Follicle-stimulating hormone induces spermatogenesis mediated by androgen production in Japanese eel, Anguilla japonica

Follicle-stimulating hormone (FSH) plays important roles in spermatogenesis. However, the biologic activity of FSH can vary in different vertebrate classes, and the definitive function of FSH has not been established. In this study, we investigated the functions of FSH on spermatogenesis using an in vitro culture system for Japanese eel testis. The eel Fsh receptor was expressed in testis tissue during the whole process of spermatogenesis, mainly by Leydig cells that produce steroid hormones and by Sertoli cells surrounding type A spermatogonia and early type B spermatogonia. In an in vitro organ culture, recombinant eel Fsh (r-eFsh) induced complete spermatogenesis from the proliferation of spermatogonia to spermiogenesis during 36 days of culture; also, spermatozoa were observed in the testicular fragments. Spermatogenesis induced by r-eFsh was inhibited by trilostane, a specific inhibitor of 3beta-hydroxysteroid dehydrogenase. However, trilostane did not inhibit spermatogenesis induced by 11-ketotestosterone. These results clearly show that the main function of FSH in eel is to induce spermatogenesis via stimulating androgen production.     

Hormonal induction of all stages of spermatogenesis in germ-somatic cell coculture from immature Japanese eel testis

In cultivated male eel, spermatogonia are the only germ cells present in testis. Our previous studies using an organ culture system have shown that gonadotropin and 11-ketotestosterone (11-KT, a potent androgen in teleost fishes) can induce all stages of spermatogenesis in vitro. for detailed investigation of the control mechanisms of spermatogenesis, especially of the interaction between germ cells and testicular somatic cells during 11-KT-induced spermatogenesis in vitro, we have established a new culture system in which germ cells and somatic cells are cocultured after they are aggregated into pellets by centrifugation. Germ cells (spermatogonia) and somatic cells (mainly Sertoli cells) were isolated from immature eel testis. Coculture of the isolated germ cells and somatic cells without forming aggregation did not induce spermatogenesis, even in the presence of 11-KT. In contrast, when isolated germ cells and somatic cells were formed into pellets by centrifugation and were then cultured with 11-KT for 30 days, the entire process of spermatogenesis from premitotic spermatogonia to spermatozoa was induced. However, in the absence of 11-KT in the culture medium spermatogenesis was not induced, even when germ cell and somatic cells were aggregated. These results demonstrate that physical contact of germ cells to Sertoli cells is required for inducing spermatogenesis in response to 11-KT.     

Impaired spermatogenesis in the Japanese eel, Anguilla japonica: Possibility of the existence of factors that regulate entry of germ cells into meiosis

In the cultivated male Japanese eel, spermatogonia are the only germ cells present in the testis. Weekly injections of human chorionic gonadotropin (HCG) can induce complete spermatogenesis from proliferation of spermatogonia to spermiogenesis. In some cases, however, HCG injection fails to induce complete spermatogenesis. Testicular morphological observations revealed that HCG-injected eels could be classified into three types based on their testicular conditions. Type 1 eels had a well-developed testis and the milt could be acquired by hand-stripping. In type 2 eels, spermatogenesis was also induced by HCG injection, but testicular size was remarkably smaller than that of type 1 eels, and the milt could not be hand-stripped. At the end of the experiment, type 2 fish had only spermatogonia and a small amount of spermatozoa, but no spermatocytes or spermatids, in their testis. Type 3 eels had thready testis, which did not develop any germ cells during the experimental period. These results suggest that, despite elevations of plasma 11–ketotestosterone levels, HCG injections were not successful in inducing the completion of spermatogenesis in type 2 and type 3 eels. In most spermatogonia of type 2 eels, meiosis was not induced by HCG injections. Furthermore, only few mitotic divisions had occurred as evidenced by the presence of 2³ to 2⁶ late type B spermatogonia in most cysts. This suggests that spermatogonial stem cells undergo four or five, and occasionally six, mitotic divisions before the interruption of spermatogenesis in type 2 eels. It is proposed that those numbers of mitotic divisions are related to a mediator that regulates entry of spermatogonia of the Japanese eel into meiosis.     

Recombinant human insulin-like growth factor I stimulates all stages of 11-ketotestosterone-induced spermatogenesis in the Japanese eel, Anguilla japonica, in vitro.

In this study, we examined the in vitro effects of insulin-like growth factor I (IGF-I) in the presence or absence of 11-ketotestosterone (11-KT: the spermatogenesis-inducing hormone) on the proliferation of Japanese eel (Anguilla japonica) testicular germ cells. Initially, a short-term culture (15 days) of testicular tissue with only type A and early type B spermatogonia (preproliferated spermatogonia) was carried out in Leibovitz-15 growth medium supplemented with different concentrations of recombinant human IGF (rhIGF)-I or -II in the presence or absence of 10 ng/ml of 11-KT. Late type B spermatogonia (proliferated spermatogonia) were observed in treatments of 100 ng/ml of both rhIGF-I and -II in combination with 11-KT, indicating the onset and progression of spermatogenesis. In all tested rhIGF-I concentrations (except 0.1 ng/ml) supplemented with 11-KT, late type B spermatogonia were detected in at least one individual. Then, we proceeded with an in vitro 45-day culture of testicular tissue with 100 ng/ml of rhIGF-I in the presence or absence of 10 ng/ml of 11-KT to test the long-term effects of rhIGF-I on the spermatogenetic cycle. The presence of all types of germ cells, including spermatozoa, in the testis cultured with the admixture of the two hormones indicated that the germ cells underwent complete spermatogenesis whereas no germ cell proliferation was observed when the rhIGF-I was applied alone. These results suggest that IGF-I in the presence of 11-KT plays an essential role in the onset, progress, and regulation of spermatogenesis in the testis of the Japanese eel.     

Spermatogenesis-preventing substance in Japanese eel

Under fresh-water cultivation conditions, spermatogenesis in the Japanese eel is arrested at an immature stage before initiation of spermatogonial proliferation. A single injection of human chorionic gonadotropin can, however, induce complete spermatogenesis, which suggests that spermatogenesis-preventing substances may be present in eel testis. To determine whether such substances exist, we have applied a subtractive hybridisation method to identify genes whose expression is suppressed after human chorionic gonadotropin treatment in vivo. We found one previously unidentified cDNA clone that was downregulated by human chorionic gonadotropin, and named it 'eel spermatogenesis related substances 21' (eSRS21). A homology search showed that eSRS21 shares amino acid sequence similarity with mammalian and chicken Müllerian-inhibiting substance. eSRS21 was expressed in Sertoli cells of immature testes, but disappeared after human chorionic gonadotropin injection. Expression of eSRS21 mRNA was also suppressed in vitro by 11-ketotestosterone, a spermatogenesis-inducing steroid in eel. To examine the function of eSRS21 in spermatogenesis, recombinant eSRS21 produced by a CHO cell expression system was added to a testicular organ culture system. Spermtogonial proliferation induced by 11-ketotestosterone in vitro was suppressed by recombinant eSRS21. Furthermore, addition of a specific anti-eSRS21 antibody induced spermatogonial proliferation in a germ cell/somatic cell co-culture system. We conclude that eSRS21 prevents the initiation of spermatogenesis and, therefore, suppression of eSRS21 expression is necessary to initiate spermatogenesis. In other words, eSRS21 is a spermatogenesis-preventing substance.     

A rapid activation of immature testis of Japanese eel (Anguilla japonica) by a single injection of human chorionic gonadotropin

The testis of Japanese eel (Anguilla japonica) consists of type A and early type B spermatogonia together with inactive Leydig and Sertoli cells. A single injection of human chorionic gonadotropin induced marked changes in the morphology of the testis and in the serum androgen levels within a period of 72 h. Morphological changes include spermatogonial proliferation, activation of Leydig and Sertoli cells, organization of seminiferous lobules and formation of lobular lumen in the testis. Leydig cells were enlarged, exhibiting characteristics of steroid-producing cells. Sertoli cells become elongated, show signs of high cellular activity and remain in close contact with spermatogonia. The lobular organization was achieved much earlier than the progression of spermatogenesis to late type B spermatogonia. Even 6 h after hCG injection, a significant increase in plasma levels of 11-ketotestosterone was observed, followed by a further time dependent increase. Plasma testosterone levels were also increased after injection, but the increase was much less than that of 11-ketotestosterone.     

A novel stage-specific antigen is expressed only in early stages of spermatogonia in Japanese eel,Anguilla japonica testis

We produced an antibody that recognized only early stages of spermatogonia in Japanese eel testis. This antibody (anti-spermatogonia-specific antigen-1, anti-SGSA-1) recognized a band of about 38 kDa in Western blot analysis of extracts from eel testis. This antigen was observed by immunohistochemistry only in type-A and early type-B spermatogonia and could not be seen in the late type-B spermatogonia, which appeared after the initiation of spermatogenesis by a single injection of human chorionic gonadotropin. Immunoreactive SGSA-1 was absent in spermatocytes, spermatids, spermatozoa, Sertoli cells, and interstitial Leydig cells. Similarly, this antigen was also detected only in type-A/primary spermatogonia in the testes of two species of teleosts, medaka (Oryzias latipes) and tilapia (Oreochromis niloticus), as well as a toad (Xenopus laevis). These results imply that the disappearance of SGSA-1 in late type-B/secondary spermatogonia is a critical step in the progression of spermatogenesis, and indicate that anti-SGSA-1 is a useful marker for analysis of the molecular mechanism controlling the differentiation of spermatogonia in lower vertebrates. Mol. Reprod. Dev. 51:355–361, 1998. © 1998 Wiley-Liss, Inc.     

Tolerance of spermatogonia to oxidative stress is due to high levels of Zn and Cu/Zn superoxide dismutase

Background

Spermatogonia are highly tolerant to reactive oxygen species (ROS) attack while advanced-stage germ cells such as spermatozoa are much more susceptible, but the precise reason for this variation in ROS tolerance remains unknown.

Methodology/Principal Findings

Using the Japanese eel testicular culture system that enables a complete spermatogenesis in vitro, we report that advanced-stage germ cells undergo intense apoptosis and exhibit strong signal for 8-hydroxy-2′-deoxyguanosine, an oxidative DNA damage marker, upon exposure to hypoxanthine-generated ROS while spermatogonia remain unaltered. Activity assay of antioxidant enzyme, superoxide dismutase (SOD) and Western blot analysis using an anti-Copper/Zinc (Cu/Zn) SOD antibody showed a high SOD activity and Cu/Zn SOD protein concentration during early spermatogenesis. Immunohistochemistry showed a strong expression for Cu/Zn SOD in spermatogonia but weak expression in advanced-stage germ cells. Zn deficiency reduced activity of the recombinant eel Cu/Zn SOD protein. Cu/Zn SOD siRNA decreased Cu/Zn SOD expression in spermatogonia and led to increased oxidative damage.

Conclusions/Significance

These data indicate that the presence of high levels of Cu/Zn SOD and Zn render spermatogonia resistant to ROS, and consequently protected from oxidative stress. These findings provide the biochemical basis for the high tolerance of spermatogonia to oxidative stress.     

PCNA protein expression during spermatogenesis of the Japanese eel (Anguilla japonica)

Spermatogenesis can be initiated by a single injection of human chorionic gonadotropin (hCG) into the cultivated Japanese eel, which produces only spermatogonia in the testis. To isolate the genes responsible for regulating spermatogenesis, we performed a differential mRNA display using poly (A)+ RNA extracted from the testes at different time points after hCG injection. Among several cDNA clones, the expression of which was initiated before the onset of meiosis, one clone has high homology with the proliferating cell nuclear antigen (PCNA). In this study, we investigated the protein expression of eel PCNA and found for the first time in any species that two forms (32-kDa and 36-kDa) of PCNA are present in the testis. Although the 36-kDa form existed in both the testis and spleen, the 32-kDa form was specifically expressed in the testis. In contrast to the appearance of 36-kDa PCNA 1 day after the hCG treatment, the 32-kDa PCNA appeared only 9 days after the hCG treatment, at which time active spermatogonial proliferation occurred in the testis. Both the 32- and 36-kDa forms were recognized by antibodies raised against different epitopes of PCNA, and their N-terminal amino acid sequences were identical. The 36-kDa form, but not the 32-kDa form, was recognized by antibodies against phosphoamino acids. These results suggest that the two PCNA proteins are the same molecule with different chemical modifications, including phosphorylation. We discuss the roles of these two forms of PCNA in the spermatogenesis of the Japanese eel.     

Comparative studies between in vivo and in vitro spermatogenesis of Japanese eel (Anguilla japonica)

In order to check the quality of in vitro spermatogenesis of Japanese eel, in vitrol 1-ketotestosterone (11-KT) induced spermatogenesis was compared with in vivo spermatogenesis induced by a single injection of human chorionic gonadotropin (hCG) in detail. DNA contents of germ cells from in vitro and in vivo testicular fragments were compared using flow cytometry. Since the in vitro result of flow cytometry showed prominent 1C peak including spermatozoa and spermatids, the reduction of DNA by meiosis was assumed to progress normally, (i.e., haploid spermatozoa were produced in this in vitro system). In the testes of in vitro culture, however, spermatozoa were not released into lumen. Furthermore, the number of mitotic divisions of the in vitro experiment (6 divisions) was fewer than that of in vivo (10 divisions). In electron microscopy observations, both of in vivo and in vitro spermatozoon had a crescent-shaped nucleus with a flagellum, and a single large spherical mitochondrion. However, the elongation of the sperm head was not sufficient and the mitochondrion was not always located at the anterior end as is observed for the spermatozoa obtained from hCG injected eels. Eel spermatogenesis related substance-11 (eSRS11) is homologue of histone H1 which is up-regulated during spermatogenesis. Using this probe, in vitro spermatogenesis was also evaluated in molecular levels. In Northern blot analysis, eSRS11 mRNA was detected in both in vivo and in vitro testes. However, the expression of in vitro was much weaker than that of in vivo. These differences indicate that the stimulation of 11-KT is not sufficient, and another factors are needed to induce complete spermatogenesis in vitro.     

The cloning of cyclin B3 and its gene expression during hormonally induced spermatogenesis in the teleost, Anguilla japonica

We cloned cyclin B1, B2, and B3 cDNAs from the eel testis. Northern blot analysis indicated that these cyclin B mRNAs were expressed and increased from day 3 onward after the hormonal induction of spermatogenesis, and that cyclin B3 was most dominantly expressed during spermatogenesis. In situ hybridization showed that cyclin B1 and B2 were present from the spermatogonium stage to the spermatocyte stage. On the other hand, cyclin B3 mRNA was present only in spermatogonia. Although mouse cyclin B3 is expressed specifically in the early meiotic prophase, these results indicate that eel cyclin B3 expression is limited during spermatogenesis to spermatogonia, but is not present in spermatocytes. These facts together suggest that eel cyclin B3 is specifically involved in spermatogonial proliferation (mitosis), but not in meiosis.     

The synthesis and role of taurine in the Japanese eel testis

In teleost fish, the progestin 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) is an essential component of the spermatogenesis pathway. In a series of investigations on the mechanisms underlying progestin-stimulated spermatogenesis, we have found that DHP up-regulates the expression of cysteine dioxygenase1 (CDO1) in the Japanese eel testis. CDO1 is one of the enzymes involved in the taurine biosynthesis pathway. To evaluate whether taurine is synthesized in the eel testis, cysteine sulfinate decarboxylase (CSD), another enzyme involved in taurine synthesis, was isolated from this species. RT-PCR and in vitro eel testicular culture revealed that although CSD was also expressed in eel testis, neither DHP nor other sex steroids affect CSD mRNA expression in a similar manner to CDO1. Using an in vitro eel testicular culture system, we further investigated the effects of DHP on taurine synthesis in the eel testis. HPLC analysis showed that DHP treatment significantly increases the taurine levels in the eel testis. These results suggest that DHP promotes taurine synthesis via the up-regulation of CDO1 mRNA expression during eel spermatogenesis. Furthermore, we observed from our analysis that although taurine does not induce complete spermatogenesis, it promotes spermatogonial DNA synthesis and the expression of Spo11, a meiosis-specific marker. These data thus suggest that taurine augments the effects of sex steroids in the promotion of spermatogonial proliferation and/or meiosis and hence that taurine plays important roles in spermatogenesis.     

Autonomous differentiation of Drosophila spermatogonia in vitro

Spermatogenesis is a complex process that produces functional sperm by establishing male germline stem cells (mGSCs) in adult testes. To study Drosophila spermatogenesis in vitro , we examined various culture conditions of spermatogonia. Spermatogonia from larval testes began to differentiate soon after culture, whereas mGSCs did not undergo self-renewal division. Strikingly, 16-cell spermatogonia from early and late larval testes differentiated into motile spermatids autonomously. Furthermore, individual spermatogonia developed into motile spermatids even after mechanical dissociation from encapsulating cyst cells. This is the first study to report that spermatogonia in larval testes retain the ability to differentiate into spermatids in the absence of gonadal tissue. Our in vitro system should provide an excellent opportunity to study spermatogenesis in detail and apply genetic manipulation.     

Influence of gonadotropins on adrenalectomy induced changes in the testis of albino mouse

Effects of adrenalectomy and administration of gonadotropins on cell counts of different cell types of spermatogenesis and morphology of the Leydig cells were studied in 30 day old mice. Adrenalectomy (duration, 12 days; age at autopsy 42 days) caused a significant decrease in the diameters of seminiferous tubules and Leydig cell nucleus and, cell counts of intermediate spermatogonia, round and elongated spermatids. Administration of FSH (75 micrograms/0.1 ml saline) + LH (25 micrograms/0.1 ml saline) everyday for 12 days to adrenalectomized mice restored testicular activity as revealed by significant increases in mean diameter of the Leydig cell nuclei and cell counts of intermediate spermatogonia and elongated spermatids over those of adrenalectomized mice. The results indicate that (i) testis of adrenalectomized mouse responds to gonadotropin treatment and (ii) impairment in gonadotropin secretion is possibly a major factor in inducing testicular regression following adrenalectomy.     

Cultivation of boar spermatogonia on Sertoli cells

We studied the in vitro effect of Sertoli cells on boar spermatogonia isolated from the testes of 60-day-old crossbred boars. In order to enrich the culture with spermatogonia, the cells were purified by density gradient centrifugation with the use of Percoll gradient followed by separation based on adhesive capacities of cells. We found lipid drops stained by Oil Red O in Sertoli cells. The experiments showed that the cultivation of boar spermatogonia in the presence of Sertoli cells (for up to 35 days) provide the same way of differentiation as in testes in natural conditions. After 10 days of cultivation, spermatogenic cells form groups, chains, and suspension clusters. By this time, spermatogenic colonies are formed; we analyzed the expression of Nanog and Plzf genes in these colonies by real-time PCR. The expression rate of Nanog gene in experimental cell clones obtained by the short-term cultivation of spermatogonia cells in the presence of Sertoli cells was 200 times higher than in freshly isolated spermatogonia cells. The product of Plzf gene expression was found both in freshly isolated spermatogenic cells and in cell clones obtained in vitro. After long-term cultivation of spermatogonia on Sertoli cells, we observed in vitro differentiation to the lineage of spermatogenesis and formation of separate motile sperm cells after 30–33 days. At this stage, the cell population was heterogeneous. In the absence of Sertoli cells, the differentiation of boar spermatogonia cells in culture stopped after 7 days of cultivation. The data show that the cultivation of boar spermatogonia cells on Sertoli cells contributes to their in vitro differentiation to the lineage of spermatogenesis and can help to obtain boar sperm cell culture.