Using specific recombinant gonadotropins to induce spermatogenesis and spermiation in the European eel (Anguilla anguilla)

Effects of cold seawater pre-treatments on induction of early sexual maturation and sperm production in European eel (Anguilla anguilla)

To induce sexual maturation in captivity, eels rely on hormonal treatments, but this process is costly and time-consuming. As an alternative, different types of conditioning, also referred as pre-treatment, have been assessed to ease hormonal treatment response. Recent studies have shown that migrating eels experience a wide range of temperatures, varying from 12 °C at night to as low as to 8 °C during the day. Therefore, this study evaluates the effects of low-temperature (10 °C) seawater pre-treatments of different durations (2 and 4 weeks) on male eel reproduction. The eye, gonadosomatic and hepatosomatic indexes from control (without thermic seawater pre-treatment) and pre-treated fish were measured. Blood and testis samples were also collected for sex steroid and histology analysis, respectively. Eels pre-treated for 2 weeks demonstrated increased progestin levels, comparing with the control group. Eels pre-treated for 4 weeks showed significantly higher gonadosomatic index and elevated androgens and estradiol levels in comparison with the remaining groups. In eels pre-treated for 2 and 4 weeks, there was an increase in the proportion of spermatogonia type B cells compared to undifferentiated spermatogonia type A, a differentiation process that was not observed in the control group. Cold seawater pre-treatment induced early sexual maturation, including steroid production, which consequently stimulated biometric changes and increased spermatogonia differentiation. Following the pre-treatments, eels started receiving standard hormonal treatment (with recombinant human chorionic gonadotropin at 20 °C). Pre-treated males started to spermiate earlier than the control group. In some treatment weeks, pre-treated individuals registered higher values of sperm density, motility, and kinetic parameters. Moreover, an economic evaluation was carried out relating the investment made in terms of hormone injections with the volume of high-quality sperm obtained from each experimental group. The low temperature pre-treatments demonstrated their economic effectiveness in terms of hormone treatment profitability, increasing the production of high-quality sperm in the European eel. Thus, this in vivo study suggests that cold seawater pre-treatment may increase sensitivity to the hormone applied during standard maturation treatment.

Characterization of potential spermatogonia biomarker genes in the European eel (Anguilla anguilla)

Identification of specific molecular markers for spermatogonial stem cells in teleost is crucial for enhancing the efficacy of reproductive biotechnologies in aquaculture, such as transplantation and surrogate production in fishes. Since it is not yet possible to distinguish spermatogonial stem cells of European eel (Anguilla anguilla) using specific molecular markers, we isolated spermatogonial cells from immature European eels to find these potential markers. We attempted this by studying three candidate genes: vasa, nanos2, and dnd1. Two vasa (vasa1 and vasa2) genes, nanos2, and dnd1 were identified, characterized, and studied in the muscle, testis, and isolated spermatogonia. Our results showed that vasa1 and vasa2 had the highest levels of expression when measured by qPCR. In situ hybridization and immunochemistry assays showed that the four genes were localized explicitly in type A spermatogonia. However, vasa1 and vasa2 exhibited stronger signals in the immature testicular tissue than the other two potential markers. According to this, vasa1 and vasa2 were found to be the most effective markers for spermatogonial cells in the European eel.

In vivo effect of recombinant Fsh and Lh administered to meagre (Argyrosomus regius) at the initial stages of sex differentiation

Recombinant gonadotropins, follicle stimulating (rFsh) and luteinizing hormone (rLh), offer the potential to induce gametogenesis in prepubertal fish. This study aimed to determine the in vivo effect of the administration of Argyrosomus regius rFsh and rLh on the reproductive development of prepubertal meagre juveniles at the initial stages of sexual differentiation. Juvenile meagre, 9-months old with mean weight of 219 ± 3.9 g (mean ± SEM) were randomly distributed into nine groups (n = 8 per group). Experimental groups were treated weekly with an acute injection of either rFsh or rLh. Control groups were injected with saline solution. In a 3-week experiment, different groups were administered with different doses 6, 12 or 18 µg kg-1 of rFsh or rLh or saline solution. In a 6-week experiment a group was administered with 12 µg kg-1 of rFsh and a second group with saline solution. The fish were held in a single 10 m3 tank with natural photoperiod (Feb. - March) and temperature 16.1 ± 0.4 °C. At the start of the experiment (n = 8) and at the end of the 3-week experiment, fish were blood sampled and sacrificed. Blood was analysed for 17β-estradiol (E2) and 11-ketotestosterone (11-KT). Gonads and liver were dissected and weighed. Gonads were fixed in Bouińs solution and processed for histological analysis. Juvenile meagre at the start of the experiment were in the initial stages of sexual differentiation, indicated by the presence of the ovarian cavity or testes duct that was surrounded by undifferentiated embryonic germ stem cells and somatic cells. At the end of the 3-week experiment, there was no significant difference in gonadosomatic index (GSI) amongst control (initial and saline treated) and the experimental groups. After three weeks of application of rFsh, rLh or saline all fish presented a similar gonadal structure as at the start of the experiment. However, the incidence of sporadic developing germ cells (principally spermatogonia, spermatocytes, spermatids, but also perinucleolar stage oocytes) generally increased in rGth treated meagre. A mean of 44 % of meagre treated with rFsh or rLh presented sporadic isolated developing germ cells, mainly male cells. Plasma steroid levels of E2 decreased significantly from the start of the experiments to the end. At the end of the experiments there were no differences in plasma E2 amongst Control fish and rGth treated fish. Plasma 11-KT showed no change from the start of the experiment to week 3. However, a significant increase was observed in a proportion of the rFsh group after six weeks of treatment compared to the start of the experiment and the saline control group on week 6. The application of rFsh or rLh to meagre at the initial stages of sex differentiation did not stimulate steroid production until week six (11-KT) and had a limited, but evident effect on the development of sporadic isolated germ cells. However, we conclude that rGth, rFsh or rLh did not stimulate large developmental changes in sexually undifferentiated meagre gonads.

Recent insights into egg quality and larval vitality of the European eel Anguilla anguilla

To date, the eel industry still depends on wild-caught juveniles that are grown to marketable size. There is an urgent need to close the eel life cycle in captivity to make aquaculture independent of the natural population. With this artificial reproduction protocol, yolk-sac larvae can be produced but egg quality may be impaired. Low survival rates and high deformity rates are frequently observed during the first week after hatching. Over the past four years, we have conducted studies with the aim to optimize the artificial reproduction protocol, thereby focussing on increasing egg and larval quality. Weekly carp or salmon pituitary extract (PE) treatment was successfully replaced with recombinant gonadotropins (rGTHs) to mature female eels and produce larvae. 17α,20β-dihydroxy-4-pregnen-3-one (DHP) was replaced with upstream precursor progesterone (P) to induce the endogenous production of DHP by the female eel. DHP and P were found equally potent in inducing oocyte maturation and ovulation. The effects of antibiotics on larval survival and the occurrence of deformities were investigated. Antibiotic treatment increased survival and decreased the occurrence of deformities indicating bacterial infection as an important cause. A deformity determination key for young eel larvae has been developed that provides a framework of reference for larval deformities which will be instrumental with gaining insights on the reasons behind each larval deformity. These improvements of the artificial reproduction protocol and hatchery practices will contribute to the production of robust eel larvae that survive, grow and metamorphose into juveniles that will later be able to reproduce in captivity.

Superoxidase dismutases (SODs) in the European eel: Gene characterization, expression response to temperature combined with hormonal maturation and possible migratory implications

Superoxide dismutases (SODs) are antioxidant enzymes that protect cells from oxidation. Three SODs have been identified in mammals, but there is limited information in teleosts. This study investigates SODs in the European eel and their expression patterns during testis maturation. Phylogenetic and synteny analyses revealed SODs paralogs and their evolution in vertebrates. The eel possesses one SOD1 and two SOD2/3 (a and b), indicating SOD2 and SOD3 duplication in elopomorphs. SODs expression were then evaluated in various male and female tissues. SOD1 is more expressed in females, while SOD2a and SOD2b dominate brain-pituitary-gonad tissues in both sexes. SOD3a showed predominant expression in the ovary and the male livers, whereas SOD3b was found in the pituitary and brain of both sexes. The effects of different maturation protocols (standard hormonal treatment vs. same protocol preceded with cold seawater pre-treatment) on SODs expression during testis maturation were evaluated. Salinity increase at the onset of standard treatment at 20 °C, simulating early migration, upregulated SOD1, SOD2a, and SOD2b, coinciding with spermatogonia type A differentiated cells dominance. Thereafter, SOD2a and SOD3a decreased, while SOD2b increased during hormonal treatment-induced spermatogenesis. Pre-treatment with seawater at 10 °C, mimicking the conditions at the beginning of the seawater migration, downregulated SOD1 but increased SOD3a expression. Finally, the standard hormonal treatment, replicating spawning at higher temperatures, downregulated SOD1 in eels without any pre-treatment while SOD2a expression increased in pre-treated eels. This study revealed tissue-specific, sex-dependent, and maturation-related SOD expression patterns, predicting SODs dynamic expression profiles during their reproductive migration.

Comparative evaluation of the effects of different activating media and temperatures on European eel (Anguilla anguilla) sperm motility assessed by computer assisted sperm analysis

The European eel is a critically endangered teleost fish with very poor success rate for captive breeding and artificial reproduction. Therefore, to support its conservation, new strategies are needed to ensure fertilization. Objective analysis of sperm motility may be critical as it potentially represents one of the most important reproductive quality parameters. Spermatozoa acquire motility once in contact with hyperosmotic solutions as saltwater, yet the exact mechanisms and the role of temperature are still to be clarified. The main aim of the study was to assess the effects of 3 activating media (artificial sea water, tank water and commercial Actifish®) at 4 and 20 °C on sperm motility, by means of computer assisted sperm analysis. Secondary aim was to test 2 different concentrations of Actifish® mimicking sea water pH/osmolality, at 4 °C. The results suggested how both temperature and activating media have effects on spermatozoa motility and kinematics, with temperature mainly acting upon interaction with the media type. The samples activated with tank water at 20 °C showed the poorest motility outcomes (mean 38.1%), while the ones activated with Actifish® diluted 1:4 and artificial sea water, at 4 °C, the highest (means 51.8 and 51.5% respectively). Additionally, diluting Actifish® to reach same pH and osmolality of seawater led to worse motility outcomes, suggesting that composition may be the critical factor for activation rather than osmolality itself.

Regulation of steroid production and key genes in catfish Heteropneustes fossilis using recombinant gonadotropins

The two gonadotropins, FSH and LH, stimulate growth and development of the gonads through gonadal biosynthesis of steroid hormones and growth factors. To date, cDNA sequences encoding gonadotropin subunits have been isolated and characterized from a large number of fish species. Recently, we successfully cloned and characterized gonadotropins (LHβ, FSHβ, and GPα) from the pituitary glands of the catfish, Heteropneustes fossilis. In the present study, we describe herein the production of recombinant stinging catfish, H. fossilis (hf) FSH (rhfFSH) and LH (rhfLH) using the methylotrophic yeast P. pastoris expression system. We further explored the hypothesis that the recombinant gonadotropins can modulate the hypothalamus-pituitary-ovarian (HPO) axis genes (avt, it, gnrh2, kiss2, and cyp19a1a) and regulate their transcriptional profile and steroid levels in relation to their annual developmental stage during preparatory and pre-spawning phases under in-vitro conditions. We found that the different concentrations of recombinant rhfFSH and rhfLH significantly stimulated E2 levels in the preparatory and prespawning season, and also upregulated gonadal aromatase gene expression in a dose dependent manner. Our results demonstrate that the yeast expression system produced biologically active recombinant catfish gonadotropins, enabling the study of their function in the catfish.

Establishment of cell-lines stably expressing recombinant Japanese eel follicle-stimulating hormone and luteinizing hormone using CHO-DG44 cells: fully induced ovarian development at different modes

The gonadotropins (Gth), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play central roles in gametogenesis in vertebrates. However, available information on their differential actions in teleost, especially in vivo, is insufficient. In this study, we established stable CHO-DG44 cell lines expressing long-lasting recombinant Japanese eel Fsh and Lh with extra O-glycosylation sites (Fsh-hCTP and Lh-hCTP), which were produced in abundance. Immature female eels received weekly intraperitoneal injections of Gths. Fsh-hCTP induced the entire ovarian development by 8 weeks from the beginning of injection; thus, the ovaries of most fish were at the migratory nucleus stage while the same stage was observed in eels after 4 weeks in the Lh-hCTP-treated group. In contrast, all pretreated and saline-injected eels were in the pre-vitellogenic stage. Gonadosomatic indices in the Fsh-hCTP-treated group were significantly higher than those in the Lh-hCTP group at the migratory nucleus stage because of the significantly higher frequency of advanced ovarian follicles. Ovarian mRNA levels of genes related to E2 production (cyp11a1, cyp17a1, cyp19a1, hsd3b, fshr, and lhr) were measured using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). All genes were induced by both Fsh-hCTP and Lh-hCTP, with a peak at either the mid- or late vitellogenic stages. Transcript abundance of cyp19a1 and fshr in the Lh-hCTP group were significantly higher than those in the Fsh-hCTP group, whereas no difference in the expression of other genes was observed between the groups. Fluctuations in serum levels of sex steroid hormones (estradiol-17β, 11-ketotestosterone, and testosterone) in female eels were comparable in the Fsh-hCTP and Lh-hCTP groups, thus increasing toward the maturational phase. Furthermore, the fecundity of the eels induced to mature by Fsh-hCTP was significantly higher than that induced by Lh-hCTP. These findings indicate that Fsh and Lh can induce ovarian development in distinctively different modes in the Japanese eel.

Male germ cell proliferation and apoptosis in sexually immature meagre Argyrosomus regius (Asso, 1801) treated with recombinant follicle stimulating hormone

The meagre Argyrosomus regius (Asso, 1801) is a marine fish species that has an increasing aquaculture production in Europe. Lowering the age at maturity of hatchery-produced juveniles would support meagre aquaculture by reducing time between generations in selective breeding programs and reducing industrial costs for broodstock maintenance. The aim of this work was to assess the effects of a treatment with recombinant follicle stimulating hormone (rFsh), produced in ovarian cells of Chinese hamsters, on male germ cell proliferation and apoptosis in sexually immature meagre. The rFsh-treated fish had higher gonadosomatic index, larger seminiferous tubules, more abundant luminal spermatozoa, a lower density of anti-PCNA positive single A spermatogonia, a higher density of anti-PCNA positive spermatocysts and a lower incidence of germ cell apoptosis than control groups. The present study demonstrated the effectiveness of the produced rFsh in stimulating testis development and spermatogenesis in pre-pubertal meagre. Moreover, the rFsh treatment proved to be highly efficient in removing the apoptotic block of spermatogenesis observed in juvenile meagre, allowing spermatogonial survival and progress towards meiosis. The administration of rFsh did not stimulate spermatogonial self-renewal, a process whose control still needs to be elucidated.

Recombinant Fsh and Lh therapy for spawning induction of previtellogenic and early spermatogenic arrested teleost, the flathead grey mullet (Mugil cephalus)

With the expansion and diversification of global aquaculture, efforts continue to develop new bio-technologies for assisted reproduction in species that present reproductive dysfunctions. Flathead grey mullet (Mugil cephalus) males held in intensive conditions in the Mediterranean region do not produce fluent milt and most females are arrested at previtellogenesis. The weekly injections of recombinant follicle stimulating hormone (rFsh) and luteinizing hormone (rLh) induced and completed vitellogenesis in treated females (n = 21), and treated males produced fluent sperm (n = 9). The application of a priming dose of 30 µg kg^-1 rLh and resolving dose of 40 mg kg^-1 Progesterone, or priming and resolving doses of 30 µg kg^-1 rLh, resulted in the induction of maturation, ovulation, and spontaneous spawns with a spawning success of the 85% (8 of 9 females) and 100% (n = 6), respectively. The eggs collected had 63 ± 21% fertilization with embryo development and 58 ± 23% hatching. In comparison, control individuals did not show advances in gonadal development and did not produce fluent sperm. The present results confirm the possibility of controlling oogenesis from previtellogenesis to the completion of maturation and fertilised tank spawning using exclusively rFsh and rLh in a teleost species.

Comparative transcriptomic analysis of gonadal development and renewal in the ovoviviparous black rockfish (Sebastes schlegelii)

BACKGROUND

The black rockfish (Sebastes schlegelii) has an ovoviviparous reproductive pattern and long-term sperm storage, resulting in asynchronous gonadal development between the sexes. However, the comprehensive understanding of gonadal development in black rockfish has not yet been achieved. Here, we studied gonadal development and germ cell renewal using histology and RNA-seq.

RESULTS

In this study, RNA-seq was performed on testes and ovaries to characterize key pathways and genes that are active during development and gamete maturation in black rockfish. Differentially expressed genes (DEGs) were identified and annotated in 4 comparisons (F_III vs. F_IV, F_IV vs. F_V, M_III vs. M_IV and M_IV vs. M_V). Based on analysis of DEGs enriched in the testis, 11 and 14 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were mapped to the M_III vs. M_IV group and the M_IV vs. M_V group, respectively. DEGs in ovarian development were also classified into 10 groups according to their biological functions. The expression patterns of the selected genes determined by qPCR were significantly correlated with the RNA-Seq results, supporting the reliability and accuracy of the RNA-Seq analysis. E2 levels showed down regulation from previtellogenesis to mature stage in female and T level showed down regulation from spermatogenesis to regressed stage in the male.

CONCLUSIONS

The categories "intercellular interaction and cytoskeleton", "molecule amplification" and "repair in the cell cycle" were revealed to be crucial in testis development and spermatogenesis, as was the biosynthesis of a series of metabolites. Our results provide comprehensive insight into black rockfish gonadal development and provide a basis for further study of reproductive physiology and molecular biology in ovoviviparity teleosts.

C-terminal peptide (hCTP) of human chorionic gonadotropin enhances in vivo biological activity of recombinant Japanese eel follicle-stimulating hormone and luteinizing hormone produced in FreeStyle 293-F cell lines

Gonadotropins (Gths), follicle-stimulating hormone (Fsh), and luteinizing hormone (Lh) play central roles in the reproductive biology of vertebrates. In this study, recombinant single-chain Japanese eel Gths (rGth: rFsh and rLh), and recombinant chimeric Gths (rGth-hCTPs: rFsh-hCTP and rLh-hCTP; rGth-eCTPs: rFsh-eCTP and rLh-eCTP) with an extra O-glycosylation site (either a C-terminal peptide of human (hCTP) or equine (eCTP) chorionic gonadotropin), which are known to prolong the half-life of glycoprotein were produced in HEK293 cells and highly purified. Lectin blot analyses demonstrated that all these recombinant Gths contained N-glycans of the high mannose and complex types. In contrast, only rGth-hCTPs and rGth-eCTPs possessed highly sialylated O-linked oligosaccharides. Further analyses of glycans by liquid chromatography-mass spectrometry suggested that the species, amount, and degree of sialylation of N-glycans were comparable among recombinant Fshs and recombinant Lhs, while the amount of O-glycans with sialic acids in rGth-hCTPs was higher than that in the corresponding rGth-eCTPs. The serum levels of recombinant Gths in male eels significantly increased 12-24 h after a single injection of the Gths. The levels of rGth-hCTPs tended to be higher than those of the corresponding rGths and rGth-eCTPs throughout the experimental period, coinciding with the serum fluctuations of 11-ketotestosterone (11KT). The long-term treatment of male eels with these recombinant Gths also revealed the superiority of rGth-hCTPs in assisted reproduction; thus, the serum levels of 11KT and gonadosomatic indices in eels treated with rGth-hCTPs were higher than those in eels treated with the corresponding rGths and rGth-eCTPs. The induction of the entire process of spermatogenesis was only histologically observed in rGth-hCTPs-treated eels. These findings strongly suggest that hCTP enhances the in vivo biological activity of recombinant Japanese eel Gths due to the high abundance of O-linked glycans with sialylated antennae.

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Generation and use of recombinant gonadotropins in fish

Understanding the differential roles of the pituitary gonadotropins Fsh and Lh in gonad maturation is crucial for a successful manipulation of the reproductive process in fish, and requires species-specific tools and appropriate active hormones. With the increasing availability of fish cDNAs coding for gonadotropin subunits, the production of recombinant hormones in heterologous systems has gradually substituted the approach of isolating native hormones. These recombinant hormones can be continually produced without depending on the fish as starting material and no cross-contamination with other pituitary glycoproteins is assured. Recombinant gonadotropins should be produced in eukaryotic cells, which have glycosylation capacity, but this post-translational modification varies greatly depending on the cell system, influencing hormone activity and stability. The production of recombinant gonadotropin beta-subunits to be used as antigens for antibody production has allowed the development of immunoassays for quantification of gonadotropins in some fish species. The administration in vivo of dimeric homologous recombinant gonadotropins has been used in basic studies and as a biotechnological approach to induce gametogenesis. In addition, gene-based therapies using somatic transfer of the gonadotropin genes have been tested as an alternative for hormone delivery in vivo. In summary, the use of homologous hormonal treatments can open new strategies in aquaculture to solve reproductive problems or develop out-of-season breeding programs.

Eel sperm cryopreservation: An overview

The eels are teleost fishes from the order Anguilliformes that includes several species with high commercial value. Due to the high interest for aquaculture production of some eel species and for the need to restore eel species that are endangered, several research groups have directed their research toward developing protocols to cryopreserve the spermatozoa of Japanese eel (Anguilla japonica) and European eel (Anguilla anguilla). In this review, we provide an overview on the different protocols that have been developed so far. The first developed protocols used DMSO as cryoprotectant in both species with good success, obtaining sperm motilities of over 45% in Japanese eel and over 35% in European eel. Moreover, sperm cryopreserved using DMSO was successfully used in fertilization trials, although with low fertilization rates. However, recent studies show that DMSO produce epigenetic changes in eel sperm and therefore, the last developed protocols used methanol as cryoprotectant instead. Cryopreservation protocols using methanol as cryoprotectant, showed improved motility values in both Japanese and European eel. In addition, the latest protocols have been adapted to cryopreserve larger volumes of sperm of up to 5 mL, which is useful for larger scale fertilization trials. The present study introduces the state of the art and future perspectives of the eel sperm cryopreservation to be applied in aquaculture and biological conservation programs.

Functional analysis of recombinant single-chain Japanese eel Fsh and Lh produced in FreeStyle 293-F cell lines: Binding specificities to their receptors and differential efficacy on testicular steroidogenesis

Pituitary gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play central roles in the control of gonadal development of vertebrates. In mammals, Fsh and Lh exclusively activate their respective cognate receptors: Fsh receptor (Fshr) in the Sertoli cell and Lh/choriogonadotropin receptor (Lhcgr) in the Leydig cell. In teleosts, the distinct functions of Fsh and Lh and information on cellular localization of their receptors are still poorly understood. Recently we established FreeStyle 293-F cell lines producing recombinant Japanese eel Fsh and Lh (reFsh and reLh), which form a single chain consisting of a common α-subunit and β-subunits. In this study, we conducted functional analyses of reFsh and reLh, focusing on the binding specificities to their receptors and effects on testicular steroidogenesis in vitro. Assays with gonadotropin receptors-expressing COS-7 cells indicated reFsh stimulated its cognate receptor, meanwhile reLh activated both receptors. Although results of in vitro incubations showed that reFsh and reLh induced testicular 11-ketotestosterone production in a dose and time-dependent manner by upregulating expression of steroidogenic enzymes, the effective doses of reLh were apparently lower and the effects of reLh emerged faster in comparison with reFsh. Results of quantitative real-time PCR using testicular cell fractions showed that fshr and lhcgr1 mRNA were detected both in Sertoli and Leydig cells. These analyses revealed that reFsh and reLh were biologically active and hence will be useful for future studies. Moreover, our data showed that both eel Fsh and Lh acted as steroidogenic hormones through their receptors in testicular somatic cells; however, Lh was more potent on androgen production, implying differential functions on spermatogenesis.

Development of specific enzyme-linked immunosorbent assay for yellowtail kingfish (Seriola lalandi) follicle stimulating hormone using recombinant gonadotropins

We developed a specific competitive enzyme-linked immunosorbent assay (ELISA) for yellowtail kingfish (Seriola lalandi) follicle stimulating hormone (FSH). We previously produced a full-length single chain recombinant yellowtail kingfish FSH using the Pichia pastoris expression system. We used the same method to produce the β subunit of the hormone, against which polyclonal antibodies were raised in rabbits. We first confirmed immunoreactivity of the polyclonal antibodies with the recombinant full length FSH and FSHβ as well as plasma and pituitary FSH of sexually immature and mature yellowtail kingfish by Western blot analysis. We then developed a precise and reproducible ELISA for yellowtail kingfish FSH and validated the assay in plasma and pituitary extracts. The intra- and inter-assay coefficients of variation was <2.2% and 10.2%, respectively. The sensitivity of the assay was 78 pg/ml. For further validation of the assay, we measured the plasma FSH in immature yellowtail kingfish treated with increasing doses (blank, 50, 100 and 150 µg/kg) of kisseptin2-10 peptide from a previous study. The dose response observed in treated females was not significant, however the increased plasma FSH levels coincided with the significantly higher estradiol levels we previously reported in the treated groups. We assessed the applicability of the assay in measuring circulating FSH in other species. We observed parallelism between the linearized FSH standard curve and displacement curves of serially diluted plasma from Atlantic bluefin tuna (Thunnus thynnus) and tilapia (Oreochromis niloticus). We also observed similar parallelism with full length recombinant giant grouper (Epinephelus lanceolatus) FSH. The ELISA we developed for yellowtail kingfish FSH will be useful in understanding the reproductive biology of the species as well as enhancing its aquaculture.

Expressional regulation of gonadotropin receptor genes and androgen receptor genes in the eel testis

Receptors for follicle-stimulating hormone (Fshr), luteinizing hormone (Lhcgr1 and Lhcgr2) and androgens (Ara and Arb) transduce the hormonal signals that coordinate spermatogenesis, but the factors that regulate the abundance of these transducers in fish testes remain little-understood. To mend this paucity of information, we first determined changes in transcript abundance for these receptors (fshr, lhcgr1, ara and arb) during spermatogenesis induced by human chorionic gonadotropin (hCG) injection in the eel, Anguilla australis. We related our findings to testicular production of the fish androgen, 11-ketotestosterone (11-KT), and to the levels of the transcripts encoding steroidogenic acute regulatory protein (star) and 11β-hydroxylase (cyp11b), and subsequently evaluated the effects of hCG or 11-KT on mRNA levels of these target genes in vitro. Testicular 11-KT production was greatly increased by hCG treatment, both in vivo and in vitro, and associated with up-regulation of star and cyp11b transcripts. In situ hybridization indicated that testicular fshr mRNA levels were higher in the early stages of hCG-induced spermatogenesis, while lhcgr1 transcripts were most abundant later, once spermatids were observed. In vitro experiments further showed that hCG and its steroidal mediator 11-KT significantly increased fshr transcript abundance. These data provide new angles on the interactions between gonadotropin and androgen signaling during early spermatogenesis. Increases in levels of 11-KT following hCG injection elevated testicular fshr mRNA levels augmenting Fsh sensitivity in the testis. This evidence is suggestive of a positive feedback loop between gonadotropins and 11-KT that may be key to regulating early spermatogenesis in fish.

Cold seawater induces early sexual developmental stages in the BPG axis of European eel males

BACKGROUND

The impossibility of closing the life cycle of the European eel (Anguilla anguilla) in captivity troubles the future of this critically endangered species. In addition, the European eel is a highly valued and demanded resource, thus the successful closing of its life cycle would have a substantial economic and ecological impact. With the aim of obtaining the highest gamete quality, the study of the effects of environmental factors, such as temperature, on reproductive performance may prove valuable. This is especially true for the exposure to cold water, which has been reported to improve sexual development in multiple other Actinopterygii species.

RESULTS

European eel males treated with cold seawater (10 °C, T10) for 2 weeks showed an increase in the proliferation and differentiation of spermatogonial cells until the differentiated spermatogonial type A cell stage, and elevated testosterone and 11-ketotestosterone plasma levels. Transcriptomes from the tissues of the brain-pituitary-gonad (BPG) axis of T10 samples revealed a differential gene expression profile compared to the other experimental groups, with clustering in a principal component analysis and in heat maps of all differentially expressed genes. Furthermore, a functional analysis of differentially expressed genes revealed enriched gene ontology terms involved in the regulation of circadian rhythm, histone modification, meiotic nuclear division, and others.

CONCLUSIONS

Cold seawater treatment had a clear effect on the activity of the BPG-axis of European eel males. In particular, our cold seawater treatment induces the synchronization and increased proliferation and differentiation of specific spermatogonial cells. In the transcriptomic results, genes related to thermoception were observed. This thermoception may have caused the observed effects through epigenetic mechanisms, since all analysed tissues further revealed differentially expressed genes involved in histone modification. The presented results support our hypothesis that a low temperature seawater treatment induces an early sexual developmental stage in European eels. This hypothesis is logical given that the average temperature experienced by eels in the early stages of their oceanic reproductive migration is highly similar to that of this cold seawater treatment. Further studies are needed to test whether a cold seawater treatment can improve the response of European eels to artificial hormonal treatment, as the results suggest.

Sperm quality in fish: Determinants and affecting factors

Fish sperm quality assessment is helpful for optimizing production and for monitoring the environmental state. Sperm can be monitored relatively easy and, to date, various analyses have been applied and proven to be helpful in this task. Among them, sperm motility parameters such as sperm speed are one of the main performance traits during assisted fish reproduction. Apart from motility the sperm concentration, volume, and seminal plasma pH and osmolality are also frequently evaluated and are the main sperm quality indicators measured in fish sperm. However, other parameters also determine sperm fertilization potential. Recent knowledge reveals several additional parameters of high importance for sperm function. Among them are DNA integration, membrane stability, mitochondria status and enzymatic activity. Measuring all these parameters in fish sperm provides complex knowledge regarding male fertility and helps to improve broodstock maintenance protocols as well as gamete handling and fertilization processes. This review focuses on the presentation of the sperm quality measures for freshwater and marine species of the fish and provides information regarding recent methods of sperm quality evaluation.

Effect of hCG and Ovaprim™ on reproductive characteristics of male Levantine scraper, Capoeta damascina (Valenciennes, 1842)

Species richness and abundance within the genus Capoeta has been depleted. As such, there is great need for developing assisted reproductive technologies for controlling reproduction in captivity. Here, we conducted in vivo studies with single administrations of human chorionic gonadotropin (hCG) and Ovaprim™ [(D-Arg⁶, Pro⁹NEt)-sGnRH + domperidone] in wild-caught Levantine scraper, Capoeta damascina and then evaluated milt characteristics, fertilization success, serum sex steroids, and spermatogenesis via histological testicular development. Spermiation responses were significantly stronger for Ovaprim injected fish than those injected with hCG or saline. hCG had a negative effect on milt quality by reducing the percentage of motile sperm and fertilization success at 12-48 h post injection (hpi), which was not observed after treatment with Ovaprim or the saline injection. Hormonal therapy resulted in higher sperm densities and spermatocrit, although sperm longevity was not impacted. Sex steroids were not impacted by hCG or saline injection, but Ovaprim effectively induced androgen and progestin release, as evident by higher serum levels of testosterone, and 17α,20β-dihydroxy-4-pregnen-3-one. Consequently, their levels peaked at 12 hpi, which coincided with maximal milt production. Histological analysis of the testes and quantification of germ cell types revealed that Ovaprim significantly stimulated spermiogenesis, as a higher number of accumulated spermatozoa were observed at 12 h and 24 hpi. Testes from saline and hCG-injected fish remained unchanged through the experiment, and contained all stages of germ cells, predominantly spermatocytes with few spermatozoa. In conclusion, Ovaprim treatment successfully induced steroidogenesis and maturation of spermatogenic germ cells, leading to spermiation and milt production without having any negative impacts on sperm quality and fertility in wild-caught C. damascina.