c-fos- and c-jun-like mRNA expression in frog (Rana esculenta) testis during the annual reproductive cycle

Kisspeptin regulates steroidogenesis and spermiation in anuran amphibian

Kisspeptin (Kp) system has a recognized role in the control of gonadotropic axis, at multiple levels. Recently, a major focus of research has been to assess any direct activity of this system on testis physiology. Using the amphibian anuran, Pelophylax esculentus, as animal model, we demonstrate - for the first time in non-mammalian vertebrate - that testis expresses both Kiss-1 and Gpr54 proteins during the annual sexual cycle and that ex vivo 17B-estradiol (E₂, 10^-6 M) increases both proteins over control group. Since the interstitium is the main site of localization of both ligand and receptor, its possible involvement in the regulation of steroidogenesis has been evaluated by ex vivo treatment of testis pieces with increasing doses of Kp-10 (10^-9-10^-6 M). Treatments have been carried out in February - when a new wave of spermatogenesis occurs - and affect the expression of key enzymes of steroidogenesis inducing opposite effects on testosterone and estradiol intratesticular levels. Morphological analysis of Kp-treated testes reveals higher number of tubules with spermatozoa detached from Sertoli cells than control group and the expression of connexin 43, the main junctional protein in testis, is deeply affected by the treatment. In spite of the effects on spermatozoa observed ex vivo, in vivo administration of Kp-10 has been unable to induce sperm release in cloacal fluid. In conclusion, we demonstrate Kp-10 effects on steroidogenesis with possible involvement in the balance between testosterone and estradiol levels, and report new Kp-10 activities on spermatozoa-Sertoli cell interaction.

Effect of estradiol on apoptosis, proliferation and steroidogenic enzymes in the testes of the toad Rhinella arenarum (Amphibia, Anura)

Estrogens inhibit androgen production and this negative action on amphibian steroidogenesis could be related to the regulation of steroidogenic enzymes. Estrogens are also involved in the regulation of amphibian spermatogenesis by controlling testicular apoptosis and spermatogonial proliferation. The Bidder's organ (BO) is a structure characteristic from the Bufonidae family and in adult males of Rhinella arenarum it is one of the main sources of plasma estradiol (E2). The purpose of this study is to analyze the effect of E2 on testicular steroidogenic enzymes, apoptosis and proliferation in the toad R. arenarum. For this purpose, testicular fragments were treated during 24h with or without 2 or 20nM of E2. After treatments, the activities of cytochrome P450 17α-hydroxylase-C17-20 lyase (CypP450c17) and 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) were measured by the transformation of radioactive substrates into products, and CypP450c17 expression was determined by Western blot analysis. Apoptosis in testicular sections was detected with a commercial fluorescent kit based on TUNEL method, and proliferation was evaluated by BrdU incorporation. Results indicate that E2 has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-HSD/I activity during the post reproductive season. Furthermore, although E2 has no effect on apoptosis during the pre and the post reproductive seasons, it stimulates testicular apoptosis during the reproductive season, mostly in spermatocytes. Finally, E2 has no effect on testicular proliferation all year long. Taken together, these results suggest that E2 is involved in the regulation of testicular steroidogenesis and spermatogenesis.

Hypothalamus-pituitary axis: an obligatory target for endocannabinoids to inhibit steroidogenesis in frog testis

Endocannabinoids - primarily anandamide (AEA) and 2-arachidonoylglycerol (2-AG) - are lipophilic molecules that bind to cannabinoid receptors (CB1 and CB2). They affect neuroendocrine activity inhibiting gonadotropin releasing hormone (GnRH) secretion and testosterone production in rodents, through a molecular mechanism supposed to be hypothalamus dependent. In order to investigate such a role, we choose the seasonal breeder, the anuran amphibian Rana esculenta, an experimental model in which components of the endocannabinoid system have been characterized. In February, at the onset of a new spermatogenetic wave, we carried out in vitro incubations of frog testis with AEA, at 10(-9)M dose. Such a treatment had no effect on the expression of cytochrome P450 17alpha hydroxylase/17,20 lyase (cyp17) nor 3-β-hydroxysteroid dehydrogenase/Δ-5-4 isomerase (3β-HSD), key enzymes of steroidogenesis. To understand whether or not the functionality of the hypothalamus-pituitary axis could be essential to support the role of endocannabinoids in steroidogenesis, frogs were injected with AEA, at 10(-8)M dose. Differently from in vitro experiment, the in vivo administration of AEA reduced the expression of cyp17 and 3β-HSD. Whereas the effect on 3β-HSD was counteracted by SR141716A (Rimonabant) - a selective antagonist of CB1, thus indicating a CB1 dependent modulation - the effect on cyp17 was not, suggesting a possible involvement of receptors other than CB1, probably the type-1 vanilloid receptor (TRPV1), since AEA works as an endocannabinoid and an endovanilloid as well. In conclusion our results indicate that endocannabinoids, via CB1, inhibit the expression of 3β-HSD in frog testis travelling along the hypothalamus-pituitary axis.

Estrogens and spermiogenesis: new insights from type 1 cannabinoid receptor knockout mice

Spermatogenesis is a complex mechanism which allows the production of male gametes; it consists of mitotic, meiotic, and differentiation phases. Spermiogenesis is the terminal differentiation process during which haploid round spermatids undergo several biochemical and morphological changes, including extensive remodelling of chromatin and nuclear shape. Spermiogenesis is under control of endocrine, paracrine, and autocrine factors, like gonadotropins and testosterone. More recently, emerging pieces of evidence are suggesting that, among these factors, estrogens may have a role. To date, this is a matter of debate and concern because of the agonistic and antagonistic estrogenic effects that environmental chemicals may have on animal and human with damaging outcome on fertility. In this review, we summarize data which fuel this debate, with a particular attention to our recent results, obtained using type 1 cannabinoid receptor knockout male mice as animal model.

The contribution of lower vertebrate animal models in human reproduction research

Many advances have been carried out on the estrogens, GnRH and endocannabinoid system that have impact in the reproductive field. Indeed, estrogens, the generally accepted female hormones, have performed an unsuspected role in male sexual functions thanks to studies on non-mammalian vertebrates. Similarly, these animal models have provided important contributions to the identification of several GnRH ligand and receptor variants and their possible involvement in sexual behavior and gonadal function regulation. Moreover, the use of non-mammalian animal models has contributed to a better comprehension about the endocannabinoid system action in several mammalian reproductive events. We wish to highlight here how non-mammalian vertebrate animal model research contributes to advancements with implications on human health as well as providing a phylogenetic perspective on the evolution of reproductive systems in vertebrates.

Non-mammalian vertebrate models and the endocannabinoid system: relationships with gonadotropin-releasing hormone

Endocannabinoids, via cannabinoid receptors (CB1 and CB2), affect reproductive functions at both local and central level. Due to the high complexity of the endocannabinoid system, to the widespread distribution outside the nervous system and to the high degree of evolutionary conservation, a deep CB1 molecular characterization among species may be useful to elucidate the activity of endocannabinoids at multiple levels. In this review we report CB1 characterization in non-mammalian animal models and, in particular, in the anuran amphibian, the frog, Rana esculenta; we also describe its expression during the annual sexual cycle. Moreover, since reproductive functions are under control of gonadotropin releasing hormone (GnRH), cb1 mRNA and protein expression profile in the forebrain has been compared to those of GnRH-I, the mammalian form primarily involved in gonadotropin release.

Type-1 cannabinoid receptor expression in the frog,Rana esculenta, tissues: A possible involvement in the regulation of testicular activity

Endogenous cannabinoids and type-1 cannabinoid receptor (CB1) are widely produced and distributed in the central nervous system (CNS) and peripheral nerves in mammals. In addition, the detection of endocannabinoids and corresponding receptors in non nervous peripheral tissues indicates an involvement of the system in the control of a wide range of physiological activities, including reproduction. Recently, the existence of CB1 was also observed in lower vertebrates and in urochordate suggesting that the endocannabinoid system is phylogenetically conserved. Using RT-PCR, CB1 mRNA expression profiles were characterized in a wide range of tissues of the anuran amphibian, the frog, Rana esculenta. Besides a strong expression in the CNS, CB1 was also present in testis, kidney, liver, ovary, muscle, heart, spleen, and pituitary. The CB1 expression pattern has been characterized in both testis and CNS during the annual sexual cycle. In testis, CB1 is poorly expressed during the winter stasis of the spermatogenesis rising during the breeding season and resumption period. An expression profile mismatching to that observed in testis was detected in whole-brain preparations during the sexual cycle; in particular in the diencephalon, the encephalic area mainly involved in the control of reproductive functions. Furthermore, fluctuations inside isolated encephalic areas and spinal cord were observed all over the reproductive cycle. In conclusion, CB1 receptor is expressed in R. esculenta CNS and testis. As far as the gonad it concerns, our results suggest the involvement of the endocannabinoids in the control of reproductive function.

Testicular Activity of Mos in the Frog, Rana esculenta: A New Role in Spermatogonial Proliferation1

Mos is a MAPK kinase kinase with an expression that is highly restricted to the gonads. Its function is mainly associated to the meiotic metaphase II arrest occurring during female gametogenesis, whereas to our knowledge, its role during spermatogenesis has not yet clarified. In the present paper, we report the isolation of c-mos cDNA and the identification of a 60-kDa Mos protein from the testis of the anuran amphibian, Rana esculenta. Both the transcript and the protein are always present at low levels in the testis during the frog annual sexual cycle, with single significant peaks of expression in March and May, respectively. Mos is mainly localized in the cytoplasm of primary and secondary spermatogonia (SPG). Therefore, we have used treatments with ethane-dimethane sulphonate (EDS), which blocks spermatogonial mitosis in frogs. Four days after a single EDS injection, Mos expression in SPG highly increases concomitantly with the temporary arrest of mitosis. From 8 to 28 days after the injection, the normal proliferative activity of SPG is restored, and Mos expression gradually decreases to control levels. These results strongly indicate that the c-mos proto-oncogene exerts a new role associated to the regulation of spermatogonial proliferation.

Jun localization in cytosolic and nuclear compartments in brain-pituitary system of the frog, Rana esculenta: an analysis carried out in parallel with GnRH molecular forms during the annual reproductive cycle

The presence of c-jun like mRNA was assessed in the brain of the frog, Rana esculenta, during the annual sexual cycle. In parallel, Jun protein and GnRH molecular form (mammalian and chicken II also indicated as GnRH1 and GnRH2, respectively) activity was studied in order to establish possible relationships. Northern blot analysis of total RNA reveals the presence of a 2.7 kb c-jun-like mRNA. Western blots, carried out on cytoplasmic and nuclear protein extracts, show the presence of Jun immunoreactive band of 39 kDa in brain and pituitary. Fluctuations of c-jun-like mRNA and Jun immunoreactive protein (cytoplasmic and nuclear) levels in brains during the year indicate relationships among transcription, translation, and nuclear activity. In particular, mRNA levels increase gradually from September until November when Jun protein concentration peaks in cytosolic extracts. Conversely, the nuclear protein reaches highest concentration in July when the cytosolic level shows low values. Immunocytochemical studies confirm the presence of Jun immunoreactivity in both cytoplasmic and nuclear compartments of several brain areas, including those primarily involved in gonadotropin discharge (e.g., anterior preoptic area and preoptic nucleus). GnRH molecular forms and Jun are colocalized in anterior preoptic area and preoptic nucleus. Moreover, during the period characterized by GnRH release, Jun levels strongly decrease in nuclei. Finally, we show that treatments with a GnRH analog (buserelin, Hoechst, Frankfurt) increase Jun levels in brain nuclear extracts.

Cytoplasmic versus nuclear localization of Fos-related proteins in the frog, Rana esculenta, testis: in vivo and direct in vitro effect of a gonadotropin-releasing hormone agonist

Evidence has been accumulated indicating that GnRH-like peptides are present in a variety of extrabrain areas of mammalian and nonmammalian vertebrates. A pioneer study carried out in the frog, Rana esculenta, demonstrated that testicular GnRH induced spermatogonial proliferation. Recently, we have shown that in proliferating spermatogonia (SPG) of frogs, a change of localization of the oncoprotein Fos, from the cytoplasm to the nucleus, occurs. This leads to the hypothesis that one or more testicular GnRH peptides may regulate SPG proliferation through Fos family proteins. Therefore, in vivo experiments in intact R. esculenta and in vitro incubations of testis fragments have been carried out using GnRH agonist (GnRHa; buserelin) and GnRH antagonist (D-pGlu(1),D-Phe(2),D-Trp(3,6)-GnRH). Cytoplasmic and nuclear Fos-like protein localization has been found by Western blot analysis in testicular extracts. Immunocytochemistry confirmed that cytoplasmic immunostaining was restricted to SPG; change of localization into the nuclear compartment was observed after GnRHa treatment. Northern blot analysis showed that treatments of testis fragments with GnRHa did not modify testicular c-fos mRNA expression. On the contrary, a Fos-like protein of 52 kDa, while not affected in vivo, disappeared from testicular cytosolic extracts after in vitro treatment with GnRHa. Contemporaneously, a 55-kDa Fos-related signal appeared in nuclear extracts. The GnRH antagonist counteracted the effects of GnRHa. Furthermore, in vivo treatments showed that GnRHa acted negatively on a 43-kDa nuclear Fos-related signal and that gonadotropins caused the decrease of 52-kDa cytoplasmic signal. In conclusion, we show, to our knowledge for the first time, that Fos is regulated by GnRHa directly (not through the pituitary) at the testicular level. The main effect appears to be related to Fos translocation from cytoplasmic to nuclear compartments of SPG.

The amphibian testis as model to study germ cell progression during spermatogenesis

Testicular morphology of vertebrate testis indicates requirement of local control. In urodeles, the testis is organized in lobes of increasing maturity throughout the cephalocaudal axis. The anuran testis is organized in tubules. Spermatogenesis occurs in cysts composed by Sertoli cells enveloping germ cells at synchronous stages. Moreover, in numerous species germ cell progression lasts a year which defines the sexual cycle. Due to the above quoted features, research on factors regulating germ cell progression in amphibians may reach greater insight as compared with mammalian animal models. In particular, studies on endocrine and paracrine/autocrine factors involved in the regulation of germ cell functions reveal that fos activation and a J protein, previously specifically found in mouse testis, exert an important role in spermatogonial proliferation and maturation of post-meiotic stages, respectively.

Expression of FTZ-F1alpha in frog testicular cells

Fushi tarazu transcription factor-1 (FTZ-F1), a member of a nuclear hormone receptor superfamily, is a transcriptional regulator for fushi tarazu gene expression in Drosophila (Ueda et al., '90). We have cloned a homologue (rrFTZ-F1alpha) of the FTZ-F1 gene of the frog Rana rugosa. The gene, in frogs, has been shown to have high expression level in the testis (Nakajima et al., 2000). In this study, the RT-PCR analysis showed that the FTZ-F1alpha mRNA level in adult frogs did not change throughout the year, even during hibernation. However, when immunohistological studies using the anti-rrFTZ-F1alpha antibody were employed to examine which testicular cells expressed this gene, Sertoli cells were found to produce rrFTZ-F1alpha in the two seasons: the breeding season (from March through May) and the pre-hibernating season (from October through November). Interstitial cells, however, did it in only the breeding season (from April through May). Taken together, the results suggest that the rrFTZ-F1alpha expression is regulated at the post-transcriptional step, and that the rrFTZ-F1alpha may play an important role(s) in the seasonal activities of Sertoli and interstitial cells in the frog testis. J. Exp. Zool. 290:182-189, 2001.

Fos localization in cytosolic and nuclear compartments in neurones of the frog, Rana esculenta, brain: an analysis carried out in parallel with GnRH molecular forms

C-fos activity was determined in the brain of the frog, Rana esculenta, during the annual sexual cycle. The localization of GnRH molecular forms (mammalian- and chicken-GnRHII) was also carried out to determine whether or not the proto-oncogene and the peptides showed a functional relationship. Northern blot analysis of total RNA revealed the presence of a single strong signal of c-fos like mRNA of 1.9 Kb during February and April. This was followed by expression of c-Fos protein (Fos) in several brain areas during March and July shown by immunocytochemistry. In particular, the olfactory region, the lateral and medial pallium, the nucleus lateralis septi, the ventral striatum, the caudal region of the anterior preoptic area, the suprachiasmatic nucleus, the ventral thalamus, tori semicircularis and ependymal layers of the tectum were immunostained. There was no overlap between Fos immunoreactive perikarya and GnRH immunoreactive perikarya (e.g. gonadotrophin-releasing hormone (GnRH) in the rostral part and Fos in the caudal region of the anterior preoptic area). Interestingly, a cytoplasmic localization of Fos was also observed by immunocytochemistry and gel retardation experiments supported this observation. Cytoplasmic extracts from September-October animals bound the AP1 oligonucleotide. The complex was not available in the nuclear extracts from the same preparation, suggesting that, besides Fos, Jun products were also present. Conversely, nuclear but not cytosolic binding was detected in the brain of animals collected in July. In conclusion, we show that Fos and GnRH activity does not correlate in the frog brain and, for the first time in a vertebrate species, we give evidence of a cytoplasmic AP1 complex in neuronal cells.

c-fos activity in Rana esculenta testis: seasonal and estradiol-induced changes

Estradiol-17beta (E2) is suspected to exert a role in the regulation of testicular activity. Using a nonmammalian vertebrate model (the frog, Rana esculenta), we have investigated whether c-fos activity is detectable in the testis during the annual sexual cycle and whether E2 exerts a regulatory role on spermatogenesis through fos activity. FOS protein is available in testicular nuclear extracts (about 60 kDa) and, surprisingly, also in cytosolic extracts (about 60, 80, and 100 kDa). Estradiol induces primary spermatogonia (ISPG) proliferation [this effect is counteracted by antiestrogens (Tamoxifen and ICI 182-780)] and FOS appearance in testicular cytosolic extracts as well as c-fos transcription. Also, this effect is counteracted by ICI 182-780. Interestingly, the number of FOS immunopositive nuclei of ISPG strongly increases after E2 treatment, whereas a great increase of immunopositivity in the cytoplasm of ISPG is observed with the contemporaneous treatment with antiestrogens. In conclusion, our results demonstrate that E2 induces ISPG multiplication in the frog, R. esculenta, and, for the first time in a vertebrate species, that it triggers c-fos activity in the testis. Moreover, E2 may be involved in mechanisms related to FOS transport in the nucleus of ISPG to induce the mitotic activity.