSPG mRNA expression is HSPG-specific and depends on the Sertoli cell developmental stage.

Identification and cellular localisation of voltage-operated calcium channels in immature rat testis

Sertoli cells regulate the spermatogenic process mainly through the secretion of a complex fluid into the lumen of the seminiferous tubules behind the blood-testis barrier, containing many of the essential proteins necessary for maintenance and maturation of male germ cells. Thus, the study of Sertoli cell secretory processes is strictly correlated with the understanding of the regulatory mechanisms of spermatogenesis. In this work the authors have explored the voltage-sensitive calcium channel variety in the immature rat testis, their localisation and distribution within the seminiferous epithelium and peritubular and interstitial tissues as well as the possible role in the control of Sertoli cell secretion. The results reported in this paper, obtained by in situ hybridisation, immunohistology of rat testicular sections and Western blot analysis of Sertoli cell plasma membranes, show that mammalian Sertoli cells express mRNA encoding for several voltage-operated calcium channel subunits and express such proteins on their surface. Experiments performed on Sertoli cell monolayers cultured in the presence of specific toxins indicate that both N and P/Q-type Ca(2+) channels are involved in the regulation of protein secretion.