Previtellogenic oocyte growth in salmon: relationships among body growth, plasma insulin-like growth factor-1, estradiol-17beta, follicle-stimulating hormone and expression of ovarian genes for insulin-like growth factors, steroidogenic-acute regulatory protein and receptors for gonadotropins, growth hormone, and somatolactin

Seasonal changes in gonadal expression of gonadotropin receptors, steroidogenic acute regulatory protein and steroidogenic enzymes in the European sea bass

The endocrine regulation of gametogenesis, and particularly the roles of gonadotropins, is still poorly understood in teleost fish. This study aimed to investigate transcript levels of both gonadotropin receptors (FSHR and LHR) during an entire reproductive cycle in male and female sea bass (Dicentrarchus labrax). To have a more comprehensive understanding of how different key factors interact to control sea bass gonadal function, changes in the transcript abundance of two important steroidogenic enzymes, P450 11beta-hydroxylase (CYP11B1) and P450 aromatase (CYP19A1), and the steroidogenic acute regulatory protein (StAR), were also studied. These expression profiles were analysed in relation to changes in the plasma levels of important reproductive hormones and histological data. Expression of the FSHR was connected with early stages of gonadal development, but also with the spermiation/maturation-ovulation periods. The expression profile of the LHR seen in both sexes supports the involvement of LH in the regulation of the final stages of gamete maturation and spermiation/ovulation. In both sexes StAR expression was strongly correlated with LHR expression. In females high magnitude increments of StAR expression levels were observed during the maturation-ovulation stage. In males, gonadotropin receptors and CYP11B1 mRNA levels were found to be correlated. In females, the expression profiles of FSHR and CYP19A1 and the changes in plasma estradiol (E2) indicate that the follicular production of E2 could be under control of FSH through the regulation of aromatase expression. This study supports the idea that FSH and LH may have different roles in the control of sea bass gonadal function.

Expression of gonadotropin subunits in roach (Rutilus rutilus, Cyprinidae) infected with plerocercoids of the tapeworm Ligula intestinalis (Cestoda)

Plerocercoids of the tapeworm Ligula intestinalis (Cestoda: Bothriocephalidea) have been reported to inhibit gametogenesis of their intermediate fish hosts. However, mechanistic studies are rare and the proximate cues leading to impaired reproduction still remain unknown. In the present study we investigated the effects of infection by L. intestinalis on reproductive parameters of roach (Rutilus rutilus, Cyprinidae), a common fish host of this parasite. Field studies on roach demonstrated that in both genders infection prevented gonad development. As revealed by quantitative PCR, infection was accompanied by essentially lower pituitary expression of follicle-stimulating hormone beta-subunit (FSHbeta) and luteinizing hormone beta-subunit (LHbeta) mRNA compared with uninfected roach, providing clear evidence for gonadotropin-insufficiency as the cause of arrested gametogenesis. Under controlled laboratory conditions infected roach showed lower mRNA levels of FSHbeta but not of LHbeta, despite histology revealing similar gonad stages as in uninfected conspecifics. These findings indicate the involvement of FSH rather than LH in mediating effects of infection early during gonad development in roach. Moreover, the impact of L. intestinalis on reproductive parameters of roach appeared to be independent of the parasite burden. Together, these data provide valuable information on the role of FSH and LH as mediators of parasite-induced sterilization in a vertebrate and implicate the selective inhibition of host reproduction by L. intestinalis as a natural source of endocrine disruption in fish.

Activity of the pituitary-gonadal axis is increased prior to the onset of spawning migration of chum salmon

The activity of the pituitary-gonadal axis (PG axis) in pre-migratory and homing chum salmon was examined because endocrine mechanisms underlying the onset of spawning migration remain unknown. Pre-migratory fish were caught in the central Bering Sea in June, July and September 2001, 2002 and 2003, and in the Gulf of Alaska in February 2006. They were classified into immature and maturing adults on the basis of gonadal development. The maturing adults commenced spawning migration to coastal areas by the end of summer, because almost all fish in the Bering Sea were immature in September. In the pituitaries of maturing adults, the copy numbers of FSHbeta mRNA and the FSH content were 2.5- to 100-fold those of the immature fish. Similarly, the amounts of LHbeta mRNA and LH content in the maturing adults were 100- to 1000-fold those of immature fish. The plasma levels of testosterone, 11-ketotestosterone and estradiol were higher than 10 nmol l(-1) in maturing adults, but lower than 1.0 nmol l(-1) in immature fish. The increase in the activity of the PG-axis components had already initiated in the maturing adults while they were still in the Gulf of Alaska in winter. In the homing adults, the pituitary contents and the plasma levels of gonadotropins and plasma sex steroid hormones peaked during upstream migration from the coast to the natal hatchery. The present results thus indicate that the seasonal increase in the activity of the PG axis is an important endocrine event that is inseparable from initiation of spawning migration of chum salmon.

Unilateral ovariectomy increases egg size and reduces follicular atresia in the semelparous coho salmon, Oncorhynchus kisutch

Unilateral ovariectomy (ULO, removal of one ovary) is a powerful technique for studying aspects of reproductive physiology, including follicular recruitment and growth. To examine effects of ULO for the first time in a semelparous species, coho salmon (Oncorhynchus kisutch) were unilaterally ovariectomized during mid-vitellogenesis approximately 3 months before spawning. At termination of the study (79 days post-surgery), single ovaries of ULO fish were gravimetrically equivalent to paired ovaries of sham surgery, control fish. There was no evidence of recruitment of new vitellogenic follicles. Instead, the dramatic increase in ovary mass was attributable to hypertrophy of existing vitellogenic follicles (33% increase in volume) and increased fecundity achieved through a greater than two-fold reduction in follicular atresia. The composition of whole ovaries on a dry weight basis from ULO fish was greater in protein, but lower in lipid than that of control fish. Expressing the data on a per follicle basis, however, showed that follicles of ULO fish contained more protein, ash, water, and lipid. The results indicate that ULO of coho salmon induces compensatory hypertrophy of existing vitellogenic follicles, while maximizing fecundity through reduction of atresia. Thus, 3 months before spawning, coho salmon exhibit the ability to adjust final egg size and number when faced with significant depletion of ovarian follicles. This in vivo system provides a platform for further study of physiological mechanisms regulating follicular growth and atresia, and the trade-off between egg size and egg number.

Cloning of somatolactin alpha, beta forms and the somatolactin receptor in Atlantic salmon: seasonal expression profile in pituitary and ovary of maturing female broodstock

BACKGROUND

Somatolactin (Sl) is a fish specific adenohypophyseal peptide hormone related to growth hormone (Gh). Some species, including salmonids, possess two forms: Sl alpha and Sl beta. The somatolactin receptor (slr) is closely related to the growth hormone receptor (ghr). Sl has been ascribed many physiological functions, including a role in sexual maturation. In order to clarify the role of Sl in the sexual maturation of female Atlantic salmon (Salmo salar), the full length cDNAs of slr, Sl alpha and Sl beta were cloned and their expression was studied throughout a seasonal reproductive cycle using real-time quantitative PCR (RTqPCR).

METHODS

Atlantic salmon Sl alpha, Sl beta and slr cDNAs were cloned using a PCR approach. Gene expression of Sl alpha, SL beta and slr was studied using RTqPCR over a 17 month period encompassing pre-vitellogenesis, vitellogenesis, ovulation and post ovulation in salmon females. Histological examination of ovarian samples allowed for the classification according to the degree of follicle maturation into oil drop, primary, secondary or tertiary yolk stage.

RESULTS

The mature peptide sequences of Sl alpha, Sl beta and slr are highly similar to previously cloned salmonid forms and contained the typical motifs. Phylogenetic analysis of Atlantic salmon Sl alpha and Sl beta shows that these peptides group into the two Sl clades present in some fish species. The Atlantic salmon slr grouped with salmonid slr amongst so-called type I ghr. An increase in pituitary Sl alpha and Sl beta transcripts before and during spawning, with a decrease post-ovulation, and a constant expression level of ovarian slr were observed. There was also a transient increase in Sl alpha and Sl beta in May prior to transfer from seawater to fresh water and ensuing fasting.

CONCLUSION

The up-regulation of Sl alpha and Sl beta during vitellogenesis and spawning, with a subsequent decrease post-ovulation, supports a role for Sl during gonadal growth and spawning. Sl could also be involved in calcium/phosphate mobilization associated with vitellogenesis or have a role in energy homeostasis associated with lipolysis during fasting. The up-regulation of both Sl alpha and Sl beta prior to fasting and freshwater transfer, suggests a role for Sl linked to reproduction that may be independent of the maturation induced fasting.

Identification of differentially expressed ovarian genes during primary and early secondary oocyte growth in coho salmon, Oncorhynchus kisutch

BACKGROUND

The aim of this study was to identify differentially expressed ovarian genes during primary and early secondary oocyte growth in coho salmon, a semelparous teleost that exhibits synchronous follicle development.

METHODS

Reciprocal suppression subtractive hybridization (SSH) libraries were generated from ovaries with perinucleolus (P) or cortical alveolus (CA) stage follicles and selected genes were assessed with quantitative PCR (qPCR). An assessment of changes in RNA composition during oocyte growth and its relationship to transcript levels was also conducted.

RESULTS

SSH revealed several differentially expressed genes during early oogenesis, some which will not likely be utilized until 1-3 years later in salmon. Zona pellucida glycoprotein (zp) genes, vitellogenin receptor (vldlr) isoforms, cathepsin B (ctsba), cyclin E (ccne), a DnaJ transcript (dnaja2), and a ferritin subunit (fth3) were significantly elevated at the P stage, while a C-type lectin, retinol dehydrogenase (rdh1), and a coatomer protein subunit (cope) were upregulated at the CA stage. Putative follicle cell transcripts such as anti-Müllerian hormone (amh), lipoprotein lipase (lpl), apolipoprotein E (apoe), gonadal soma-derived growth factor (gsdf) and follicle-stimulating hormone receptor (fshr) also increased significantly at the CA stage. The analysis of RNA composition during oocyte growth showed that the total RNA yield and proportion of messenger RNA relative to non-polyadenylated RNAs declined as oogenesis progressed. This influenced apparent transcript levels depending on the type of RNA template used and normalization method.

CONCLUSION

In coho salmon, which exhibit a dramatic change in oocyte size and RNA composition during oogenesis, use of messenger RNA as template and normalization of qPCR data to a housekeeping gene, ef1a, yielded results that best reflected transcript abundance within the ovarian follicle. Synthesis of zp transcripts and proteins involved in yolk incorporation and processing occurred during primary growth, while increased expression of a CA component and genes related to lipid incorporation occurred concomitant with the appearance of CA, but prior to lipid accumulation. Significant increases in transcripts for fshr, gsdf, and amh at the CA stage suggest a role of FSH and TGFbeta peptides in previtellogenic oocyte growth and puberty onset in female salmon.

Temporal dynamics of oocyte development, plasma sex steroids and somatic energy reserves during seasonal ovarian maturation in captive Murray cod Maccullochella peelii peelii

The temporal dynamics of oocyte growth, plasma sex steroids and somatic energy stores were examined during a 12 month ovarian maturation cycle in captive Murray cod Maccullochella peelii peelii under simulated natural photothermal conditions. Ovarian function was found to be relatively uninhibited in captivity, with the exception that post-vitellogenic follicles failed to undergo final maturation, resulting in widespread pre-ovulatory atresia. Seasonal patterns of oocyte growth were characterised by cortical alveoli accumulation in March, deposition of lipids in April, and vitellogenesis between May and September. Two distinct batches of vitellogenic oocytes were found in Murray cod ovaries, indicating a capacity for multiple spawns. Plasma profiles of 17beta-oestradiol and testosterone were both highly variable during the maturation period suggesting that multiple roles exist for these steroids during different stages of oocyte growth. Condition factor, liver size and visceral fat stores were all found to increase prior to, or during the peak phase of vitellogenic growth. Murray cod appear to strategically utilise episodes of high feeding activity to accrue energy reserves early in the reproductive cycle prior to its deployment during periods of rapid ovarian growth.

11-Ketotestosterone and IGF-I increase the size of previtellogenic oocytes from shortfinned eel, Anguilla australis, in vitro

Previtellogenic ovarian fragments from eel,Anguilla australis, were culturedin vitroin a chemically defined medium containing steroids and/or peptide hormones for 18 days in order to investigate their involvement in control of early oocyte growth. 11-Ketotestosterone (11-KT), but not estradiol-17β, induced a significant 10–20% increase in diameters of previtellogenic oocytes and oocyte nuclei in a dose-dependent manner. Effects were greatest for 100 nM 11-KT, a dose that is within the physiological range seen in very early vitellogenic eels in the wild. The effect was not accompanied by obvious ultrastructural changes in the oocytes other than an apparent increase in nuclear size. Similarly, treatment with recombinant human IGF-I resulted in increased oocyte diameters, whereas no such effect was seen after treatment with heterologous insulin, GH, leptin, or human chorionic gonadotropin. Interestingly, lipid supplementation also resulted in an increase in oocyte diameter, and greater radioactivity in ovarian explants following incubation with14C-triglycerides and 11-KT, but not FSH, suggesting that the androgen may play a role in lipid accumulation into the oocyte. Our results implicate hormones from both the reproductive and the metabolic axes in control of previtellogenic oocyte growth in a teleost fish.

Beta-sitosterol and 17beta-estradiol alter gonadal steroidogenic acute regulatory protein (StAR) expression in goldfish, Carassius auratus

Fish exposed to the phytosterol beta-sitosterol (beta-sit) have decreased circulating hormone and cholesterol concentrations, and decreased gonadal intra-mitochondrial cholesterol pools. The current study examined the potential for beta-sit to alter abundance of the key cholesterol transport protein, steroidogenic acute regulatory (StAR) protein, which delivers cholesterol to the first and rate-limiting steroidogenic enzyme P450 side chain cleavage (P450(scc)) inside the mitochondria. Plasma testosterone (T) and lipids (cholesterol, lipoproteins and triglycerides) were also measured. Goldfish were exposed to 200 microg/g beta-sit (97% pure or as a 72.6% pure phytosterol mixture) and 10 microg/g 17beta-estradiol (E(2); estrogenic control) by intra-peritoneal Silastic implants for 21-days or for five-months. Plasma T was significantly decreased in male fish exposed to the phytosterol mixture following the long-term exposure (p<0.001). There were no differences in total cholesterol concentrations among treatments in the short- or long-term exposure, but male fish in the long-term exposure had significantly lower HDL as compared to control fish (p<0.025) with a corresponding increase in LDL. StAR transcript levels were unchanged following the short-term exposure, but were reduced after five months in male beta-sit fish (p=0.05) and E(2)-treated female fish (p=0.05). This reduction in StAR transcript abundance in conjunction with decreased plasma T and altered plasma lipoprotein fractions demonstrates a non-estrogenic effect of beta-sit. This is the first study to show that beta-sit has the capacity to alter gonadal StAR transcript abundance, offering a mechanism by which beta-sit disrupts reproductive endocrine endpoints.

Cloning and characterization of cDNAs for hormones and/or receptors of growth hormone, insulin-like growth factor-I, thyroid hormone, and corticosteroid and the gender-, tissue-, and developmental-specific expression of their mRNA transcripts in fathead minnow (Pimephales promelas)

Growth hormone (GH), insulin-like growth factor-I (IGF-I), thyroid hormones, and corticosteroids play central roles in a wide range of body functions but, in fish, information on their interactions is limited. These axes of the endocrine system are also potential targets for disruption of signaling pathways by hormone-mimicking chemicals, but have received little study. Molecular approaches offer an effective way to help unravel these endocrine interactions but require the appropriate gene-specific assays to do so. In this study, the cDNAs for a suite of hormones and/or receptors involved in signaling for the effects of GH and IGF-I [GH, GH receptor (GHR), IGF-I, IGF-I receptor (IGF-IR)], thyroid hormones [thyroid hormone receptor alpha (TRalpha) and beta (TRbeta)], and corticosteroids [glucocorticoid receptor (GR)] were cloned from the fathead minnow (Pimephales promelas; fhm), and the tissue-, developmental-, and gender-related expression of their mRNA transcripts established. By polymerase chain reaction (PCR) strategy, we obtained full-length 1123-bp GH, 817-bp IGF-I, 1584-bp TRbeta, and 2571-bp GR cDNAs, coding for 210 amino acid (aa) GH, 161 aa IGF-I, 378 aa TRbeta, and 745 aa GR putative proteins, and partial-length 158-bp GHR, 811-bp IGF-IR, and 446-bp TRalpha cDNAs. Real-time PCR analyses revealed broad tissue expression for the target mRNAs; all targets were expressed in brain, pituitary, gill, liver, gonad, intestine, and muscle, with the exception of GH that was expressed only in the pituitary and gonad. Expression patterns in both juvenile and adult fhm were complex, with both temporal-, tissue-, and sex-specific characteristics. For example, hepatic expressions of GHR, IGF-I, and IGF-IR were far higher in males than in females, possibly reflecting the sex-related dimorphism in growth that occurs in this species, and TRalpha and TRbeta showed divergent expression patterns during development (where TRbeta predominated) and in adult tissues implying some distinct roles for the two TR subtypes.

Investigation of de novo cholesterol synthetic capacity in the gonads of goldfish (Carassius auratus) exposed to the phytosterol beta-sitosterol

Total and intra-mitochondrial gonadal cholesterol concentrations are decreased in fish exposed to the phytoestrogen beta-sitosterol (beta-sit). The present study examined the potential for beta-sit to disrupt de novo cholesterol synthesis in the gonads of goldfish exposed to 200 microgram/g beta-sit and 10 microgram/g 17beta-estradiol (E2; estrogenic control) by intra-peritoneal Silastic implants for 21 days. The de novo cholesterol synthetic capacity was estimated by incubating gonadal tissue with 14C-acetate for a period of 18 hours, followed by chloroform/methanol lipid extraction and thin layer chromatography (TLC) lipid separation. Lipid classes were confirmed using infrared spectroscopy. Plasma testosterone (T) and total cholesterol concentration were measured and gonadosomatic index (GSI) was calculated. Plasma T was significantly reduced in male beta-sit-treated fish compared to control and E2-treated fish (p < 0.001). 14C-Acetate incorporation into cholesterol and cholesterol esters was not significantly different among treatment groups for male and female fish, however, 14C-enrichment was higher than expected in both triglycerides (TG) and free fatty acids (FFA). FFA incorporation was significantly higher in male control fish than either beta-sit or E2 treatments (p = 0.005). Plasma cholesterol concentration was significantly increased in the male beta-sit treatment group compared to controls (p = 0.027). These results indicate gonadal de novo cholesterol biosynthetic capacity is not disrupted by beta-sit or E2 treatment in early recrudescing male or female goldfish, while plasma cholesterol and steroid concentrations are sensitive to beta-sit exposure.