Identification of an inhibitory factor from a Sertoli clonal cell line (TM4) that modulates adult rat Leydig cell steroidogenesis

Wt1 dictates the fate of fetal and adult Leydig cells during development in the mouse testis

Wilms' tumor 1 (Wt1) is a tumor suppressor gene encoding ∼24 zinc finger transcription factors. In the mammalian testis, Wt1 is expressed mostly by Sertoli cells (SCs) involved in testis development, spermatogenesis, and adult Leydig cell (ALC) steroidogenesis. Global knockout (KO) of Wt1 is lethal in mice due to defects in embryogenesis. Herein, we showed that Wt1 is involved in regulating fetal Leydig cell (FLC) degeneration and ALC differentiation during testicular development. Using Wt1(-/flox);Amh-Cre mice that specifically deleted Wt1 in the SC vs. age-matched wild-type (WT) controls, FLC-like-clusters were found in Wt1-deficient testes that remained mitotically active from postnatal day 1 (P1) to P56, and no ALC was detected at these ages. Leydig cells in mutant adult testes displayed morphological features of FLC. Also, FLC-like cells in adult mutant testes had reduced expression in ALC-associated genes Ptgds, Sult1e1, Vcam1, Hsd11b1, Hsd3b6, and Hsd17b3 but high expression of FLC-associated genes Thbs2 and Hsd3b1. Whereas serum LH and testosterone level in mutant mice were not different from controls, intratesticular testosterone level was significantly reduced. Deletion of Wt1 gene also perturbed the expression of steroidogenic enzymes Star, P450c17, Hsd3b6, Hsd3b1, Hsd17b1, and Hsd17b3. FLCs in adult mutant testes failed to convert androstenedione to testosterone due to a lack of Hsd17b3, and this defect was rescued by coculturing with fetal SCs. In summary, FLC-like cells in mutant testes are putative FLCs that remain mitotically active in adult mice, illustrating that Wt1 dictates the fate of FLC and ALC during postnatal testis development.

Toll-like receptor 3-initiated antiviral responses in mouse male germ cells in vitro

The testis is an immunoprivileged site where local cell-initiated innate immunity plays a crucial role in antimicrobial responses. Toll-like receptors (TLRs) mediate innate immune responses in testicular somatic cells. Although several TLRs are expressed in some stages of male germ cells, the potential role of TLRs in triggering antimicrobial responses in the germ cells has yet to be exclusively studied. The current study demonstrates that TLR3 is constitutively expressed in spermatogonia and spermatocytes and can be activated by a synthetic double-strained RNA analog, polyinosinic-polycytidylic acid. TLR3 activation in these male germ cells up-regulates the expression of proinflammatory cytokines, such as interleukin IL1B, IL6, and tumor necrosis factor alpha, through activation of nuclear factor kappa B; it also induces production of type 1 interferons (IFNA and IFNB) through the activation of IFN regulatory factor 3. In addition, TLR3 activation increases the production of two major antiviral proteins, namely, double-stranded RNA-activated protein kinase and MX1 protein, by germ cells. Data in this article describe an antiviral response of male germ cells through the activation of TLR3 in vitro.

Role of tissue inhibitor of metalloproteases-1 in junction dynamics in the testis

Using multiple high-performance liquid chromatography steps, we have identified and purified a polypeptide to apparent homogeneity from primary Sertoli cell conditioned culture medium that consisted of 2 molecular variants of 31 and 29 kDa when electrophoresed on a sodium dodecyl sulfate-polyacrylamide gel run under reducing conditions. Partial N-terminal amino acid sequence analysis of these 2 proteins revealed a sequence of NH(2)-IKMAKMLKGFDAVGNATG, which is homologous to tissue inhibitor of metalloproteases-1 (TIMP-1). Studies by semiquantitative reverse transcription-polymerase chain reaction using a primer pair specific to rat TIMP-1 demonstrated that both Sertoli and germ cells express TIMP-1. During maturation, the steady-state TIMP-1 mRNA level in the testis increased significantly from 40 to 60 days of age, which suggests its role in the restructuring of the epithelium during spermiation. This increase in testicular TIMP-1 expression was apparently not due to the increase in germ cell number, because TIMP-1 expression decreased approximately fivefold in germ cells isolated from testes of aging rats. Using Sertoli cells cultured at low (0.05 x 10(6) cells/cm(2)) and high (0.5 x 10(6) cells/cm(2)) densities, it was found that TIMP-1 expression increased transiently but significantly during junction assembly. A similar induction of TIMP-1 mRNA was also detected in Sertoli-germ cell cocultures during germ cell adhesion onto Sertoli cells. More important, the inclusion of either alpha(2)-macroglobulin (a protease inhibitor produced by Sertoli cells) or aprotinin (a serine protease inhibitor) into an in vitro germ cell adhesion assay facilitated the attachment of fluorescently labeled germ cells onto the Sertoli cell epithelium when compared to control, which suggests that the assembly of adherens junctions may involve protease inhibitors.

Is the cadherin/catenin complex a functional unit of cell-cell actin-based adherens junctions in the rat testis?

Much controversy exists regarding the presence of the cadherin/catenin complex and its intracellular attachment site in the testis, which is the functional unit for actin-based cell-cell adherens junctions (AJs) in multiple epithelia. Furthermore, whether germ and Sertoli cells are equipped with the necessary AJ-associated signaling molecules to regulate this cadherin/catenin complex during spermatogenesis is not known. In the present study, it was shown that both Sertoli and germ cells indeed express N-cadherin, E-cadherin, alpha-catenin, beta-catenin, and p120(ctn) by semiquantitative reverse transcription-polymerase chain reaction and immunoblotting. Furthermore, the assembly of AJs between Sertoli and germ cells was associated with a transient induction in the steady-state mRNA and protein levels of cadherins and catenins. These analyses reveal, to our knowledge for the first time, that the testis may indeed be using the cadherin/catenin complex as one of the functional units to regulate AJ dynamics between Sertoli and germ cells in addition to alpha(6)beta(1) integrin and the nectin/afadin complex. To further confirm the existence of such a complex between Sertoli and germ cells, immunoprecipitation experiments were performed using Sertoli-germ cell lysates during AJ assembly. An anti-N-cadherin antibody can pull out beta-catenin, whereas N-cadherin can also be pulled out using an anti-beta-catenin antibody. To further expand and validate these in vitro biochemical studies, immunofluorescent histochemistry was performed, which colocalized N-cadherin and beta-catenin to the same site of Sertoli-Sertoli and Sertoli-germ cell AJs, possibly ectoplasmic specializations near the basal compartment, at the lower third of the seminiferous epithelium in vivo as well as between Sertoli cells cultured in vitro. Furthermore, studies by cross-linking using dithiobis(succinimidylpropionate) confirmed that the cadherin/catenin complex between Sertoli cells as well as between Sertoli and germ cells indeed structurally linked to actin but not to vimentin (an intermediate filament protein) or to tubulin (a microtubule protein). These results thus unequivocally demonstrate that the cadherin/catenin complex, which can be up-regulated by testosterone, is indeed present between Sertoli and germ cells and is used for the assembly of functional AJs.

The interplay of collagen IV, tumor necrosis factor-alpha, gelatinase B (matrix metalloprotease-9), and tissue inhibitor of metalloproteases-1 in the basal lamina regulates Sertoli cell-tight junction dynamics in the rat testis

During spermatogenesis, preleptotene and leptotene spermatocytes must translocate across the blood-testis barrier formed by inter-Sertoli cell-tight junctions (TJs) from the basal compartment of the seminiferous epithelium adjacent to the basement membrane to the adluminal compartment at stages VIII-IX for further development. Because of the close proximity between extracellular matrix (ECM) that constitutes the basement membrane and the blood-testis barrier, we sought to investigate the role of ECM in Sertoli cell TJ dynamics. When Sertoli cells were cultured in vitro to initiate the assembly of the Sertoli cell TJ-permeability barrier, the presence of an anticollagen IV antibody indeed perturbed the barrier. Because ECM is known to maintain a pool of cytokines and TNFalpha has been shown to regulate TJ dynamics in other epithelia, we investigated whether TNFalpha can regulate Sertoli cell TJ function via its effects on collagen alpha3(IV) and other proteins that maintain the homeostasis of ECM. As expected, recombinant TNFalpha perturbed the Sertoli cell TJ-barrier assembly in vitro dose dependently. TNFalpha also inhibited the timely induction of occludin, which is known to associate with the Sertoli cell TJ-barrier assembly. Furthermore, TNFalpha induced the expression of Sertoli cell collagen alpha3(IV), gelatinase B (matrix metalloprotease-9, MMP-9) and tissue inhibitor of metalloproteases-1 but not gelatinase A (matrix metalloprotease-2), and promoted the activation of pro-MMP-9. These results thus suggest that the activated MMP-9 induced by TNFalpha is used to cleave the existing collagen network in the ECM, thereby perturbing the TJ-barrier. This in turn creates a negative feedback that causes TNFalpha to induce collagen alpha3(IV) and tissue inhibitor of metalloproteases-1 expression so as to replenish the collagen network in the disrupted TJ-barrier and limit the activity of MMP-9. Taken collectively, these observations strengthen the notion that ECM is involved in the regulation of junction dynamics in addition to its structural role in the testis.

Molecular genetics of male infertility: stem cell factor/c-kit system

PROBLEM

Infertility, affects about 5% of human males and genetic factors are recognized in approximately 30% of them. The mouse represents a good model to study autosomal genes that might play a role in spermatogenesis. In mice, mutations in the c-kit gene and in the gene encoding stem cell factor (SCF) cause pleiotropic defects among which sterility. A possible involvement of the SCF/c-kit system in human spermatogenesis was investigated.

METHODS OF STUDY

A group of 65 idiopathic azoospermic patients was screened for the presence of mutations in the human c-kit gene codon encoding tyrosine 721 (Y721), analogous to Y719 in the murine c-kit gene (a residue known to be essential for a normal spermatogonial proliferation). Furthermore we have used a mouse model for studying the molecular mechanisms that regulate the transcription of the endogenous SCF gene.

RESULTS

No mutations have been detected on codon encoding Y721 of the human c-kit gene, in our group of infertile patients.

CONCLUSIONS

A larger group of azoospermic patients, including preferentially patients affected by Sertoli-cell-only syndrome, should be screened in order to exclude a role of c-kit mutations in Y721 in spermatogenesis defects. In this study we also show that the murine SCF promoter is transcriptionally active and stimulated by follicle stimulating hormone (FSH), 3'-5' cyclic adenosine monophosphate (cAMP) analogs, and IBMX in primary mouse Sertoli cells, and that the cAMP effect is cell-specific, as the SCF promoter is not stimulated in other SCF-expressing cell types tested.

Sertoli cell conditioned media modulate the androgen biosynthetic pathways in rat Leydig cell primary cultures

Increasing evidence suggests the presence of local modulators that contribute to testicular function regulation in rats. The seminiferous tubules and Sertoli cell factors influence the steroidogenic activity of Leydig cells. Addition of Sertoli cell conditioned media to primary cultures of adult rat Leydig cells produces in a dose-dependent manner an inhibition of testosterone production with a concomitant decrease in the C21 delta 4-intermediates of the steroidogenic pathway, modifying the C21/C19 ratios in normal cells. The nature and mechanisms of action of this putative modulating activity are currently under study.

Evidence that heparin binding autocrine factors modulate testosterone production by the adult rat Leydig cell

Androgen production by adult rat Leydig cells is stimulated by pituitary LH but can also be modulated in vitro by paracrine stimulatory and inhibitory factors, many of which belong to growth factor families. Their actions are mediated through cell surface or extracellular matrix proteoglycans and the aim of this study was to determine the role of cell surface heparan sulfate proteoglycans in the regulation of testosterone secretion by adult rat Leydig cells. The presence of sodium chlorate (25 mM) and protamine sulfate (10 micrograms/ml) inhibited testosterone production by LH stimulated cells by over 50%, but had no effect on unstimulated cells. The LH responsiveness and testosterone production returned to normal after these agents were removed from the culture media. No significant difference in LH receptor numbers at the end of the culture period was seen between sodium chlorate treated and untreated cells. Testosterone production by dibutryl-cAMP stimulated Leydig cells was also inhibited by sodium chlorate. The addition of heparin inhibited testosterone production by LH stimulated cells in a dose-dependent manner, however, in unstimulated Leydig cells heparin stimulated testosterone production to up to 50% of that seen in LH stimulated cells. These data suggest that cell surface heparan sulfate proteoglycans modulate testosterone production by adult Leydig cells in vitro, and that this may involve the autocrine actions of heparin binding growth factors on the Leydig cells.

Purine suppression of proliferation of Sertoli-like TM4 cells in culture

The effect of adenosine and related compounds on the proliferation of cultured TM4 cells, a Sertoli-like cell line, has been examined. Adenosine, as well as A1 and A2 adenosine receptor agonists (cyclohexyladenosine and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine) inhibited cell proliferation. These effects were prevented by 8-cyclopentyl theophylline, 1,3-dimethyl-propargylxanthine and caffeine, antagonists at the A1, A2 and both receptors, respectively. The xanthines had no effect by themselves and, consistent with this, the bathing medium was found not to contain detectable levels of adenosine. It is concluded that TM4 cell proliferation can be regulated by receptors for adenosine.

Rat seminiferous tubular culture medium contains a biological factor that inhibits Leydig cell steroidogenesis: its purification and mechanism of action

Seminiferous tubules prepared from adult rats cultured for 48 h in serum-free conditions produce multiple biological factors that modulate Leydig cell steroidogenic function in vitro. Using gel filtration chromatography, it was shown that seminiferous tubular culture medium (STCM) contained at least three inhibitory activities designated AI, AII, and AIII that inhibited testosterone production by purified Leydig cells. The factor that induced AIII activity, designated Leydig cell inhibitor (LCI), was further purified to apparent homogeneity by sequential HPLC using gel permeation, C8-, C18-, C2/C18-reversed-phase, and microbore anion exchange columns. When this batch of purified factor was resolved by SDS-PAGE under reducing conditions, only a single silver stained band with an apparent M(r) of 21,000 was detected. Protein sequence analysis using about 100 pmol of purified LCI revealed that its N-terminus was blocked. Incubation of this highly purified factor with Percoll gradient purified Leydig cells induced a dose-dependent inhibition of hCG-stimulated testosterone production. LCI inhibited the basal testosterone production and hCG-stimulated cAMP production by Leydig cell dose-dependently. It also inhibited the forskolin- and cholera toxin-stimulated testosterone and cAMP production but had no apparent effect on the binding of 125I-labeled hCG to LH receptors. These data suggest that this LCI exerts its inhibitory action at steps beyond the LH receptors but prior to the cAMP formation by affecting the adenylate cyclase activity directly or indirectly through inhibition of the stimulatory G-protein (Gs-protein); however, it is also possible that it decreases the coupling of the receptors to the Gs-protein. LCI also inhibited the conversion of exogenously added 22R-hydroxycholesterol, pregnenolone, progesterone, and 17 alpha-hydroxyprogesterone to testosterone. However, it had no effect on the conversion of dehydroepiandrostenedione and androstenedione to testosterone. These data strongly suggest that LCI affects the steroidogenic enzymes metabolizing cholesterol to testosterone, the cytochrome P-450 side-chain cleavage (P-450SCC), and cytochrome P-450 17 alpha-hydroxylase/17,20-lyase (P-450C17). However, it has no effect on the 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) enzyme activities. Based on the results of the present study, it is apparent that this LCI is distinct from other known potent Leydig cells inhibitors such as interleukin-1 (IL-1) and transforming growth factor-beta (TGF-beta). The LCI appears to involve in the paracrine regulation of Leydig cell function.

Human chorionic gonadotropin (hCG) increases cytosolic free calcium in adult rat Leydig cells

Regulation of steroidogenesis by luteinizing hormone is mediated by cAMP and calcium. We have investigated changes in cytosolic free calcium ([Ca2+]i) in Leydig cells by using Fura-2 as the fluorescent calcium indicator. Purified Leydig cells were plated on polylysine coated glass coverslips, cultured for 24 h in DMEM/F12 and loaded with Fura-2 at 37 degrees C. [Ca2+]i measurements were made fluorometrically by placing coverslips into 3 ml cuvettes with PBS+calcium. Addition of hCG increased [Ca2+]i gradually after a lag of about 2-3 min and plateaued by 5-6 min. The plateau level was sustained for at least 15 min. Absence of external Ca2+ in the medium or presence of diltiazem or nicardipine or cobalt chloride abolished the rise. Addition of BAY K 8644 or KCl caused a small but significant increase of [Ca2+]i. 8-Br-cAMP, forskolin or cholera toxin produced a gradual rise in [Ca2+]i that plateaued after 5-6 min similar to that observed with hCG. The action of hCG was inhibited by protein kinase A inhibitor (20-residues peptide) but not by protein kinase C inhibitor (staurosporine). We conclude that binding of hCG to its receptors would transmit the signal through G proteins to adenylyl cyclase to increase cAMP which would increase Ca2+ influx into cytosol across plasma membrane Ca2+ channels. Therefore, it appears that the primary action of hCG is to increase cytosolic cAMP which would then regulate [Ca2+]i as well as steroidogenesis.

Presence in mouse Sertoli cell-conditioned medium of a factor that expresses AVP-like inhibition of steroidogenesis by mouse Leydig cells in long-term culture

The influence of spent medium from immature mouse Sertoli cells (SCM) on testosterone production by purified Leydig cells was investigated and compared to that of AVP, a potent local modulator of Leydig cell steroidogenesis. SCM inhibited in a dose-dependent manner the hCG-stimulated testosterone production, but was ineffective in basal conditions. As is known for AVP, (i) a lag period of 72 h was prerequisite for SCM to inhibit Leydig cell function; (ii) the main effect of SCM was located at a step beyond the receptor-adenylate cyclase system, since the hCG- and 8-bromo-cAMP-stimulated testosterone productions were similarly affected. The possibility that the effect of SCM may be related to AVP-like molecule(s) is also supported by the observations that at maximal concentrations the inhibitory effects of AVP and SCM were not additive and that the inhibition of testosterone production was largely (65%) reversed by the presence of [(beta-mercapto-beta, beta-cyclopentamethylenepropionyl, O-Me-Tyr2,Arg8)-vasopressin], a selective vasopressor antagonist. These data indicate that Sertoli cells produce in vitro potent inhibitory factors of Leydig cell steroidogenesis. They provide additional evidence that one of these bioactive factors has an effect on Leydig cell function similar to that of AVP.