Identification of new participants in the rainbow trout (Oncorhynchus mykiss) oocyte maturation and ovulation processes using cDNA microarrays

Aromatase expression in Xenopus oocytes: a three cell-type model for the ovarian estradiol synthesis

In contrast to the classical model describing the synthesis of androgens and estrogens as restricted to somatic cells, a previous study demonstrated that Xenopus laevis oocytes participate in androgen synthesis. The objective of our study was to determine whether Xenopus oocytes are also involved in estrogen synthesis. More precisely, we analyzed aromatase expression by in situ hybridization and RT-QPCR and measured aromatase activity. Aromatase, the enzyme responsible for estrogen synthesis, appears to be expressed and active not only in the follicular cells but also in the vitellogenic oocytes. During late oogenesis, aromatase oocyte expression and activity decreased concomitantly with the trend observed in surrounding follicular layers. In order to investigate the role of estradiol-17β (E(2)), we studied its effect on oocyte meiotic resumption. It appears that, as in Rana pipiens, E(2) inhibited the follicle-enclosed maturation of Xenopus oocytes, likely through inhibition of LH-induced maturation-inducing steroid synthesis. In addition, E(2) exerted a slight enhancing action on denuded oocyte maturation whose biological significance remains unclear. Together, our results demonstrate that Xenopus oocyte significantly participates in ovarian E(2) synthesis and this may be a common feature of vitellogenic vertebrates.

In vivo induction of oocyte maturation and ovulation in zebrafish

The maturation of fish oocytes is a well-characterized system induced by progestins via non-genomic actions. In a previous study, we demonstrated that diethylstilbestrol (DES), a non-steroidal estrogen, induces fish oocyte maturation via the membrane progestin receptor (mPR). Here, we attempted to evaluate the effect of DES as an environmental endocrine disrupting chemical (EDC) upon fish oocyte maturation using live zebrafish. DES triggered oocyte maturation within several hours in vivo when administrated directly into the surrounding water. The natural teleost maturation-inducing hormone, 17alpha, 20beta-dihydroxy-4-pregnen-3-one (17,20beta-DHP) also induced oocyte maturation in vivo. Steroids such as testosterone, progesterone or 17alpha-hydroxyprogesterone were also effective in vivo. Further studies indicated that externally applied 17,20beta-DHP even induced ovulation. In contrast to 17,20beta -DHP, DES induced maturation but not ovulation. Theoretically this assay system provides a means to distinguish pathways involved in the induction of ovulation, which are known to be induced by genomic actions from the pathway normally involved in the induction of oocyte maturation, a typical non-genomic action-dependent pathway. In summary, we have demonstrated the effect of EDCs on fish oocyte maturation in vivo. To address the effects, we have explored a conceptually new approach to distinguish between the genomic and non-genomic actions induced by steroids. The assay can be applied to screens of progestin-like effects upon oocyte maturation and ovulation for small molecules of pharmacological agents or EDCs.

Molecular evolution of the transferrin family and associated receptors

BACKGROUND

In vertebrates, serum transferrins are essential iron transporters that have bind and release Fe(III) in response to receptor binding and changes in pH. Some family members such as lactoferrin and melanotransferrin can also bind iron while others have lost this ability and have gained other functions, e.g., inhibitor of carbonic anhydrase (mammals), saxiphilin (frogs) and otolith matrix protein 1 (fish).

SCOPE OF REVIEW

This article provides an overview of the known transferrin family members and their associated receptors and interacting partners.

MAJOR CONCLUSIONS

The number of transferrin genes has proliferated as a result of multiple duplication events, and the resulting paralogs have developed a wide array of new functions. Some homologs in the most primitive metazoan groups resemble both serum and melanotransferrins, but the major yolk proteins show considerable divergence from the rest of the family. Among the transferrin receptors, the lack of TFR2 in birds and reptiles, and the lack of any TFR homologs among the insects draw attention to the differences in iron transport and regulation in those groups.

GENERAL SIGNIFICANCE

The transferrin family members are important because of their clinical significance, interesting biochemical properties, and evolutionary history. More work is needed to better understand the functions and evolution of the non-vertebrate family members. This article is part of a Special Issue entitled Molecular Mechanisms of Iron Transport and Disorders.

Relationships between the transcriptome and physiological indicators of reproduction in female rainbow trout over an annual cycle

Normal transcriptomic patterns along the brain-pituitary-gonad-liver (BPGL) axis should be better characterized if endocrine-disrupting compound-induced changes in gene expression are to be understood. Female rainbow trout were studied over a complete year-long reproductive cycle. Tissue samples from pituitary, ovary, and liver were collected for microarray analysis using the 16K Genomic Research on Atlantic Salmon Project (GRASP) microarray and for quantitative polymerase chain reaction measures of estrogen receptor (ER) isoform messenger RNA (mRNA) levels. Plasma was collected to determine levels of circulating estradiol-17β (E2), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). As an a priori hypothesis, changes in gene expression were correlated to either circulating levels of E2, FSH, and LH, or ER mRNAs quantified by quantitative polymerase chain reaction. In the liver, most transcriptomic patterns correlated to levels of either E2, LH, or ERs. Fewer ovarian transcripts could be correlated to levels of E2, ERα, or FSH. No significant associations were obvious in the pituitary. As a post hoc hypothesis, changes in transcript abundance were compared with microarray features with known roles in gonadal maturation. Many altered transcripts in the ovary correlated to transcript levels of estradiol 17-beta-dehydrogenase 8 or 17 B HSD12, or to glycoprotein alpha chain 1 or 2. In the pituitary, genes involved with the growth axis (e.g., growth hormone, insulin-related growth factor binding protein) correlated with the most transcripts. These results suggest that transcriptional networks along the BPGL axis may be regulated by factors other than circulating steroid hormones.

Molecular and functional characterization of catfish (Heteropneustes fossilis) aquaporin-1b: changes in expression during ovarian development and hormone-induced follicular maturation

The oocytes of the freshwater catfish Heteropneustes fossilis hydrate during hormone-induced meiotic maturation. To investigate if this process may be mediated by aquaporins (AQPs), as it occurs in marine fish producing highly hydrated eggs, the cloning of ovarian AQPs in catfish was carried out. Using degenerate primers for conserved domains of the major intrinsic protein (MIP) family, and 5' and 3'end amplification procedures, a full-length cDNA encoding for an AQP1-like protein was isolated. The predicted protein showed the typical six transmembrane domains and two Asn-Pro-Ala (NPA) motifs conserved among the members of the AQP superfamily. Phylogenetic analysis indicated that the catfish AQP clustered with the teleost-specific aquaporin-1b subfamily, and accordingly it was termed HfAqp1b. Heterologous expression in Xenopus laevis oocytes indicated that HfAqp1b encoded for a functional AQP, water permeability being enhanced by cAMP. Site-directed mutagenesis revealed that cAMP induced the translocation of HfAqp1b into the oocyte plasma membrane most likely through the phosphorylation of HfAqp1b Ser(227). In adult catfish, hfaqp1b transcripts were detected exclusively in ovary and brain and showed significant seasonal variations; in the ovary, hfaqp1b was maximally expressed during the pre-spawning period, whereas in the brain the highest expression was detected during spawning. In vitro stimulation of isolated catfish ovarian follicles with vasotocin (VT) or human chorionic gonadotropin (hCG), which induce oocyte maturation and hydration, elevated the hfaqp1b transcript levels after 6 or 16 h of incubation, respectively. These results suggest that HfAqp1b may play a role during VT- and hCG-induced oocyte hydration in catfish, and that VT may regulate HfAqp1b at the transcriptional and post-translational level in a manner similar to the vasopressin-dependent mammalian AQP2.

Prochloraz-induced oocyte maturation in rainbow trout (Oncorhynchus mykiss), a molecular and functional analysis

In the present study, we aimed at characterizing the effect of prochloraz, an imidazole fungicide, on the oocyte meiotic maturation process in a freshwater teleost species, the rainbow trout (Oncorhynchus mykiss). Full-grown post-vitellogenic ovarian follicles were incubated in vitro with prochloraz, Luteinizing Hormone (LH), or a combination of prochloraz and LH. The occurrence of oocyte maturation was assessed by monitoring germinal vesicle breakdown (GVBD) after 62-h in vitro incubation. Experiments were repeated in presence of actinomycin D, cycloheximide, or trilostane. The effect of prochloraz on the production of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), the natural maturation-inducing steroid, was quantified by radioimmunoassay. In addition, the effect of prochloraz on ovarian expression of 12 genes was monitored by real-time PCR. Prochloraz (10(-5)M) administered alone was able to induce 100% GVBD in the most responsive females. The occurrence of GVBD observed after prochloraz stimulation of follicles originating from various females was similar and highly correlated with the occurrence of GVBD observed after stimulation with low LH concentration. In addition, oocyte maturation induced by LH or prochloraz was totally inhibited by actinomycin D, cycloheximide, and trilostane. Similarly to LH, prochloraz was able to trigger 17,20βP production by the ovarian follicle. Finally, prochloraz induced the overexpression of genes participating in 17,20βP production, intercellular communication, and paracrine control of preovulatory follicular differentiation such as igf, igf2, connexin 43, and 20β hydroxysteroid dehydrogenase (hsbd20). Together, our results demonstrate that prochloraz administered alone is able to trigger oocyte maturation through the induction of specific genes, some of them being also triggered by LH. Finally, our results clearly indicate that the effects of prochloraz and LH on oocyte maturation are synergistic.

Studies on feminization, sex determination, and differentiation of the Southern catfish, Silurus meridionalis--a review

The sex ratio of the feral Southern catfish was reported to be about 1:1, while the fish obtained by artificial fertilization were always female. Hence, we examined the possible influence of the micro-environment during artificial insemination (pH of the ovarian fluid and concentration of the semen) and early development (feed, hatching temperature, and water) on the sex ratio of Southern catfish fry. In order to examine the possibility of the occurrence of gynogenesis during artificial propagation, cytological observations on the insemination processes and the artificial induction of gynogenesis were also performed. However, no male fish were obtained even in these experiments, excluding the possibilities of these micro-environmental changes on catfish sex ratio and the occurrence of gynogenesis during artificial propagation. Female-to-male sex reversal was achieved by treatment with fadrozole (an aromatase inhibitor) and tamoxifen (an estrogen receptor antagonist). Histological analyses on the gonadal development of both female and induced male fish were subsequently performed. Moreover, several genes involved in sex differentiation, such as dmrt1, foxl2, and cyp19, and three subunits of gonadotropin (gth), i.e., gthalpha, lhbeta, and fshbeta, were isolated. Their expression patterns were studied under normal gonadal development and sex reversal conditions. The results revealed that dmrt1, foxl2, and cyp19a were closely related to catfish sex differentiation, and the gth subunits were possibly related to ovarian differentiation and oocyte development. Taken together, we hypothesized that estrogen was highly responsible for the ovarian differentiation and feminization of catfish fry under artificial propagation, although the mechanism involved remains elusive.

Effects of pulp and paper mill effluent extractives on aromatase CYP19a gene expression and sex steroid levels in juvenile triploid rainbow trout

We evaluated plasma testosterone (T) and 17beta-estradiol (E2) levels and ovarian aromatase CYP19a gene expression following a single intraperitoneal injection of Chilean pulp and paper mill effluent extracts into juvenile triploid rainbow trout. Fish injected with untreated effluent extracts had increased plasma T after 4 days, while plasma E2 concentration was increased in fish injected with both primary and secondary treated effluent extracts at the same sampling period. Ovarian CYP19a gene expression as measured by qRT-PCR was significantly induced in fish injected with the untreated, primary and secondary treated pulp and paper mill effluent extracts. Similar induction of CYP19a expression was found in fish injected with the androgens androstenedione (ADD) and T. A Principal Component Analysis (PCA) was conducted in order to identify structure in relationships between all measured variables and identifying which factors were most responsible for the variance observed within the plasma steroid levels, upregulation of ovarian CYP19a gene expression and the final estrogenic effect of increased plasma VTG levels. This analysis indicated a cluster correlation between plasma T levels and CYP19a gene expression (Factor 1, explaining 27.2% of total variance), a cluster including condition factor and liver somatic index (Factor 2, explaining 17.3%) and an additional cluster including plasma E2 and vitellogenin levels (Factor 3, explaining an additional 15.8%). The present results indicate that Chilean pulp and paper mill effluent extracts cause estrogenic effects in triploid rainbow trout. These effects could be related to the compounds present in the effluent that act as estrogen receptor agonists, or that induce changes leading to increased amounts of endogenous estrogens, reflected by increased E2 levels and induced aromatase expression/activity.

Saving energy to fuel exercise: swimming suppresses oocyte development and downregulates ovarian transcriptomic response of rainbow trout Oncorhynchus mykiss

Metabolic processes and sexual maturation closely interact during the long-distance reproductive migration of many fish species to their spawning grounds. In the present study, we have used exercise experimentally to investigate the effects on sexual maturation in rainbow trout. Pubertal autumn-spawning seawater-raised female rainbow trout Oncorhynchus mykiss (n = 26; 50 cm, 1.5 kg) were rested or swum at a near optimal speed of 0.75 body lengths per second in a 6,000-liter swim flume under natural reproductive conditions (16 degrees C fresh-water, starvation, 8:16-h light-dark photoperiod). Fish were sampled after arrival and subsequently after 10 days (resting or swimming 307 km) and 20 days (resting or swimming 636 km). Ovarian development was significantly reduced in the swimmers. Analysis of the expression of key factors in the reproductive axis included pituitary kiss1-receptor, lh, and fsh and ovarian lh-receptor, fsh-receptor, aromatase, and vitellogenin-receptor (vtgr). Swimmers had lower pituitary lh and ovarian vtgr expression than resters. Furthermore, the number of late vitellogenic oocytes was lower in swimmers than in resters, probably resulting from the lower vtgr expression, and vitellogenin plasma levels were higher. Therefore, swimming exercise suppresses oocyte development possibly by inhibiting vitellogenin uptake. Transcriptomic changes that occurred in the ovary of exercised fish were investigated using a salmonid cDNA microarray platform. Protein biosynthesis and energy provision were among the 16 functional categories that were all downregulated in the ovary. Downregulation of the transcriptomic response in the ovary illustrates the priority of energy reallocation and will save energy to fuel exercise. A swimming-induced ovarian developmental suppression at the start of vitellogenesis during long-term reproductive migration may be a strategy to avoid precocious muscle atrophy.

An involvement of vasotocin in oocyte hydration in the catfish Heteropneustes fossilis: A comparison with effects of isotocin and hCG

In the present investigation, in vitro effects of vasotocin (VT) on oocyte (follicular) hydration during germinal vesicle breakdown (GVBD) and ovulation were demonstrated in hCG-primed and non-primed catfish. The data were compared with that of groups incubated with isotocin, and hCG alone or in combination with VT. The priming with hCG resulted in significant increases on percentage GVBD and ovulation, and stimulated follicular hydration, as judged by the increase in diameter, volume, water content, osmolality and Ca(2+) concentration. However, Na(+), K(+) ATPase activity, and concentrations of Na(+), K(+) and Mg(2+) did not alter significantly. The incubations with hCG or VT stimulated all the above parameters. In the non-primed fish, the response of hCG was significantly higher on follicular diameter, volume and osmolality, and that of VT on ovulation. In the primed fish, the VT response was significantly higher on GVBD, ovulation, Na(+), K(+) ATPase activity and divalent cation concentrations. The co-incubation with both hCG and VT produced maximal increases in all the parameters with significantly higher effects in the primed fish. The effects of IT on various parameters were relatively low compared to hCG or VT effects. The results indicate that VT may play an important role in oocyte (follicular) hydration, which is consistent with its role in osmoregulation of fish.

Cellular and molecular evidence for a role of tumor necrosis factor alpha in the ovulatory mechanism of trout

BACKGROUND

The relevance of immune-endocrine interactions to the regulation of ovarian function in teleosts is virtually unexplored. As part of the innate immune response during infection, a number of cytokines such as tumor necrosis factor alpha (TNF alpha) and other immune factors, are produced and act on the reproductive system. However, TNF alpha is also an important physiological player in the ovulatory process in mammals. In the present study, we have examined for the first time the effects of TNF alpha in vitro in preovulatory ovarian follicles of a teleost fish, the brown trout (Salmo trutta).

METHODS

To determine the in vivo regulation of TNF alpha expression in the ovary, preovulatory brook trout (Salvelinus fontinalis) were injected intraperitoneally with either saline or bacterial lipopolysaccharide (LPS). In control and recombinant trout TNF alpha (rtTNF alpha)-treated brown trout granulosa cells, we examined the percentage of apoptosis by flow cytometry analysis and cell viability by propidium iodide (PI) staining. Furthermore, we determined the in vitro effects of rtTNF alpha on follicle contraction and testosterone production in preovulatory brown trout ovarian follicles. In addition, we analyzed the gene expression profiles of control and rtTNF alpha-treated ovarian tissue by microarray and real-time PCR (qPCR) analyses.

RESULTS

LPS administration in vivo causes a significant induction of the ovarian expression of TNF alpha. Treatment with rtTNF alpha induces granulosa cell apoptosis, decreases granulosa cell viability and stimulates the expression of genes known to be involved in the normal ovulatory process in trout. In addition, rtTNF alpha causes a significant increase in follicle contraction and testosterone production. Also, using a salmonid-specific microarray platform (SFA2.0 immunochip) we observed that rtTNF alpha induces the expression of genes known to be involved in inflammation, proteolysis and tissue remodeling. Furthermore, the expression of kallikrein, TOP-2, serine protease 23 and ADAM 22, genes that have been postulated to be involved in proteolytic and tissue remodeling processes during ovulation in trout, increases in follicles incubated in the presence of rtTNF alpha.

CONCLUSIONS

In view of these results, we propose that TNF alpha could have an important role in the biomechanics of follicle weakening, ovarian rupture and oocyte expulsion during ovulation in trout, primarily through its stimulation of follicular cell apoptosis and the expression of genes involved in follicle wall proteolysis and contraction.

Aquaporin expression dynamics in osmoregulatory tissues of Atlantic salmon during smoltification and seawater acclimation

Osmotic balance in fish is maintained through the coordinated regulation of water and ion transport performed by epithelia in intestine, kidney and gill. In the current study, six aquaporin (AQP) isoforms found in Atlantic salmon (Salmo salar) were classified and their tissue specificity and mRNA expression in response to a hyperosmotic challenge and during smoltification were examined. While AQP-1a was generic, AQP-1b had highest expression in kidney and AQP-3 was predominantly found in oesophagus, gill and muscle. Two novel teleost isoforms, AQP-8a and -8b, were expressed specifically in liver and intestinal segments, respectively. AQP-10 was predominantly expressed in intestinal segments, albeit at very low levels. Transfer from freshwater (FW) to seawater (SW) induced elevated levels of intestinal AQP-1a, -1b and -8b mRNA, whereas only AQP-8b was stimulated during smoltification. In kidney, AQP-1a, -3 and -10 were elevated in SW whereas AQP-1b was reduced compared with FW levels. Correspondingly, renal AQP-1a and -10 peaked during smoltification in April and March, respectively, as AQP-1b and AQP-3 declined. In the gill, AQP-1a and AQP-3 declined in SW whereas AQP-1b increased. Gill AQP-1a and -b peaked in April, whereas AQP-3 declined through smoltification. These reciprocal isoform shifts in renal and gill tissues may be functionally linked with the changed role of these organs in FW compared with SW. The presence and observed dynamics of the AQP-8b isoform specifically in intestinal sections suggest that this is a key water channel responsible for water uptake in the intestinal tract of seawater salmonids.

Oogenesis in teleosts: how eggs are formed

One of the major objectives of the aquaculture industry is the production of a large number of viable eggs with high survival. Major achievements have been made in recent years in improving protocols for higher efficiency of egg production and viability of progeny. Main gaps remain, however, in understanding the dynamic processes associated with oogenesis, the formation of an egg, from the time that germ cells turn into oogonia, until the release of ova during spawning in teleosts. Recent studies on primordial germ-cells, yolk protein precursors and their processing within the developing oocyte, the deposition of vitamins in eggs, structure and function of egg envelopes and oocyte maturation processes, further reveal the complexity of oogenesis. Moreover, numerous circulating endocrine and locally-acting paracrine and autocrine factors regulate the various stages of oocyte development and maturation. Though it is clear that the major regulators during vitellogenesis and oocyte maturation are the pituitary gonadotropins (LH and FSH) and sex steroids, the picture emerging from recent studies is of complex hormonal cross-talk at all stages between the developing oocyte and its surrounding follicle layers to ensure coordination of the various processes that are involved in the production of a fertilizable egg. In this review we aim at highlighting recent advances on teleost fish oocyte differentiation, maturation and ovulation, including those involved in the degeneration and reabsorption of ovarian follicles (atresia). The role of blood-borne and local ovarian factors in the regulation of the key steps of development reveal new aspects associated with egg formation.

Comparative transcriptomic analysis of follicle-enclosed oocyte maturational and developmental competence acquisition in two non-mammalian vertebrates

BACKGROUND

In vertebrates, late oogenesis is a key period during which the oocyte acquires its ability to resume meiosis (i.e. maturational competence) and to develop, once fertilized, into a normal embryo (i.e. developmental competence). However, the molecular mechanisms involved in these key biological processes are far from being fully understood. In order to identify key mechanisms conserved among teleosts and amphibians, we performed a comparative analysis using ovarian tissue sampled at successive steps of the maturational competence acquisition process in the rainbow trout (Oncorhynchus mykiss) and in the clawed toad (Xenopus laevis). Our study aimed at identifying common differentially expressed genes during late oogenesis in both species. Using an existing transcriptomic analysis that had previously been carried out in rainbow trout, candidate genes were selected for subsequent quantitative PCR-based comparative analysis.

RESULTS

Among the 1200 differentially expressed clones in rainbow trout, twenty-six candidate genes were selected for further analysis by real-time PCR in both species during late oogenesis. Among these genes, eight had similar expression profiles in trout and Xenopus. Six genes were down-regulated during oocyte maturation (cyp19a1, cyp17a1, tescalcin, tfr1, cmah, hsd11b3) while two genes exhibited an opposite pattern (apoc1, star). In order to document possibly conserved molecular mechanisms, four genes (star, cyp19a1, cyp17a1 and hsd11b3) were further studied due to their known or suspected role in steroidogenesis after characterization of the orthology relationships between rainbow trout and Xenopus genes. Apoc1 was also selected for further analysis because of its reported function in cholesterol transport, which may modulate steroidogenesis by regulating cholesterol bioavailability in the steroidogenic cells.

CONCLUSIONS

We have successfully identified orthologous genes exhibiting conserved expression profiles in the ovarian follicle during late oogenesis in both trout and Xenopus. While some identified genes were previously uncharacterized during Xenopus late oogenesis, the nature of these genes has pointed out molecular mechanisms possibly conserved in amphibians and teleosts. It should also be stressed that in addition to the already suspected importance of steroidogenesis in maturational competence acquisition, our approach has shed light on other regulatory pathways which may be involved in maturational and developmental competence acquisitions that will require further studies.

Stressors, glucocorticoids and ovarian function in teleosts

The purpose of this overview is to re-examine the postulated direct and indirect actions of glucocorticoids on ovarian function in teleosts. The re-examination is undertaken in light of recent advances in the understanding of the stress response itself, the mode of action of the hypothalamus-pituitary gland-ovarian (HPO) axis, the mechanisms of control of oestrogen-dependent hepatic vitellogenin (VtG) secretion and the apparent roles of corticotrophin-releasing hormone (CRH) and CRH-related factors in the regulation of feeding activity. Many of the results of different studies, particularly whole-animal studies, are conflicting, and little is known as to whether the hormone acts directly on various components of the HPO axis or indirectly by virtue of redirection of energy resources away from ovarian growth to provide a source of metabolic resources for other organ systems involved in the physiological stress response. In vitro studies provide some new insights into the direct actions of glucocorticoid on hepatic VtG synthesis and ovarian follicle steroidogenesis, but even here, in some studies the cellular sites of action of these hormones is not altogether clear. The overview emphasizes the complexity of the stress response, the complexity of the regulation of glucocorticoid-dependent gene expression and the extensive interactive nature of the HPO with other hypothalamus-pituitary gland-peripheral endocrine gland axes, such as the thyroid (HPT), 'somatic' (GH-IGF) and interrenal tissue (HPI) axes.

Molecular pathways during marine fish egg hydration: the role of aquaporins

The pre-ovulatory hydration of the oocyte of marine teleosts, a unique process among vertebrates that occurs concomitantly with meiosis resumption (oocyte maturation), is a critical process for the correct development and survival of the embryo. Increasing information is available on the molecular mechanisms that control oocyte maturation in fish, but the identification of the cellular processes involved in oocyte hydration has remained long ignored. During the past few years, a number of studies have identified the major inorganic and organic osmolytes that create a transient intra-oocytic osmotic potential for hydrating the oocytes, whereas water influx was believed to occur passively. Recent work, however, has uncovered the role of a novel molecular water channel (aquaporin), designated aquaporin-1b (Aqp1b), which facilitates water permeation and resultant swelling of the oocyte. The Aqp1b belongs to a teleost-specific subfamily of water-selective aquaporins, similar to mammalian aquaporin-1 (AQP1) that has possibly evolved by duplication of a common ancestor and further neofunctionalization in oocytes of marine teleosts for water uptake. Strikingly, Aqp1b shows specific regulatory domains at the cytoplasmic tail, which are key to the vesicular trafficking and temporal insertion of Aqp1b in the oocyte plasma membrane during the phase of maximal hydration. These findings are revealing that the mechanism of oocyte hydration in marine teleosts is a highly regulated process based on the interplay between the generation of inorganic and organic osmolytes and the controlled insertion of Aqp1b in the oocyte surface. The discovery of Aqp1b in teleosts provides an important insight into the molecular basis of the production of viable eggs in marine fish.

Endocrine/paracrine control of zebrafish ovarian development

Ovarian differentiation and the processes of follicle development, oocyte maturation and ovulation are complex events, requiring the coordinated action of regulatory molecules. In zebrafish, ovarian development is initiated at 10 days after hatching and fish become sexually mature at 3 months. Adult zebrafish have asynchronous ovaries, which contain follicles of all stages of development. Eggs are spawned daily under proper environmental conditions in a population of zebrafish, with individual females spawning irregularly every 4-7 days in mixed sex conditions. Maximal embryo viability is achieved when sexually isolated females are bred in 10-day intervals [Niimi, A.J., LaHam, Q.N., 1974. Influence of breeding time interval on egg number, mortality, and hatching of the zebra fish Brachydanio verio. Can. J. Zool. 52, 515-517]. Similar to other vertebrates, hormones from the hypothalamus-pituitary-gonadal axis play important roles in regulating follicle development. Follicle stimulating hormone (FSH) stimulates estradiol production, which in turn, promotes viteollogenesis. Luteinizing hormone (LH) stimulates the production of 17,20beta-dihydroxy-4-pregnen-3-one (17,20betaP) or maturation inducing hormone (MIH) which acts through membrane progestin receptors to activate maturation promoting factor, leading to oocyte maturation. Recent studies in zebrafish have also provided novel insights into the functions of ovary-derived growth factors in follicle development and oocyte maturation. The present review summarizes the current knowledge on how endocrine and paracrine factors regulate ovarian development in zebrafish. Special emphasis is placed on how follicle development and oocyte maturation in adult females is regulated by gonadotropins, ovarian steroids and growth factors produced by the ovary.

Stress transcriptomics in fish: a role for genomic cortisol signaling

The physiological responses to stressors, including hormonal profiles and associated tissue responsiveness have been extensively studied in teleosts, but the molecular mechanisms associated with this adaptive response are not well understood. The advent of cDNA microarray technology has transformed the field of functional genomics by revealing global gene expression changes in response to stressor exposures even in non-mammalian vertebrates, including fish. A unifying response in studies related to stressor exposure is activation of the hypothalamus-pituitary-interrenal (HPI) axis in fish, leading to cortisol release into the circulation. Here we will discuss the implications of some of the gene expression changes observed in response to acute stress in fish, while highlighting a role for cortisol in this adaptive stress response. As liver is a key organ for metabolic adjustments to stressors and also is a major target for cortisol action, the genomic studies pertaining to stress and glucocorticoid regulation have focused mainly on this tissue. The studies have identified several genes that are altered transiently after an acute stressor exposure in fish. A number of these stress-responsive genes were also modulated by glucocorticoid receptor activation, suggesting that elevation in cortisol levels during stressor exposure may be a key signal for target tissue molecular programming, essential for stress adaptation. The identification of regulatory gene networks that are stress activated, and modulated by cortisol, both in hepatic and extra-hepatic tissues, including gonads, brain, immune cells and gills, will provide a mechanistic framework to characterize the multifaceted role of cortisol during stress adaptation.

New insights into molecular pathways associated with flatfish ovarian development and atresia revealed by transcriptional analysis

Background

The Senegalese sole (Solea senegalensis) is a marine flatfish of increasing commercial interest. However, the reproduction of this species in captivity is not yet controlled mainly because of the poor knowledge on its reproductive physiology, as it occurs for other non-salmonid marine teleosts that exhibit group-synchronous ovarian follicle development. In order to investigate intra-ovarian molecular mechanisms in Senegalese sole, the aim of the present study was to identify differentially expressed genes in the ovary during oocyte growth (vitellogenesis), maturation and ovarian follicle atresia using a recently developed oligonucleotide microarray.

Results

Microarray analysis led to the identification of 118 differentially expressed transcripts, of which 20 and 8 were monitored by real-time PCR and in situ hybridization, respectively. During vitellogenesis, many up-regulated ovarian transcripts had putative mitochondrial function/location suggesting high energy production (NADH dehydrogenase subunits, cytochromes) and increased antioxidant protection (selenoprotein W2a), whereas other regulated transcripts were related to cytoskeleton and zona radiata organization (zona glycoprotein 3, alpha and beta actin, keratin 8), intracellular signalling pathways (heat shock protein 90, Ras homolog member G), cell-to-cell and cell-to-matrix interactions (beta 1 integrin, thrombospondin 4b), and the maternal RNA pool (transducer of ERBB2 1a, neurexin 1a). Transcripts up-regulated in the ovary during oocyte maturation included ion transporters (Na⁺-K⁺-ATPase subunits), probably required for oocyte hydration, as well as a proteinase inhibitor (alpha-2-macroglobulin) and a vesicle calcium sensor protein (extended synaptotagmin-2-A). During follicular atresia, few transcripts were found to be up-regulated, but remarkably most of them were localized in follicular cells of atretic follicles, and they had inferred roles in lipid transport (apolipoprotein C-I), chemotaxis (leukocyte cell-derived chemotaxin 2,), angiogenesis (thrombospondin), and prevention of apoptosis (S100a10 calcium binding protein).

Conclusion

This study has identified a number of differentially expressed genes in the ovary that were not previously found to be regulated during ovarian development in marine fish. Specifically, we found evidence, for the first time in teleosts, of the activation of chemoattractant, angiogenic and antiapoptotic pathways in hypertrophied follicular cells at the onset of ovarian atresia.

Identification of novel oocyte and granulosa cell markers

Here we present novel gene expression patterns in the ovary as part of an ongoing assessment of published micro-array data from mouse oocytes and embryos. We present the expression patterns of 13 genes that had been determined by micro-array to be expressed in the mature egg, but not during subsequent preimplantation development. In-situ hybridization of sectioned ovaries revealed that these genes were expressed in one of two distinct patterns: (1) oocyte-specific or (2) expressed in both the oocyte and surrounding granulosa cells. Despite the fact that micro-array data demonstrated expression in the egg, several of these genes are expressed at low levels in the oocyte, but strongly expressed in granulosa cells. Eleven of these genes have no reported function or expression during oogenesis, indicating that this approach is a necessary step towards functional annotation of the genome. Also of note is that while some of these gene products have been well characterized in other tissues and cell types, others are relatively unstudied in the literature. Our results provide novel gene expression information that may provide insights into the molecular mechanisms of follicular recruitment, oocyte maturation and ovulation and will direct further experimentation into the role these genes play during oogenesis.

Effects of hCG and ovarian steroid hormones on vasotocin levels in the female catfish Heteropneustes fossilis

Effects of hCG, ovariectomy and estradiol replacement on brain, plasma and/or ovarian vasotocin in vivo, and estradiol, progesterone, 17alpha, 20beta-hydroxy-4-pregnen-3-one and hCG on ovarian vasotocin in vitro were investigated in the catfish. A 100IU/fish of hCG induced ovulation and elicited both periovulatory and post-ovulatory changes in vasotocin concentrations with a significant increase up to 8h in the brain and up to 16h in both plasma and ovary. After stripping the fish at 16h, the peptide concentration decreased significantly with time, up to 4 days. Ovariectomy in early pre-spawning phase resulted in a duration-dependent significant reduction of both brain and plasma vasotocin. Estradiol replacement in 3-week ovariectomized fish produced dosage-dependent biphasic effects: the lower dosage (0.1microg/g) restored the vasotocin level while the higher dosage (0.5microg/g) decreased it significantly below the control level. In vitro incubation of ovarian tissues with estradiol produced season-dependent effects on vasotocin. The incubation of pre-vitellogenic ovarian pieces with estradiol (1, 10, and 100ng/ml) elevated vasotocin level in a dose- and duration-dependent manner while that of post-vitellogenic follicles resulted in a significant decrease. The incubation of intact post-vitellogenic follicles or follicular envelope with progesterone and 17alpha, 20beta-hydroxy-4-pregnen-3-one (1microg/ml) or hCG (20IU/ml) for 8 and 16h significantly increased vasotocin in a duration-dependent manner. The results show that both gonadotropin and ovarian steroids modulate vasotocin titer, which may influence follicular growth, ovulation and spawning in the catfish.

Ovarian function of the trout preovulatory ovary: new insights from recent gene expression studies

During the preovulatory period the follicle-enclosed oocyte progressively acquires maturational and developmental competence. In addition, the follicle is also preparing for the release of the oocyte from the follicle at ovulation. Using real-time PCR and cDNA microarrays we have investigated the molecular mechanisms of oocyte competence acquisition and ovulation in rainbow trout (Oncorhynchus mykiss) by monitoring gene expression in the preovulatory ovary. These studies have demonstrated that many molecular events related to maturational competence and developmental competence acquisition, and ovulation occur concomitantly in the preovulatory ovarian follicle. Oocyte maturational competence acquisition is associated with a decrease of estrogen synthesis and signaling capacities. We also observed a differential expression of genes encoding for igfs and related binding protein, members of the TGF beta superfamily, proteins involved in ion and water transport, bone morphogenetic proteins, and cathepsins. In addition, our observation of a strong up-regulation, prior to ovulation, of genes encoding for proteins putatively involved in proteolysis, inflammation, coagulation, vasodilatation, and angiogenesis further supports the hypothesis comparing ovulation with an inflammatory-like reaction. Together, our results suggest that a finely tuned cross-talk exists between oocyte and follicular layers and between the ovulatory process and the oocyte maturational and developmental competence acquisition processes.

Immunocytochemical localization, HPLC characterization, and seasonal dynamics of vasotocin in the brain, blood plasma and gonads of the catfish Heteropneustes fossilis

Immunocytochemical distribution and dynamics of vasotocin (VT) were studied in the air-breathing catfish Heteropneustes fossilis in relation to the reproductive cycle. Vasotocin was localized in the brain and ovary by streptavidin-biotin immunocytochemistry. The immunoreactivity was found throughout the hypothalamo-hypophysial neurosecretory system consisting of the magnocellular and parvocellular neurons of the nucleus preopticus, neurosecretory axonal tract and neurohypophysis (NH). The VT neurons showed seasonal changes; they were numerically less in resting phase but increased during the recrudescent phase. The neurons were hypertrophied and degranulated in pre-spawning phase and heavily degranulated and vacuolated in spawning phase. In the NH, the density of VT fibers increased up to the pre-spawning phase and decreased thereafter. In the ovary, VT immunoreactivity was noticed in the follicular layer and varied with the growth of the follicles. Vasotocin was characterized and quantified by a high performance liquid chromatography with UV detection method in the brain, plasma and ovary. Brain and plasma VT concentrations were also assayed with an EIA method, which was more sensitive than the HPLC method with values about 2-fold higher. Vasotocin levels showed significant seasonal and sexual differences with higher concentrations in females in the recrudescent (preparative, pre-spawning and spawning) phase. Brain VT recorded the highest concentration in the preparative phase (both sexes) while plasma (both sexes) and ovarian VT in the spawning phase. The ovarian concentration of VT was 15- and 25-fold higher in the pre-spawning and spawning phases (when expressed per mg protein), respectively, than plasma but lower than brain levels. In testis, VT concentration was relatively low and apparently did not show any significant seasonal variation. The seasonal activity patterns and gonadal distribution of VT indicate a reproductive function of the peptide.

Genomic resources for a commercial flatfish, the Senegalese sole (Solea senegalensis): EST sequencing, oligo microarray design, and development of the Soleamold bioinformatic platform

Background

The Senegalese sole, Solea senegalensis, is a highly prized flatfish of growing commercial interest for aquaculture in Southern Europe. However, despite the industrial production of Senegalese sole being hampered primarily by lack of information on the physiological mechanisms involved in reproduction, growth and immunity, very limited genomic information is available on this species.

Results

Sequencing of a S. senegalensis multi-tissue normalized cDNA library, from adult tissues (brain, stomach, intestine, liver, ovary, and testis), larval stages (pre-metamorphosis, metamorphosis), juvenile stages (post-metamorphosis, abnormal fish), and undifferentiated gonads, generated 10,185 expressed sequence tags (ESTs). Clones were sequenced from the 3'-end to identify isoform specific sequences. Assembly of the entire EST collection into contigs gave 5,208 unique sequences of which 1,769 (34%) had matches in GenBank, thus showing a low level of redundancy. The sequence of the 5,208 unigenes was used to design and validate an oligonucleotide microarray representing 5,087 unique Senegalese sole transcripts. Finally, a novel interactive bioinformatic platform, Soleamold, was developed for the Senegalese sole EST collection as well as microarray and ISH data.

Conclusion

New genomic resources have been developed for S. senegalensis, an economically important fish in aquaculture, which include a collection of expressed genes, an oligonucleotide microarray, and a publicly available bioinformatic platform that can be used to study gene expression in this species. These resources will help elucidate transcriptional regulation in wild and captive Senegalese sole for optimization of its production under intensive culture conditions.

Gene expression patterns during the larval development of European sea bass (dicentrarchus labrax) by microarray analysis

During the larval period, marine teleosts undergo very fast growth and dramatic changes in morphology, metabolism, and behavior to accomplish their metamorphosis into juvenile fish. Regulation of gene expression is widely thought to be a key mechanism underlying the management of the biological processes required for harmonious development over this phase of life. To provide an overall analysis of gene expression in the whole body during sea bass larval development, we monitored the expression of 6,626 distinct genes at 10 different points in time between 7 and 43 days post-hatching (dph) by using heterologous hybridization of a rainbow trout cDNA microarray. The differentially expressed genes (n = 485) could be grouped into two categories: genes that were generally up-expressed early, between 7 and 23 dph, and genes up-expressed between 25 and 43 dph. Interestingly, among the genes regulated during the larval period, those related to organogenesis, energy pathways, biosynthesis, and digestion were over-represented compared with total set of analyzed genes. We discuss the quantitative regulation of whole-body contents of these specific transcripts with regard to the ontogenesis and maturation of essential functions that take place over larval development. Our study is the first utilization of a transcriptomic approach in sea bass and reveals dynamic changes in gene expression patterns in relation to marine finfish larval development.

A novel, functional, and highly divergent sex hormone-binding globulin that may participate in the local control of ovarian functions in salmonids

A cDNA encoding for a novel rainbow trout SHBG was identified and characterized. Phylogenetic analysis showed that this novel SHBG, named SHBGb, was a highly divergent paralog of the classical SHBG (SHBGa) form previously known in vertebrates including zebrafish, seabass, and rainbow trout. Using all available sequences, no SHBGb-like sequence could be identified in any fish species besides Atlantic salmon. Rainbow trout SHBGa and SHBGb share only 26% sequence identity at the amino acid level and exhibit totally distinct tissue distribution, thus demonstrating a functional shift of SHBGb. Indeed, shbga mRNA was predominantly expressed in liver and spleen but could not be detected in the ovary, whereas shbgb had a predominant ovarian expression but could not be detected in liver. Despite its high divergence, rainbow trout SHBGb expressed in COS-7 cells could bind estradiol and testosterone with high affinity and specificity. Both rainbow trout shbgb mRNA and proteins were localized to the granulosa cells of vitellogenic ovarian follicles, whereas SHBGb immunoreactivity was also found in theca cells. Finally, shbgb ovarian mRNA expression exhibited a significant drop between late vitellogenesis and oocyte maturation at a time when ovarian aromatase (cyp19a) gene expression and estradiol circulating levels exhibited a dramatic decrease. Together, these observations show that SHBGb is a functional and highly divergent SHBG paralog probably arising from a salmonid-specific duplication of the shbg gene.

A pig multi-tissue normalised cDNA library: large-scale sequencing, cluster analysis and 9K micro-array resource generation

Background

Domestic animal breeding and product quality improvement require the control of reproduction, nutrition, health and welfare in these animals. It is thus necessary to improve our knowledge of the major physiological functions and their interactions. This would be greatly enhanced by the availability of expressed gene sequences in the databases and by cDNA arrays allowing the transcriptome analysis of any function.

The objective within the AGENAE French program was to initiate a high-throughput cDNA sequencing program of a 38-tissue normalised library and generate a diverse microarray for transcriptome analysis in pig species.

Results

We constructed a multi-tissue cDNA library, which was normalised and subtracted to reduce the redundancy of the clones. Expressed Sequence Tags were produced and 24449 high-quality sequences were released in EMBL database. The assembly of all the public ESTs (available through SIGENAE website) resulted in 40786 contigs and 54653 singletons. At least one Agenae sequence is present in 11969 contigs (12.5%) and in 9291 of the deeper-than-one-contigs (22.8%). Sequence analysis showed that both normalisation and subtraction processes were successful and that the initial tissue complexity was maintained in the final libraries. A 9K nylon cDNA microarray was produced and is available through CRB-GADIE. It will allow high sensitivity transcriptome analyses in pigs.

Conclusion

In the present work, a pig multi-tissue cDNA library was constructed and a 9K cDNA microarray designed. It contributes to the Expressed Sequence Tags pig data, and offers a valuable tool for transcriptome analysis.

Co-localization of angiotensin-converting enzyme 2-, octomer-4- and CD34-positive cells in rabbit atherosclerotic plaques

Angiotensin‐converting enzyme 2 (ACE2) is a novel enzyme with possible implications in the treatment of blood pressure disorders. Recent evidence suggests that an upregulation of ACE2 can be stimulated by all‐trans retinoic acid (at‐RA); however, at‐RA also affects regulation of the stem‐cell marker octomer‐4 (Oct‐4) and thus cellular differentiation. We have previously shown that smooth muscle cells and macrophages present within rabbit atherosclerotic plaques are positive for ACE2, Oct‐4 and the haematopoietic stem‐cell marker CD34. Thus, to provide evidence that possible at‐RA treatment could affect both plaque cellular biology (via effects on cellular differentiation) and blood pressure (via ACE2), it is vital to show that cells with atherosclerotic plaques co‐express all three markers. Thus, we sought to provide evidence that a subset of cells within atherosclerotic plaques is positive for ACE2, Oct‐4 and CD34. We used New Zealand White rabbits that were fed a control diet supplemented with 0.5% cholesterol plus 1% methionine for 4 weeks and then allowed to consume a normal diet for 10 weeks. Immunohistochemistry was performed by standard techniques. We report that ACE2, Oct‐4 and CD34 were all present within atherosclerotic plaques. Although macrophages were positive for all three markers, spindle‐shaped cells in the media did not show all three markers. The endothelium overlying normal arterial wall showed positive Oct‐4 and ACE2 immunoreactivity, but CD34 immunoreactivity was patchy, indicating that such cells might not have fully differentiated. It is concluded that cells in atherosclerotic plaques express co‐express ACE2, Oct‐4 and CD34. Further studies aimed at establishing the effects of all‐trans retinoic acid on blood pressure and atherosclerotic cell differentiation are warranted.

Microarray-based analysis of fish egg quality after natural or controlled ovulation

Background

The preservation of fish egg quality after ovulation-control protocols is a major issue for the development of specific biotechnological processes (e.g. nuclear transfer). Depending on the species, it is often necessary to control the timing of ovulation or induce the ovulatory process. The hormonal or photoperiodic control of ovulation can induce specific egg quality defects that have been thoroughly studied. In contrast, the impact on the egg transcriptome as a result of these manipulations has received far less attention. Furthermore, the relationship between the mRNA abundance of maternally-inherited mRNAs and the developmental potential of the egg has never benefited from genome-wide studies. Thus, the present study aimed at studying the rainbow trout (Oncorhynchus mykiss) egg transcriptome after natural or controlled ovulation using 9152-cDNA microarrays.

Results

The analysis of egg transcriptome after natural or controlled ovulation led to the identification of 26 genes. The expression patterns of 17 of those genes were monitored by real-time PCR. We observed that the control of ovulation by both hormonal induction and photoperiod manipulation induced significant changes in the egg mRNA abundance of specific genes. A dramatic increase of Apolipoprotein C1 (APOC1) and tyrosine protein kinase HCK was observed in the eggs when a hormonal induction of ovulation was performed. In addition, both microarray and real-time PCR analyses showed that prohibitin 2 (PHB2) egg mRNA abundance was negatively correlated with developmental success.

Conclusion

First, we showed, for the first time in fish, that the control of ovulation using either a hormonal induction or a manipulated photoperiod can induce differences in the egg mRNA abundance of specific genes. While the impact of these modifications on subsequent embryonic development is unknown, our observations clearly show that the egg transcriptome is affected by an artificial induction of ovulation.

Second, we showed that the egg mRNA abundance of prohibitin 2 was reflective of the developmental potential of the egg.

Finally, the identity and ontology of identified genes provided significant hints that could result in a better understanding of the mechanisms associated with each type of ovulation control (i.e. hormonal, photoperiodic), and in the identification of conserved mechanisms triggering the loss of egg developmental potential.