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Inserm U418/INRA UMR 1245, Hôpital Debrousse, 29 rue soeur Bouvier, 69322 Lyon cedex 05, France
1 CNRS-UMR7079, Université Pierre et Marie Curie, 75252 Paris cedex 05, France
(Requests for offprints should be addressed to B Le Magueresse-Battistoni, Email: lemagueresse{at}lyon.inserm.fr.)
R El Ramy is now at AFSSA-LERMVD, Toxicologie Alimentaire, Fougères BP 90203, Javene 35133, France
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Epithelial–mesenchymal cell interactions are regulated by a complex array of regulatory agents including the fibroblast growth factors (FGFs). FGFs belong to a large family of heparin-binding polypeptides, and 22 members and four high affinity receptors (FGFRs) encoding trans-membrane protein tyrosine kinase receptors have been characterized. The four FGFR genes encode seven functionally distinct variants by alternative splicing, and these variants exhibit differential binding affinities toward the FGF ligands. FGFs are involved in an array of biological processes during development and in adult life, including regulation of cell proliferation, migration and differentiation. There is evidence of a functional redundancy between some FGFs and FGFRs. Furthermore, most FGFs can signal through multiple FGFRs. Consequently, targeted deficiency of an FGF ligand or one of its receptors may induce mild to severe defects. Examples have been shown, on the one hand by fgf1–fgf2 single or double (Miller et al. 2000), fgfr3 (Colvin et al. 1996) or fgfr4 (Weinstein et al. 1998) knockout mice which are viable and fertile, and on the other hand by the lethality observed in the fetal or in the early postnatal period in mice lacking either one of fgf 4, 8, 9, 10, 15 and 18 or fgfr1 and fgfr2 (Goldfarb 1996, Itoh & Ornitz 2004).
Within the testis, FGF2 and FGF9 at least have emerged as being important regulators throughout testicular development. FGF2 was first isolated and characterized from bovine testes (Ueno et al. 1987), and its presence was next confirmed in the testes of different species, including humans, cows, mice and rats. Evidence then accumulated showing that FGF2 exerted mitogenic and non-mitogenic functions in the rat testis, where several FGFRs have been localized (Mullaney & Skinner 1992, Han et al. 1993, Le Magueresse-Battistoni et al. 1994, 1996, Van Dissel-Emiliani et al. 1996, Cancilla & Risbridger 1998, Cancilla et al. 2000). The importance of FGF9 in testis functions was shown more recently by the finding that most fgf9 –/– XY reproductive systems appeared grossly female at birth. Fgf9 –/– mice also exhibited lung hypoplasia and died at birth (Colvin et al. 2001). Detailed studies have shown that FGF9 supports the normal formation of testis cords (Colvin et al. 2001, Schmahl et al. 2004), downstream of Sry, the sex-determining gene in mammals (Swain & Lovell-Badge 1999). Colvin et al.(2001) and Schmahl et al.(2004) suggested that FGF9 could be involved in the interactions between the fetal Sertoli cells and the adjacent mesonephros from which the peritubular cells arise (Buehr et al. 1993).
These observations prompted us to investigate whether FGF2 or FGF9, which are testicular products, acted as morphogenic, survival or mitogenic factors in primary cultures of SCs and PCs. Prepubertal rat testes were used as a source of cells and we examined by RT-PCR the pattern of FGFRs present in these cells. We also questioned whether FGF2 and FGF9 acted as morphogens in the formation of testis cords in two different culture systems: a coculture system using prepubertal SCs and PCs, and an organotypic culture system using XY rat embryonic gonads. Finally, we monitored the pattern of expression of several proteinases and proteinase inhibitors produced by testicular cell types (Fritz et al. 1993, Longin et al. 2001, Siu et al. 2003). Our interest stems from several considerations. It has been demonstrated that in SC–PC cocultures, cord-like structures are allowed to assemble de novo depending on the production and deposition by the two cell types of ECM proteins within a basement membrane (Tung & Fritz 1980, 1987). Proteinases and proteinase inhibitors regulate, at least in part, the homeo-stasis of ECM proteins (Vu & Werb 2000). The major components of the testicular basement membrane, i.e. collagen IV, fibronectin (FN) and laminin (Richardson et al. 1995) are putative substrates for matrix metallo-proteinases (MMPs) and plasminogen activators (PAs) (Vu & Werb 2000). PCs and/or SCs produce several MMPs and tissue inhibitors of metalloproteinases (TIMPs) as well as the PAs and the inhibitor-1 (PAI-1) (Fritz et al. 1993, Longin et al. 2001, Siu et al. 2003), which may well take part in basement membrane turnover and synthesis.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Gonads with attached mesonephroi were isolated from 13.5 days postcoitum (dpc) embryos. This age corresponds to the very onset of sexual differentiation when gonads cannot be discriminated on a morphological basis. Embryos were therefore sexed extemporaneously scoring the presence of the barr bodies in amniotic membranes, and explants were cultured for 1 day on steel grids previously coated with 2% agar, as described elsewhere (Magre & Jost 1984). Grids were placed in organ culture dishes with 0.8 ml medium necessary to wet the grid. CMRL 1066 (Life Technologies, Grand Island, NY, USA) was used as basal culture medium. FGF2 or FGF9 (TEBU, Le Perray-en-Yvelines, France) at a dose of 50 ng/ml were added to some cultures. Controls were cultures without FGF2 or FGF9. After 1 or 3 days of culture, explants were fixed for 1 h at 4 °C in 2% paraformal-dehyde in 0.1 M phosphate buffer, pH 7.4. This was followed by washes in 0.1 M phosphate-buffered 0.15 M saline (PBS), pH 7.4, and immersion in a series of cold sucrose solutions. Specimens were frozen and sectioned in a cryostat at 5 µm. To identify gonadal structures, the distribution of anti-Müllerian hormone/Müllerian-inhibiting substance (AMH/MIS), FN and keratin 8 was examined using indirect immunoperoxidase detection as described (Mazaud et al. 2002). Antibodies used were a rabbit anti-AMH/MIS raised against bovine AMH/MIS (kindly provided by Dr B Vigier, Unité Biologie du Développement et Reproduction INRA, Jouy en Josas, France), a rabbit anti-human plasma FN serum (6071 SA; Life Technologies) and a monoclonal mouse anti-keratin 8 (LE 41, kindly provided by Professor E B Lane, Cancer Research UK, University of Dundee, Dundee, UK).
Cell preparations
SCs and PCs were isolated from 20-day-old Sprague–Dawley rats and cultured at 32 °C in a humidified atmosphere of 5% CO2 as previously described (Le Magueresse-Battistoni et al. 1998). Experiments were conducted in accordance with the Guide for the Care and Use of Laboratory Animals. Briefly, decapsulated testes were minced, then subjected to sequential enzymatic treatment with collagenase-dispase (0.05%) (Roche Biochemicals, Meylan, France), hyaluronidase (0.1%) (Sigma Chemical Co.) and collagenase-dispase (0.05%) at 32 °C for 30 min each. Cells were allowed to sediment by gravity between enzymatic treatment and washes. The final SC suspension was filtered through a 60µm nylon screen. At that point, the SC preparations were contaminated with 10–15% of germ cells (spermatogonia and early spermatocytes) and 1% PCs (Le Magueresse-Battistoni et al. 1998). PCs were collected in the supernatant of the first collagenase-dispase digestion. Cells were filtered through a 20µm nylon screen and cultured at 32 °C in the culture medium (Dulbecco’s modified Eagles’ medium–Ham’s F-12; Life Technologies) supplemented with 10% fetal calf serum (FCS). An average of 20 Petri dishes (diameter 60 mm) could be prepared when starting with ten testes. Two hours after plating, culture dishes were rinsed well to remove serum and non-adhering cells, mainly the contaminating germ cells.
Cell culture and coculture
SCs (1 x 106 viable cells) were either cultured alone (SC culture) or cocultured (SC–PC) so that SCs were placed on top of PCs at a ratio of 3:1 (SC:PC) as described (Skinner & Moses 1989). PCs were also cultured alone (PC cultures). Twenty-four hours after seeding, SC and SC–PC culture dishes were exposed to a hypotonic treatment to remove the 10–15% of germ cells which contaminate the SC preparations (Galdieri et al. 1981). Culture dishes were replenished with culture media supplemented or not (control) with FCS (10%), dibutyryl cAMP (bu2cAMP) (1 mM), platelet-derived growth factor (PDGF) (50 ng/ml; TEBU), FGF9 or FGF2 (50 ng/ml, unless otherwise specified). Morphological changes were observed daily by phase contrast microscopy. On day 5 of culture, cells were scraped from the dishes and RNA or proteins were extracted. Culture and coculture media were collected for protein analysis.
MTS test
SC and PC number was examined in 96 multiwell plates (10 000 cells/well) after a 4-day treatment with increasing doses of FGF2 or FGF9 ranging from 3 to 100 ng/ml, FCS (10%) or Bu2cAMP (1 mM). Cells were quantitated using the celltiter 96 R Aqueous non-radioactive cell proliferation assay according to manufacturer’s intructions (Promega, France). The assay is based on the ability of viable cells to reduce the yellow tetrazolium salt, MTS, to water-insoluble, dark blue formazan crystals, which can be spectrophotometrically measured at 490 nm. Usually four replicate wells were used for each group. The control included untreated cells, whereas medium alone was used as a blank.
Measurement of DNA synthesis
DNA synthesis was measured as [3H]thymidine (Amersham Pharmacia Biotech Europe GMBH, Orsay, France) incorporation into trichloroacetic acid (TCA)-insoluble material. PCs were plated in 96-well plates (10 000 cells/well). On the next day, culture medium was changed and FGF2 (1.5, 15, 30 and 50 ng/ml) or FGF9 (50 ng/ml) was added per well together with 0.5 µCi/ml [3H]thymidine (50 Ci/mM). Control cells received only 0.5 µCi/ml [3H]thymidine. After a 24-h incubation period, cells were washed twice with ice-cold PBS and twice with ice-cold 5% TCA to remove unincorporated radioactivity. Cells were solubilized by adding 0.25 M NaOH and the cell lysate was transferred to scintillation fluid and counted by a spectrometer (Beckman-Coulter, Villepinte, France). Each measurement consisted of a four-well replica.
RNA extraction and RT-PCR
The procedure for RNA extraction and RT-PCR has been described previously (Longin et al. 2001, Guyot et al. 2003). Total RNA was extracted from whole testes, liver and kidney recovered from 20-day-old Sprague–Dawley rats. Total RNA was also extracted from the SCs and PCs isolated from 20-day-old Sprague–Dawley rats and cultured for 5 days. Specific primers for RT-PCR were designed with the help of the Gene-Jockey sequence processor (Biosoft, Cambridge, Cambs, UK). The optimal temperature of annealing for each pair of primers was defined (Table 1
). Negative controls contained water instead of cDNA. PCR with no RT reactions gave no band, eliminating the possibility of a genomic DNA contamination in the RNA preparations. Amplified cDNAs were visualized in a 1.5% agarose gel stained with ethidium bromide. A DNA ladder (Promega, Charbonniè res, France) was loaded on each gel, and PCR products were sequenced by Biofidal (Lyon, France).
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Culture and coculture media were concentrated ten times using Centriprep (cut-off at 10 kDa; Amicon, Beverly, MA, USA). Proteins (10–20 µg/lane measured by BCA protein assay (Pierce-Interchim, Montluçon, France)) were electrophoresed at 4 °C on 10% polyacrylamide gels containing 1 mg/ml gelatin (Sigma Chemical Co.) in the absence of any reducing agent (Longin et al. 2001, Guyot et al. 2003). Following electrophoresis, SDS was removed from the gel by exchange in Triton X-100 (two washes of 30 min at room temperature in 2.5% Triton X-100 followed by two washes of 20 min in distilled water). The gel was subsequently incubated at 37 °C for 48 h in 100 mM Tris–base, pH 7.6, containing 15 mM CaCl2. In these conditions, gelatinases present in the samples renatured and autoactivated. White zones of lysis indicating gelatin-degrading activity were revealed by staining with Coomassie Brillant blue R-250.
Plasminogen zymography
Cells scraped from the culture and coculture dishes were lysed in a Tris-buffer solution (0.5 M, pH 8.2) containing 0.5% Triton X-100 and proteins (20 µg), and then electrophoresed at 4 °C on 8% polyacrylamide gels in the absence of any reducing agent. Following electrophoresis, SDS was removed from the gel by exchange in Triton X-100 (two washes of 30 min at room temperature in 2.5% Triton X-100 followed by three washes of 30 min in distilled water). The gel was subsequently placed on a casein–agar–plasminogen underlay essentially as described (Tolli et al. 1995, Odet et al. 2004). Clear lytic zones indicating plasminogen-degrading activity appeared after a 24-h incubation at 37 °C, and gels were scanned.
Western blot analysis
SDS-PAGE and Western blotting were carried out as reported earlier (Longin et al. 2001, Guyot et al. 2003), using either (i) tenfold concentrated cell culture media for the analysis of TIMPs-1 and -2 and PAI-1 or (ii) cell lysates prepared using PBS containing 1% NP-40 and 5 mM EDTA for the analysis of TIMP-3. Briefly, proteins were separated by electrophoresis performed on 10% (15% for TIMP-2) polyacrylamide gels under reducing conditions. Precision protein standards (Biorad, Hercules, CA, USA) were loaded onto each gel for accurate estimation of the Mr of the bands. The proteins were then electrophoretically transferred to a PVDF membrane (Biorad). After treatment with a blocking solution (5% skim milk in Tris-buffered saline; TBS) for 3 h, the membrane was incubated overnight with the primary antibody at 4 °C. Primary antibodies were a rabbit anti-PAI-1 (working dilution 1:200) from American Diagnostics (Greenwich, CT, USA), a rabbit anti-TIMP-1 (working dilution, 1:2000), a rabbit anti-TIMP-2 (working dilution, 1:1000) and a rabbit anti-TIMP-3 (working dilution, 1:1000) (all three from Chemicon International, Temecula, CA, USA). The PVDF membrane was washed and incubated with peroxidase-conjugated goat anti-rabbit IgG antibody (Dako, Trappes, France). Different experiments were performed to ensure equal loading of the samples. Gels processed for Western blot analysis were systemically stained with red ponceau before proteins were transferred. In the case of TIMP-3, PVDF membranes hybridized with the anti-TIMP-3 antibody were stripped and reprobed with a rabbit anti-ß-actin antibody (working dilution, 1:1000; Sigma Chemical Co).
Data analysis
All experiments were repeated three to five times using independent cell preparations. A representative experiment from each series is presented. Data are the means±S.E.M. The significance of the results was determined by ANOVA followed by the Mann–Whitney U test. Differences are accepted as significant at P<0.05.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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After a few days in culture, SCs formed a confluent monolayer (Fig. 1). No gross morphological changes could be observed between SCs cultured for 5 days in control culture medium and SCs cultured in the presence of FGF2 or FGF9 (50 ng/ml) from day 1 to day 5 of culture. This was in contrast with the morphological aspect of the PCs which changed depending on the treatment performed. For example, FCS-treated PC cultures displayed a more flattened fibroblast-like appearance than did control cells, and bu2cAMP-treated cells remained round and exhibited neuronal-like cytoplasmic expansions. This finding extended previous results (Ailenberg et al. 1990). FGF9-treated cells appeared more retracted than control or FCS-treated cells even though they had spread and showed slender long processes. FGF2-treated cells adopted a highly retracted cell shape with fewer and shorter cytoplasmic expansions than the FGF9-treated cells (Fig. 1).
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FGF2 and FGF9 were added to primary cultures of SCs and PCs at increasing doses ranging from 3 to 100 ng/ml, and cell number was determined after a 96-h incubation period using the MTS viability test. In preliminary experiments (not shown), we ensured that FCS could enhance PC but not SC proliferation as reported (Skinner et al. 1989). We found that FGF2 and FGF9 did not alter SC number (not shown). By contrast, FGF2 but not FGF9 dose-dependently increased the number of PCs. First significant effects were seen with FGF2 (6 ng/ml) (+22%, P<0.05, n=4). Maximal effects were obtained with FGF2 (50 ng/ml) (+48%, P < 0.05, n=4). Bu2cAMP had no effect (Fig. 2A). To discriminate between cell survival and cell proliferation, we examined [3H]thymidine incorporation. FCS was used as a positive control (not shown). Data presented indicated that FGF2 enhanced the DNA labeling index of PCs after a 24-h incubation period. First significant effects were seen for FGF2 (15 ng/ml) (+38%; P<0.05) and +83% enhancement (P<0.05) was observed with FGF2 (50 ng/ml). FGF9 (50 ng/ml) had no effect (Fig. 2B).
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The nature of the receptors present on the cultured PCs as well as on the cultured SCs and in the whole testes of 20-day-old rats was determined using RT-PCR. Specific pairs of primers (Table 1
) were designed against FGFRs 1–3 isoform IIIb or IIIc and FGFR4. RT-PCR studies were also conducted using primers directed against 18S to ensure that equal amounts of material were used. RNA extracted from PCs gave RT-PCR bands for FGFRs 1 IIIc, 2 IIIb and IIIc, a very weak band for FGFR3 IIIc, and no band for the IIIb isoform of FGFRs 1 or 3, or for FGFR4. RNA extracted from SCs gave a PCR band for receptors 1–3, but not for FGFR4. An RT-PCR band was found in the whole rat testes for the seven receptors. With regard to the ligands, FGF9 was present in SC cultures but not in PC cultures (Fig. 3), whereas FGF2 has been shown to be a PC as well as an SC product in prepubertal testes (Mullaney & Skinner 1992). A PCR band for FGF9 was also detected in RNA from rat fetal testes. RNA recovered from kidney was used as a positive control for FGFR4.
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When SCs are seeded on top of PCs they reaggregate and form tubule-like structures resembling seminiferous cords of the differentiating testis (Tung & Fritz 1980). In the present study, we observed that the extent of reorganization varied depending on the factor added in the culture medium (Fig. 4). We found that FGF2 (50 ng/ml) promoted the formation of condensed nodules of aggregated SC. Cocultures exposed to FGF9 (50 ng/ml) also showed a reorganization, and its effect was comparable with that elicited by PDGF (50 ng/ml) known to stimulate cord formation (Uzumcu et al. 2002, Brennan et al. 2003, Ricci et al. 2004). However, the nodules were smaller than in the FGF2-treated cocultures and the PCs could still develop long cytoplasmic expansions between the adjacent nodules. The control cocultures and the FCS-treated cocultures were characterized by the presence of very small aggregates of SCs and whirls of PCs (Fig. 4). Cocultures treated with FGF2 (10 ng/ml) formed fewer tubule-like structures than cocultures treated with FGF2 (50 ng/ml), and cocultures treated with FGF9 (10 ng/ml) were not different in their aspect from control cocultures (not shown). The extent of reorganization was time-dependent, and large aggregates could be seen after a period of 8 days in control cocultures. However, as the nodules increased in size they exhibited a reduced ability to remain attached to the plastic dishes and finally they were detaching when culture media was changed (e.g. in the FGF2-treated cocultures older than 8 days). Consistent with previous data (Tung & Fritz 1987), no aggregation of SCs was observed in the presence of bu2cAMP (1 mM; Fig. 4).
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We also examined whether FGF2 and FGF9 (50 ng/ml) acted as morphogens for rat embryonic testes. Typically when gonads at the very onset of testis differentiation (13.5 dpc in the rat) remain associated with their mesonephroi, seminiferous cords differentiate in vitro (Magre & Jost 1984). These cords are identifiable using keratin 8 and FN to identify the epithelial and mesenchymal cells respectively. After a 24-h exposure, explants have grown, cords have differentiated and SCs could be immunostained for AMH/MIS in the three culture conditions. It is noteworthy that the extent of growth was more pronounced and the well-differentiated cords were higher in number in the FGF2-treated explants as compared with the FGF9-treated explants. Also, FGF9-treated explants were more developed than the explants in control cultures (Fig. 5). After 3 days of culture, all explants had further enlarged. FGF2- and FGF9-treated explants could no longer be discriminated on a morphological basis, and control explants remained less developed than the FGF2-or the FGF9-treated explants (data not shown).
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Two groups of gelatinolytic bands are classically found with testicular extracts: the pro- and active form of MMP-9 migrating at 92 and 84 kDa, and the pro, intermediate and active forms of MMP-2 migrating as a triplet around 72, 66 and 62 kDa (Hoeben et al. 1996, Longin et al. 2001). Gelatinases are mostly secreted proteins, and we analyzed concentrated media of SCs and SC–PC cultured for 96 h (day 1 to day 5 of culture). We found that bu2cAMP stimulated MMP-2 synthesis and activity, extending previous observations on SC and PC cultures (Fritz et al. 1993, Hoeben et al. 1996). FGF9 had no effect on the activity/synthesis of the MMPs-2 and -9 in SC–PC coculture but a slight decrease was detected in SC culture. PDGF slightly stimulated the two MMPs in coculture (Fig. 6). In fact, the most striking effect was observed in the FGF2-treated cocultures in which MMP-9 and, to a lesser extent MMP-2 were stimulated (both the pro- and active forms). By contrast, MMP-9 was not enhanced in FGF2-treated SC cultures and MMP-2 was weakly stimulated (Fig. 6A).
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). Western blot study of cognate inhibitors
Three PAIs, i.e. PAI-1, -2 and -3 have been described. However, PAI-2 is barely detectable in the testis and PAI-3 is a Leydig cell product in the immature rat testis (Odet et al. 2004). PAI-1 is a doublet of 46 and 49 kDa produced by PCs and SCs. It is downregulated by bu2cAMP and upregulated by FGF2 in both cell types (Le Magueresse-Battistoni et al. 1996, 1998). Results presented in Fig. 7 extend these previous data, and demonstrate that in SC–PC cocultures FGF9 and PDGF stimulated PAI-1 to the same extent as did FGF2.
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). TIMP-3 occurs as a minor 25 kDa unglycosylated protein and a major 30 kDa glycosylated protein in the cell lysates of untreated cocultures. Treatment of cocultures with FGF2 or FGF9 resulted in an increase of the 30 kDa glycosylated form of TIMP-3 (Fig. 7). The protein profile observed in the bu2cAMP-treated cocultures was very different as compared with the untreated or growth factor-treated cocultures, in that a high molecular weight band migrating around 60 kDa was strongly labeled in addition to the 30 kDa protein. ß-actin was used to ensure equal loading of the cell lysates proteins (Fig. 7). |
Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In a first series of experiments, we investigated whether FGF2 and FGF9 acted as morphogenic, survival or mitogenic factors in primary cultures of SCs and PCs recovered from 20-day-old rats. We observed that both FGFs were morphogenic for PCs but not for SCs. In addition, FGF2 but not FGF9 was a survival and a mitogenic factor for PCs in vitro. Dose–response studies indicated that maximal effects of FGF2 were seen with the dose of 50 ng/ml. This finding was consistent with previous publications examining the mitogenic or regulatory function of FGF2 (Hoeben et al. 1999, Van Der Wee & Hofmann 1999), FGF9 (Colvin et al. 2001) or PDGF (Uzumcu et al. 2002, Puglianiello et al. 2004, Ricci et al. 2004) on testicular cells. Hence, this dose of 50 ng/ml was chosen for the next experiments for each of the growth factors studied. We also found that FGF2 was not a mitogen for SCs, consistent with the fact that prepubertal SCs have a very low mitotic index (Skinner et al. 1989).
The next step was to determine the nature of the FGFRs present on SCs and PCs, because seven FGFR variants exhibiting differential affinity toward the FGF ligands have been described. Specifically, FGF2 preferentially binds FGFR1 IIIc, 3 IIIc and FGFR4, whereas FGF9 prefers FGFR2 IIIc, 3 IIIc and FGFR4 (Goldfarb 1996, Itoh & Ornitz 2004). RT-PCR analysis was conducted because specific antibodies necessary to discriminate the isoforms IIIb and IIIc of the FGFRs 1, 2, and 3 are still lacking. In adition, immunohistochemical data have previously shown the presence of the four FGFRs within the rat testis (Cancilla & Risbridger 1998). We found that cultured SCs exhibited signals for the FGFRs 1–3, and the isoform IIIc of the FGFR1 was the most abundant. The isoform IIIc of the FGFR1 was also the most abundant in cultured PCs. The signal for FGFR3 IIIc was weak in PCs versus the signal in SCs. Finally, we found no signal for FGFR4 in PCs and SCs, contrasting with previous immunohistochemical data showing FGFR4 in PCs (Cancilla & Risbridger 1998). This discrepancy may result from the two different experimental conditions, in situ versus cells cultured for 5 days in a chemically defined medium, in the present study. Therefore, it may be hypothesized that FGFR1 IIIc is the receptor mediating the mitogenic and morphogenic actions of FGF2 on PCs, and that the lack of effect of FGF9 on PC proliferation reflects the low abundance of FGFR3 IIIc in these cells. We are currently addressing this issue.
We then questioned whether FGF2 and FGF9 acted as morphogens for the formation of testis cords. Two different culture systems were used, a coculture system using the SCs and PCs recovered from prepubertal rat testes and an organotypic culture system using XY rat embryonic gonads attached to their mesonephroi. We observed that both FGFs were morphogens in SC–PC coculture in that they promoted reorganization into cord-like structures. In order to evaluate the potency of each FGF, other co-cultures received PDGF, which promotes cord formation (Uzumcu et al. 2002, Brennan et al. 2003, Puglianielli et al. 2004, Ricci et al. 2004), or an analog of cAMP to block cellular reorganization (Tung & Fritz 1987). FGF9- and PDGF-treated cocultures could not be distinguished on a morphological basis, whereas FGF2 was more efficient in promoting cellular reorganization after a 4-day exposure. Whether the differential effect of the FGF2 and FGF9 in cocultures reflects the differential abundance of the FGFRs remains plausible. However, it appears unlikely that the morphogenic effect of FGF2 in coculture resulted only from its mitogenic action on PCs because FCS stimulates PC proliferation. Nonetheless, FCS-treated co-cultures showed little reorganization as compared with FGF2-treated cocultures. Using rat embryonic gonads as an alternate model, we have also described a morphogen action for the two FGFs and a primary effect of FGF2 over FGF9 after a 24-h but not after a 72-h exposure. However, the expression pattern of FGFR1 IIIc and FGFR3 IIIc in the embryonic rat testes is presently unknown.
It has also been described that heparan sulfate proteoglycans (HSPGs) are low affinity receptors for FGFs and are required to efficiently activate FGFRs (Itoh & Ornitz 2004). Moreover, tissue-specific expression and modification of HSPGs have been shown to be responsible for the tissue-specific action of FGFs in other systems (Friedl et al. 1997, Park et al. 2000, Allen & Rapraeger 2003). Therefore, monitoring the expression pattern of the HSPGs in SC–PC cocultures as well as in the organotypic cultures treated by FGF2 and FGF9 should be instrumental in further elucidating the mechanisms of action of the two FGFs.
It should also be pointed out that several growth factors are morphogens during embryonic testis development, i.e., the hepatocyte growth factor (HGF) (Ricci et al. 2002), the PDGF (Uzumcu et al. 2002, Brennan et al. 2003, Puglianielli et al. 2004, Ricci et al. 2004) and the neurotropins (Cupp et al. 2000, 2003). Such an apparent redundancy may explain the lack of a severe testicular phenotype (at least with respect to embryonic testis development) in mice deleted for c-Met, the HGF receptor (Ricci et al. 2002), for PDGF receptor-ß (Soriano 1994) and for trkA and trkC neurotropin receptors (Cupp et al. 2002). FGF2 null mice also seem to have a normal testicular function and reproduction (Miller et al. 2000) although no detailed studies have been reported so far. The notable exception to date is FGF9 (Colvin et al. 2001). It is therefore really intriguing that FGF2 was a better morphogen than FGF9 in our study. Whether this is a species-related phenomenon, i.e. rat versus mouse, should be examined.
Finally, we examined whether proteinases and their cognate inhibitors might be downstream targets in the FGF-signaling cascade. Interestingly, FGF2-treated co-cultures had a unique signature of proteinase expression. Indeed, FGF2 increased the overall synthesis and activity of the two gelatinases, particularly that of MMP-9, whereas bu2cAMP could induce MMP-2 but not MMP-9. Furthermore and in addition to MMP-9, we found that TIMP-3 and PAI-1 antigen levels, but not those of TIMPs-1 and -2, correlated with the degree of reorganization in cocultures. Indeed, the 30 kDa glycosylated form of TIMP-3 was the major band in control and growth factor-treated cocultures, whereas the 60 kDa band predominated in the bu2cAMP-treated cocultures. TIMP-3 occurs normally as 25 kDa unglycosylated and 30 kDa glycosylated proteins. In addition, high molecular weight bands may also be labeled. Because TIMP-3 is an ECM-associated protein, the high molecular weight proteins may represent ECM ligands that form SDS stable complexes with TIMP-3 as reported in the rat central nervous system (Jaworski & Fager 2000). It remains that the nature of such bu2cAMP-induced ECM ligand is presently unknown. With regard to PAI-1, bu2cAMP-treated cocultures had decreased antigen levels whereas they were enhanced in the growth factor-treated co-cultures. It is also worth noting that, apart from being a fast-acting and a specific inhibitor of the PAs, PAI-1 exhibits multiple activities including the ability to regulate the attachment–detachment of a cell from the extracellular matrix components by inactivating integrins (Czekay et al. 2003). However, it is not yet known whether this activity prevails in our coculture model.
In summary, we have demonstrated that FGF2 and FGF9 are morphogens for the formation of the testis cords in two different in vitro models. FGF2 and FGF9 may therefore participate to the formation of testis cords in the rat embryonic testis as demonstrated recently for FGF9 in the mouse embryonic testis (Colvin et al. 2001). Such a morphogen action may also be important in the prepubertal testes which is characterized by important restructuring. Specifically, tight junctions are formed between neighboring SCs thus creating the blood–testis barrier, and cords develop a lumen becoming tubules. Because there is evidence that ECM components may regulate the expression of tight junction proteins (Siu et al. 2003), it may well be hypothesized that FGF2 and FGF9 mediate mesenchymal–epithelial interactions between PCs and SCs and selectively regulate the expression pattern of specific proteinases and inhibitors and thus, subsequently, affect the basement membrane remodeling.
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Acknowledgements |
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Received 7 July 2005
Accepted 26 July 2005
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