Oestrogen and insulin-like growth factors during the reproduction and growth of the tilapia Oreochromis niloticus and their interactions

Identification of Insulin-like Growth Factor (IGF) Family Genes in the Golden Pompano, Trachinotus ovatus: Molecular Cloning, Characterization and Gene Expression

Insulin-like growth factors (IGFs) are hormones that primarily stimulate and regulate animal physiological processes. In this study, we cloned and identified the open reading frame (ORF) cDNA sequences of IGF family genes: the insulin-like growth factor 1 (IGF1), insulin-like growth factor 2 (IGF2), and insulin-like growth factor 3 (IGF3). We found that IGF1, IGF2, and IGF3 have a total length of 558, 648, and 585 base pairs (bp), which encoded a predicted protein with 185, 215, and 194 amino acids (aa), respectively. Multiple sequences and phylogenetic tree analysis showed that the mature golden pompano IGFs had been conserved and showed high similarities with other teleosts. The tissue distribution experiment showed that IGF1 and IGF2 mRNA levels were highly expressed in the liver of female and male fish. In contrast, IGF3 was highly expressed in the gonads and livers of male and female fish, suggesting a high influence on fish reproduction. The effect of fasting showed that IGF1 and mRNA expression had no significant difference in the liver but significantly decreased after long-term (7 days) fasting in the muscles and started to recover after refeeding. IGF2 mRNA expression showed no significant difference in the liver but had a significant difference in muscles for short-term (2 days) and long-term fasting, which started to recover after refeeding, suggesting muscles are more susceptible to both short-term and long-term fasting. In vitro incubation of 17β-estradiol (E2) was observed to decrease the IGF1 and IGF3 mRNA expression level in a dose- (0.1, 1, and 10 μM) and time- (3, 6, and 12 h) dependent manner. In addition, E2 had no effect on IGF2 mRNA expression levels in a time- and dose-dependent manner. The effect of 17α-methyltestosterone (MT) in vitro incubation was observed to significantly increase the IGF3 mRNA expression level in a time- and dose-dependent manner. MT had no effect on IGF2 mRNA but was observed to decrease the IGF1 mRNA expression in the liver. Taken together, these data indicate that E2 and MT may either increase or decrease IGF expression in fish; this study provides basic knowledge and understanding of the expression and regulation of IGF family genes in relation to the nutritional status, somatic growth, and reproductive endocrinology of golden pompano for aquaculture development.

Sex-specific responses to growth hormone and luteinizing hormone in a model teleost, the Mozambique tilapia

Across the vertebrate lineage, sexual dimorphism in body size is a common phenomenon that results from trade-offs between growth and reproduction. To address how key hormones that regulate growth and reproduction interact in teleost fishes, we studied Mozambique tilapia (Oreochromis mossambicus) to determine whether the activities of luteinizing hormone (Lh) are modulated by growth hormone (Gh), and conversely, whether targets of Gh are affected by the presence of Lh. In particular, we examined how gonadal morphology and specific gene transcripts responded to ovine GH (oGH) and/or LH (oLH) in hypophysectomized male and female tilapia. Hypophysectomized females exhibited a diminished gonadosomatic index (GSI) concomitant with ovarian follicular atresia. The combination of oGH and oLH restored GSI and ovarian morphology to conditions observed in sham-operated controls. A similar pattern was observed for GSI in males. In control fish, gonadal gh receptor (ghr2) and estrogen receptor β (erβ) expression was higher in females versus males. A combination of oGH and oLH restored erβ and arβ in females. In males, testicular insulin-like growth factor 3 (igf3) expression was reduced following hypophysectomy and subsequently restored to control levels by either oGH or oLH. By contrast, the combination of both hormones was required to recover ovarian igf3 expression in females. In muscle, ghr2 expression was more responsive to oGH in males versus females. In the liver of hypophysectomized males, igf2 expression was diminished by both oGH and oLH; there was no effect of hypophysectomy, oGH, or oLH on igf2 expression in females. Collectively, our results indicate that gene transcripts associated with growth and reproduction exhibit sex-specific responses to oGH and oLH. These responses reflect, at least in part, how hormones mediate trade-offs between growth and reproduction, and thus sexual dimorphism, in teleost fishes.

Effects of continuous and intermittent cadmium exposure on HPGL axis, GH/IGF axis and circadian rhythm signaling and their consequences on reproduction in female zebrafish: Biomarkers independent of exposure regimes

Typical biomarkers of cadmium (Cd) pollution have well been confirmed in fish from continuous exposure pattern. However, in a natural environment, fish may be exposed to Cd intermittently. In this study, juvenile female zebrafish were exposed for 48 days to 10 μg/L Cd continuously, 20 μg/L for 1 day in every 2 days or 30 μg/L for 1 day in every 3 days. The toxic effects were evaluated using 8 various physiological and biochemical endpoints like specific growth rate (SGR), 17β-estradiol (E2) and vitellogenin (VTG) concentrations in plasma, reproductive parameters (gonadosomatic index (GSI), egg-laying amount, spawning percentage, and hatching and mortality rate of embryos). Transcription of 59 genes related to hypothalamic-pituitary-gonadal-liver (HPGL) axis, circadian rhythm signaling and insulin-like growth factor (IGF) system was examined. SGR, spawning percentage, E2 and VTG levels declined in fish exposed to 10 and 20 μg/L Cd but remained relatively stable in fish exposed to 30 μg/L Cd. Exposure to 10, 20 and 30 μg/L Cd significantly reduced GSI, hatching rate and mortality rate. Similarly, mRNA expression of 27 genes were sensitive to both continuous and intermittent Cd exposure. Among these genes, expression levels of 10 genes had more than 5-fold increase or decrease, including mRNA levels of vtg1, vtg2, vtg3, esr1, igf2a, igf2b, igfbp5b, nr1d1, gnrh3 and gnrhr4. The most sensitive molecular biomarker was vtg3 expression with 1500-3100 fold increase in the liver. The present study, for the first time, provides effective candidate biomarkers for Cd, which are independent of exposure regimes.

Igf3: a novel player in fish reproduction†

As in other vertebrates, fish reproduction is tightly controlled by gonadotropin signaling. One of the most perplexing aspects of gonadotropin action on germ cell biology is the restricted expression of gonadotropin receptors in somatic cells of the gonads. Therefore, the identification of factors conveying the action of gonadotropins on germ cells is particularly important for understanding the mechanism of reproduction. Insulin-like growth factors (Igfs) are recognized as key factors in regulating reproduction by triggering a series of physiological processes in vertebrates. Recently, a novel member of Igfs called Igf3 has been identified in teleost. Different from the conventional Igf1 and Igf2 that are ubiquitously expressed in a majority of tissues, Igf3 is solely or highly expressed in the fish gonads. The role of Igf3 in mediating the action of gonadotropin through Igf type 1 receptor on several aspects of oogenesis and spermatogenesis have been demonstrated in several fish species. In this review, we will summarize existing data on Igf3. This new information obtained from Igf3 provides insight into elucidating the molecular mechanism of fish reproduction, and also highlights the importance of Igf system in mediating the action of gonadotropin signaling on animal reproduction.

Insulin-like growth factor signalling and its significance as a biomarker in fish and shellfish research

The insulin-like growth factor signalling system comprises insulin-like growth factors, insulin-like growth factor receptors and insulin-like growth factor-binding proteins. Along with the growth hormones, insulin-like growth factor signalling is very pivotal in the growth and development of all vertebrates. In fishes, insulin-like growth factors play an important role in osmoregulation, besides the neuroendocrine regulation of growth. Insulin-like growth factor concentration in plasma can assess the growth in fishes and shellfishes and therefore widely applied in nutritional research as an indicator to evaluate the performance of selected nutrients. The present review summarizes the role of insulin-like growth factor signalling in fishes and shellfishes, its significance in aquaculture and in evaluating growth, reproduction and development, and discusses the utility of this system as biomarkers for early indication of growth in aquaculture.

Insulin-like growth factor 2 is produced by antral follicles and promotes preantral follicle development in macaques†

Insulin-like growth factors (IGFs) are known for their involvement in endocrine and paracrine regulation of ovarian function. Although IGF2 is the predominant circulating and intraovarian form of IGFs in primate species, the stage-specific follicular expression, action, and regulation of IGF2 are not well defined. Therefore, experiments were conducted to investigate the follicular IGF production in response to steroid hormone regulation and the direct IGF actions on follicular development and function in vitro. Preantral follicles were isolated from rhesus macaque ovaries and cultured to the antral stage in media supplemented with follicle-stimulating hormone and insulin. Follicles were randomly assigned to treatment groups: (a) control, (b) trilostane (a steroid synthesis inhibitor), (c) trilostane + estradiol, (d) trilostane + progesterone, and (e) trilostane + dihydrotestosterone. Media was analyzed for IGF concentrations, which were correlated to follicle growth. Follicles produced IGF2, but not IGF1, at the antral stage. Steroid depletion decreased, whereas steroid replacement increased, IGF2 production by antral follicles. Media IGF2 levels correlated positively with antral follicle diameters. Macaque preantral follicles and granulosa cells were subsequently cultured without (control) and with recombinant human IGF2 supplementation. Follicle survival, growth, and paracrine factor production, as well as granulosa cell proliferation and gonadotropin receptor gene expression, were assessed. IGF2 addition increased follicle survival rates, diameters and inhibin B production, as well as granulosa cell proliferation. These data demonstrate that IGF2 produced by antral follicles, in response to steroid hormone regulation, could act as a paracrine factor that positively impacts preantral follicle development and function in primates.

Regulation of spermatogenesis and reproductive capacity by Igf3 in tilapia

A novel insulin-like growth factor (igf3), which is exclusively expressed in the gonads, has been widely identified in fish species. Recent studies have indicated that Igf3 regulates spermatogonia proliferation and differentiation in zebrafish; however, detailed information on the role of this Igf needs further in vivo investigation. Here, using Nile tilapia (Oreochromis niloticus) as an animal model, we report that igf3 is required for spermatogenesis and reproduction. Knockout of igf3 by CRISPR/Cas9 severely inhibited spermatogonial proliferation and differentiation at 90 days after hatching, the time critical for meiosis initiation, and resulted in less spermatocytes in the mutants. Although spermatogenesis continued to occur later, more spermatocytes and less spermatids were observed in the igf3-/- testes when compared with wild type of testes at adults, indicating that Igf3 regulates spermatocyte to spermatid transition. Importantly, a significantly increased occurrence of apoptosis in spermatids was observed after loss of Igf3. Therefore, igf3-/- males were subfertile with drastically reduced semen volume and sperm count. Conversely, the overexpression of Igf3 in XY tilapia enhanced spermatogenesis leading to more spermatids and sperm count. Transcriptomic analysis revealed that the absence of Igf3 resulted in dysregulation of many genes involved in cell cycle, meiosis and pluripotency regulators that are critical for spermatogenesis. In addition, in vitro gonadal culture with 17α-methyltetosterone (MT) and 11-ketotestosterone (11-KT) administration and in vivo knockout of cyp11c1 demonstrated that igf3 expression is regulated by androgens, suggesting that Igf3 acts downstream of androgens in fish spermatogenesis. Notably, the igf3 knockout did not affect body growth, indicating that this Igf specifically functions in reproduction. Taken together, our data provide genetic evidence for fish igf3 in the regulation of reproductive capacity by controlling spermatogenesis.

17α-ethynylestradiol prevents the natural male-to-female sex change in gilthead seabream (Sparus aurata L.)

Exposure to 17α-ethynylestradiol (EE2, 5 μg/g food) impairs some reproductive events in the protandrous gilthead seabream and a short recovery period does not allow full recovery. In this study, spermiating seabream males in the second reproductive cycle (RC) were fed a diet containing 5 or 2.5 μg EE2/g food for 28 days and then a commercial diet without EE2 for the remaining RC. Individuals were sampled at the end of the EE2 treatment and then at the end of the RC and at the beginning of the third RC, 146 and 333 days after the cessation of treatment, respectively. Increased hepatic transcript levels of the gene coding for vitellogenin (vtg) and plasma levels of Vtg indicated both concentrations of EE2 caused endocrine disruption. Modifications in the histological organization of the testis, germ cell proliferation, plasma levels of the sex steroids and pituitary expression levels of the genes coding for the gonadotropin β-subunits, fshβ and lhβ were detected. The plasma levels of Vtg and most of the reproductive parameters were restored 146 days after treatments. However, although 50% of the control fish underwent sex reversal as expected at the third RC, male-to female sex change was prevented by both EE2 concentrations.

Development of a time-resolved fluoroimmunoassay for measuring plasma growth hormone in Nile tilapia (Oreochromis niloticus)

Growth hormone is a hormone secreted from the pituitary and is involved in the regulation of most major physiological processes such as growth, development and metabolism. Therefore, an accurate and sensitive detection method is needed for the detection of tilapia serum Gh level. Phage display technology is widely used in the expression of antibody fragments, in which fragments of antibodies are expressed as a fusion with phage proteins and are displayed on the phage surface for easy screening. Time-resolved fluorescence immunoassay (TRFIA) is a microanalysis method developed nearly two decades ago and is one of the most sensitive analytical techniques. With the use of a special lanthanide, the detection background can be distinguished, which can greatly improve the sensitivity of detection. In this report, we cloned the V_H and V_L DNA fragments from the lymphocytes of rabbits immunized with recombinant Gh and assembled them with a linker to form a single-chain variable fragment (scFv) gene pool. Using phage display technology, we isolated scFv DNA fragments from the pool, which encode a protein that specifically binds to tilapia Gh. We then established Eu-DTTA-based TRFIA for measuring plasma Gh in tilapia. The sensitivity of double antibody sandwich Gh-TRFIA was 0.225 ng/ml, and the linear range of the standard curve was 0.225-250 ng/ml. The intra- and interassay coefficients of variation (CVs) were <9.1 and <4.5%, respectively. The cross-reactivities (CRs) of 1 μg/ml recombinant tilapia somatolactin (rtSl), prolactin (rtPrl) and thyroid-stimulating hormone beta subunit (rtTshb) were 0.042%, 0.472% and 0.036%, respectively. The sensitivity of direct competitive Gh-TRFIA was 0.208 ng/ml, and the linear range of the standard curve was 0.208-500 ng/ml. The intra- and interassay CVs were <4.8 and <7.1%, respectively. The CRs of 1 μg/ml rtSl, rtPrl and rtTshb were 0.041%, 0.079% and 0.073%, respectively. In conclusion, Gh-TRFIA is a safe (no concerns about radioactive isotopes), economical, and efficient detection method for the quantification of plasma Gh. Thus, the application of phage display technology for antibody screening and the use of TRFIA for tilapia Gh detection are conducive to research in the field of fish endocrinology.

Examination of methods for manipulating serum 17β-Estradiol (E2) levels by analysis of blood E2 concentration in medaka (Oryzias latipes)

It is widely known that reproduction in vertebrates is regulated by the hypothalamus-pituitary-gonadal (HPG) axis. Although the mechanism of the HPG axis has been well documented in mammals, it cannot be always applied to that in non-mammalian species, which is a great disadvantage in understanding reproduction of vertebrates in general. Recently, transgenic and genome editing tools have rapidly been developed in small teleosts, and thus these species are expected to be useful for the understanding of general mechanism of reproduction in vertebrates. One of the major sex steroid hormones in female vertebrates 17β-Estradiol (E2) plays crucial roles in the formation of sexual dimorphism and the HPG axis regulation. In spite of the importance of E2 in reproductive regulation, only a few studies have analyzed blood E2 levels in small teleosts that are easily amenable to genetic manipulation. In the present study, we analyzed blood E2 concentration in medaka and demonstrated that female medaka show diurnal changes in blood E2 concentration. We then examined the best method for manipulating the circulating E2. First, we found that ovariectomy (OVX) drastically removes endogenous E2 in a day in female medaka. We examined different methods for E2 administration and revealed that feeding administration of E2-containing food is the most convenient and physiological method for mimicking the diurnal E2 changes of female medaka. On the other hand, the medaka exposed to E2 containing water showed high blood E2 concentrations, which exceeds those of environmental water, suggesting that E2 may cause bioconcentration.

Expression patterns and immunolocalisation of IGF-I and IGF-II in male and female gonads of the Neotropical characid fish Astyanax fasciatus

The insulin-like growth factor (IGF) system plays important roles in fish reproduction, but the expression pattern and cellular location of IGF-I and IGF-II during gonadal maturation are uncertain. The present study reports a stage-specific assessment of gonadal expression levels and immunolocalisation of IGF-I and IGF-II in Astyanax fasciatus, a characid fish from South America. Adult fish in different maturity stages were caught in the Furnas Reservoir, Grande River, Brazil. Gonad samples were processed for histology, immunohistochemistry, and ELISA for IGF-I and IGF-II. Ovarian levels of IGF-I were low during ripening and ripe stages, higher in totally spent, and then decreased in resting. Levels of IGF-II increased during ovarian maturation, reaching significantly higher values at stage totally spent. In males, IGF-I levels followed gonadal maturation, with higher values in ripening and ripe stages, whereas IGF-II levels showed higher values in stage ripening and partially spent. A positive correlation was found between IGF-I and gonadosomatic index (GSI) for males (r = 0.59), while females showed a negative correlation (r = - 0.43), but IGF-II showed no correlation to GSI. IGF-I was expressed mainly in oogonia nests whereas IGF-II stained the follicular cells in the perinucleolar follicles, cortical vesicles in the previtellogenic follicles, and oogonia nests. In males, IGF-I was evident in spermatogonia and spermatocytes while IGF-II stained Sertoli cells surrounding spermatids cysts and spermatogonia in late stages. Together, these findings support a hypothesis that the balance between IGF-I and IGF-II levels is important in the regulation of gonad maturation in Astyanax fasciatus.

Diethylstilbestrol arrested spermatogenesis and somatic growth in the juveniles of yellow catfish (Pelteobagrus fulvidraco), a fish with sexual dimorphic growth

In fish, spermatogenesis and somatic growth are mainly regulated by hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-somatic (HPS) axes, respectively. Xenoestrogens have been reported to impair spermatogenesis in some fishes, and arrest somatic growth in some others, whereas, whether xenoestrogens are capable of disrupting spermatogenesis and somatic growth simultaneously in fish that exhibits sexual dimorphic growth is little known, and the underlying mechanisms remain poorly understood. In this study, male juveniles of yellow catfish (Pelteobagrus fulvidraco), which exhibits a sexual dimorphic growth that favors males, were exposed to diethylstilbestrol (DES) for 28 days. After exposure, DES significantly disrupted the spermatogenesis (decreased gonadal-somatic index (GSI) and germ cell number) and arrested the somatic growth (declined body weight) of the catfish juveniles. Gene expression and plasma steroid analyses demonstrated the suppressed mRNA levels of genes in HPG axis (gnrh-II, fshβ, and lhβ in the brain and dmrt1, sf1, fshr, cyp17a1, cyp19a1a, and cyp11b2 in the testis) and decreased 17β-estrodial (E2) and 11-ketotestosterone (11-KT) levels in plasma. Further analysis revealed the arrested germ cell proliferation (cyclin d1), meiosis (dmc1, sycp3), and enhanced apoptosis (decreased bcl-2 and elevated bax/bcl-2 ratio) in the testis. Besides, DES also suppressed the mRNA levels of genes in HPS axis (ghrh, gh, and prl in the brain and ghr, igf1, igf2a, and igf2b in the liver). The suppressed HPG and HPS axes were thus supposed to disturb spermatogenesis and arrest somatic growth in yellow catfish. The present study greatly extended our understanding on the mechanisms underlying the toxicity of DES on spermatogenesis and somatic growth of fish.

Microcystin-LR retards gonadal maturation through disrupting the growth hormone/insulin-like growth factors system in zebrafish

Recent studies have documented that microcystins (MCs) have potential toxic effects on growth and reproduction in fish. However, no systematic data exist on whether MCs cause gonadal development retardation through disrupting the growth hormone/insulin-like growth factors (GH/IGFs) system. To this end, zebrafish hatchlings (5 d post-fertilization) were exposed to 0, 0.3, 3 and 30µg/L microcystin-LR (MC-LR) for 90 d until they reached sexual maturity. Life-cycle exposure to MC-LR caused delayed ovarian maturation and sperm development along with ultrapathological lesions in the brain and liver. Moreover, the retarded gonadal development was accompanied by an inhibition of the GH/IGFs system, which was characterized by significant decreases in the transcriptional levels of brain gh (males only), hepatic igf2a and igf2b as well as gonadal igf1 (males only), igf3 and igf2r. These findings for the first time point to the influence of MC-LR on fish gonadal development via the GH/IGFs system. Also, sex-differential impairments suggested that gonadal development of males is more vulnerable than that of female to MC-LR. Our results provide evidence that MC-LR at environmentally relevant concentrations is able to induce impairments on fish gonadal development.

Evidences for involvement of growth hormone and insulin-like growth factor in ovarian development of starry flounder (Platichthys stellatus)

Although gonadotrophins are major regulators of ovarian function in teleosts and other vertebrates, accumulating evidence indicates that the growth hormone (GH)-insulin-like growth factor (IGF) axis also plays an important role in fish reproduction. As a first step to understand the physiological role of the GH-IGF system in the ovarian development of starry flounder (Platichthys stellatus), the expression profiles of GH and IGF messenger RNAs (mRNAs) and plasma GH, IGF-I, estradiol-17β (E2), and testosterone (T) levels during the ovarian development were investigated. The developmental stages of ovaries were divided into five stages (II, III, IV, V, and VI) by histological analysis. The hepatosomatic index (HSI) and gonadosomatic index (GSI) values increased and peaked at stage IV and stage V, respectively, and then declined at stage VI. Pituitary GH mRNA levels decreased sharply at stage III and raised to top level at stage VI. The hepatic IGF-I mRNA levels ascended to maximum value at stage V and then declined significantly at stage VI. However, the hepatic IGF-II mRNA levels remained stable and increased significantly at stage VI. In contrast, the ovarian IGF-I mRNA levels increased gradually and peaked at stage VI. The ovarian IGF-II mRNA levels were initially stable and increased significantly at stage V until the top level at stage VI. Consistent with the pituitary GH mRNA levels, plasma GH levels reduced sharply at stage III and remained depressed until stage V and then raised remarkably at stage VI. Plasma IGF-I level peaked at stage V and then declined to initial level. Plasma E2 level peaked at stage IV and then dramatically descended to the basal level. Plasma T level peaked at stage V and then declined significantly back to the basal level. Based on statistical analysis, significant positive correlations between hepatic IGF-I mRNA and GSI, ovarian IGF-II mRNA and hepatic IGF-II mRNA, ovarian IGF-I mRNA and ovarian IGF-II mRNA, and plasma IGF-I and plasma T were observed, respectively. These results suggest that the GH-IGF system may be involved in the ovarian development of starry flounder; GH and IGFs appear to play distinct roles in the regulation of the ovarian development in paracrine/autocrine manners. These findings extend our knowledge of the roles of the GH-IGF axis on reproduction regulation in fish.

Effects of 17α-Methyltestosterone and Aromatase Inhibitor Letrozole on Sex Reversal, Gonadal Structure, and Growth in Yellow Catfish Pelteobagrus fulvidraco

Monosex populations are in demand in many fish species with sexual dimorphism, e.g., better growth performance, higher gonad value, superior ornamental value. From the point of view of research, a monosex population is one of the best materials for investigating sex-determining mechanisms, sex differentiation, and sex-linked markers. Sex reversal of females (phenotypic reversal from XX female to XX male) is the first step in all-female production in species with an XX/XY system for sex determination. In the present study, masculinization of yellow catfish, a species with XX/XY sex determination, was investigated by oral administration of various doses of 17α-methyltestosterone (MT) or an aromatase inhibitor (AI) letrozole (LZ); effects on survival, growth performance, sex ratio, and changes in gonadal structure were evaluated. Three doses (20, 50, and 100 mg kg(-1) diet) of oral MT or LZ were administered to fry from 10 days post-hatching (DPH) to 59 DPH. Oral administration of MT at all doses did not significantly change the ratio of males (45.8%, 33.3%, and 50.0% respectively) compared to the control group (37.5%), while yielding intersex fish at all doses (4.2% to 8.3%). Oral administration of LZ produced a significantly higher proportion of males in all doses (75.5%, 83.3%, and 75.0%, respectively). Additionally, the lowest dose of LZ improved the growth of treated fish compared to the control, and all doses of LZ enhanced spermatogenesis in treated males.