In silico predicted reproductive endocrine transcriptional regulatory networks during zebrafish (Danio rerio) development

The interconnected topology of transcriptional regulatory networks (TRNs) readily lends to mathematical (or in silico) representation and analysis as a stoichiometric matrix. Such a matrix can be 'solved' using the mathematical method of extreme pathway (ExPa) analysis, which identifies uniquely activated genes subject to transcription factor (TF) availability. In this manuscript, in silico multi-tissue TRN models of brain, liver and gonad were used to study reproductive endocrine developmental programming in zebrafish (Danio rerio) from 0.25h post fertilization (hpf; zygote) to 90 days post fertilization (dpf; adult life stage). First, properties of TRN models were studied by sequentially activating all genes in multi-tissue models. This analysis showed the brain to exhibit lowest proportion of co-regulated genes (19%) relative to liver (23%) and gonad (32%). This was surprising given that the brain comprised 75% and 25% more TFs than liver and gonad respectively. Such 'hierarchy' of co-regulatory capability (brain<liver<gonad) indicated presence of highly gene-specific TRNs in the brain, alluding to its role as 'master controller' of endocrine function. Second, TRN models were constrained with varying TF availabilities during zebrafish development. Normalized numbers of genes active during development showed concomitant activations between brain and gonad from 10 to 12 hpf (embryonic life stage) up to 30-90 dpf (adult life stage). This indicated a putative 'syncing' between the brain and gonad, and initiation of an early reproductive endocrine developmental program. Finally, comparison of in vivo active genes with those predicted in silico showed relatively good agreement for brain (49%), liver (27%) and gonad (32%). The multi-tissue TRN models presented can lend diagnostic insights into the effects of changing environmental and/or genetic constraints on reproductive endocrine function.

Grass Carp Follisatin: Molecular Cloning, Functional Characterization, Dopamine D1 Regulation at Pituitary Level, and Implication in Growth Hormone Regulation

Activin is involved in pituitary hormone regulation and its pituitary actions can be nullified by local production of its binding protein follistatin. In our recent study with grass carp, local release of growth hormone (GH) was shown to induce activin expression at pituitary level, which in turn could exert an intrapituitary feedback to inhibit GH synthesis and secretion. To further examine the activin/follistatin system in the carp pituitary, grass carp follistatin was cloned and confirmed to be single-copy gene widely expressed at tissue level. At the pituitary level, follistatin signals could be located in carp somatotrophs, gonadotrophs, and lactotrophs. Functional expression also revealed that carp follistatin was effective in neutralizing activin's action in stimulating target promoter with activin-responsive elements. In grass carp pituitary cells, follistatin co-treatment was found to revert activin inhibition on GH mRNA expression. Meanwhile, follistatin mRNA levels could be up-regulated by local production of activin but the opposite was true for dopaminergic activation with dopamine (DA) or its agonist apomorphine. Since GH stimulation by DA via pituitary D1 receptor is well-documented in fish models, the receptor specificity for follistatin regulation by DA was also investigated. Using a pharmacological approach, the inhibitory effect of DA on follistatin gene expression was confirmed to be mediated by pituitary D1 but not D2 receptor. Furthermore, activation of D1 receptor by the D1-specific agonist SKF77434 was also effective in blocking follistatin mRNA expression induced by activin and GH treatment both in carp pituitary cells as well as in carp somatotrophs enriched by density gradient centrifugation. These results, as a whole, suggest that activin can interact with dopaminergic input from the hypothalamus to regulate follistatin expression in carp pituitary, which may contribute to GH regulation by activin/follistatin system via autocrine/paracrine mechanisms.

Dynamic expression of tgf-β2, tgf-β3 and inhibin βA during muscle growth resumption and satellite cell differentiation in rainbow trout (Oncorhynchus mykiss)

Members of the TGF-β superfamily are involved in numerous cell functions; however, except for myostatin, their roles in the regulation of muscle growth in fish are completely unknown. We measured tgf-β1, tgf-β2, tgf-β3, inhibin βA (inh) and follistatin (fst) gene expression during muscle growth recovery following a fasting period. We observed that tgf-β1a and tgf-β2 expression were quickly down-regulated after refeeding and that tgf-β3 reached its highest level of expression 7days post-refeeding, mirroring myogenin expression. Inh βA1 mRNA levels decreased sharply after refeeding, in contrast to fst b2 expression, which peaked at day 2. No significant modification of expression was observed for tgf-β1a, tgf-β1b, tgf-β1c and tgf-β6 during refeeding. In vitro, tgf-β2 and inh βA1 expression decreased during the differentiation of satellite cells, whereas tgf-β3 expression increased following the same pattern as myogenin. Surprisingly, fst b1 and fst b2 expression decreased during differentiation, whereas no variation was observed in fst a1 and fst a2 expression levels. In vitro analyses also indicated that IGF1 treatment up-regulated tgf-β3, inh βA1 and myogenin expression, and that MSTN treatment increased fst b1 and fst b2 expression. In conclusion, we showed that the expression of tgf-β2, tgf-β3 and inh βA1 is dynamically regulated during muscle growth resumption and satellite cell differentiation, strongly suggesting that these genes have a role in the regulation of muscle growth.

Targeted gene disruption in zebrafish reveals noncanonical functions of LH signaling in reproduction

The pivotal role of gonadotropin signaling in regulating gonadal development and functions has attracted much research attention in the past 2 decades. However, the precise physiological role of gonadotropin signaling is still largely unknown in fish. In this study, we have established both LH β-subunit (lhb) and LH receptor (lhr) knockout zebrafish lines by transcription activator-like effector nucleases. Intriguingly, both homozygous lhb and lhr mutant male fish are fertile. The fertilization rate, sperm motility, and histological structure of the testis were not affected in either lhb or lhr mutant males. On the contrary, homozygous lhb mutant females are infertile, whereas homozygous lhr mutant females are fertile. Folliculogenesis was not affected in either lhb or lhr mutants, but oocyte maturation and ovulation were disrupted in lhb mutant, whereas only ovulation was affected in lhr mutant. Differential expression of genes in the ovary involved in steroidogenesis, oocyte maturation, and ovulation was found between the lhb and lhr mutants. These data demonstrate the essential role of LH signaling in oocyte maturation and ovulation, and support the notion that LH acts through the FSH receptor in the absence of LH receptor. Moreover, the defects of lhb mutant could be partially restored by administration of human chorionic gonadotropin. This in vivo evidence in the present study demonstrates, for the first time in any vertebrate species, that LH signaling is indispensable in female reproduction but not in male reproduction. LH signaling is demonstrated to control oocyte maturation and ovulation in the ovary.

Intra-ovarian roles of activins and inhibins

Granulosa cells are the main ovarian source of inhibins, activins and activin-binding protein (follistatin) while germ (oogonia, oocytes) and somatic (theca, granulosa, luteal) cells express activin receptors, signaling components and inhibin co-receptor (betaglycan). Activins are implicated in various intra-ovarian roles including germ cell survival and primordial follicle assembly; follicle growth from preantral to mid-antral stages; suppression of thecal androgen production; promotion of granulosa cell proliferation, FSHR and CYP19A1 expression; enhancement of oocyte developmental competence; retardation of follicle luteinization and/or atresia and involvement in luteolysis. Inhibins (primarily inhibin A) are produced in greatest amounts by preovulatory follicles (and corpus luteum in primates) and suppress FSH secretion through endocrine negative feedback. Together with follistatin, inhibins act locally to oppose auto-/paracrine activin (and BMP) signaling thus modulating many of the above processes. The balance between activin-inhibin shifts during follicle development with activin signalling prevailing at earlier stages but declining as inhibin and betaglycan expression rise.

Gene expression profiling of the androgen receptor antagonists flutamide and vinclozolin in zebrafish (Danio rerio) gonads

The studies presented in this manuscript focus on characterization of transcriptomic responses to anti-androgens in zebrafish (Danio rerio). Research on the effects of anti-androgens in fish has been characterized by a heavy reliance on apical endpoints, and molecular mechanisms of action (MOA) of anti-androgens remain poorly elucidated. In the present study, we examined effects of a short term exposure (24-96h) to the androgen receptor antagonists flutamide (FLU) and vinclozolin (VZ) on gene expression in gonads of sexually mature zebrafish, using commercially available zebrafish oligonucleotide microarrays (4×44K platform). We found that VZ and FLU potentially impact reproductive processes via multiple pathways related to steroidogenesis, spermatogenesis, and fertilization. Observed changes in gene expression often were shared by VZ and FLU, as demonstrated by overlap in differentially-expressed genes and enrichment of several common key pathways including: (1) integrin and actin signaling, (2) nuclear receptor 5A1 signaling, (3) fibroblast growth factor receptor signaling, (4) polyamine synthesis, and (5) androgen synthesis. This information should prove useful to elucidating specific mechanisms of reproductive effects of anti-androgens in fish, as well as developing biomarkers for this important class of endocrine-active chemicals.

Temporal mRNA expression of transforming growth factor-beta superfamily members and inhibitors in the developing rainbow trout ovary

During mammalian ovarian development transforming growth factor-beta (TGFbeta) superfamily members and their inhibitors are critical paracrine regulators, yet the intraovarian functions of these proteins have received less attention in fish. Using quantitative real-time RT-PCR, changes in ovarian mRNA expression of six TGFbeta members and two inhibitors were investigated in rainbow trout across a wide range of fish ovarian stages (i.e., early perinucleous stage through acquisition of maturational competence). Transcript changes for insulin-like growth factor 1 and 2, and five enzymes associated with steroidogenesis, as well as plasma levels of three sex steroids were also measured to provide a framework of established intraovarian regulators in trout. Expression of bone morphogenetic protein 4 (bmp4), bone morphogenetic protein7 (bmp7), and growth differentiation factor 9 (gdf9) peaked during pre-vitellogenic stages and steadily decreased through advancing stages implicating these genes in early ovarian development. A dramatic increase in inhibin beta(A) and decrease in follistatin expression occurred during early to mid-vitellogenic stages, which corresponded with increased 17beta-estradiol plasma levels suggesting a vitellogenic role for ovarian activin A. Follicles that were competent to respond to the maturation-inducing hormone had decreased levels of inhibin beta(B) and increased expression of bambi (bmp and activin membrane-bound inhibitor) suggesting their roles in maturation processes. Furthermore, bmp4, bmp7 and gdf9 are primarily expressed in the oocyte whereas the inhibin subunits, follistatin, and bambi are primarily expressed in the somatic follicle cells. These results support TGFbeta superfamily members and their inhibitors have wide-ranging and disparate roles in regulating ovarian development in fish.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

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.

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.

Direct effects, compensation, and recovery in female fathead minnows exposed to a model aromatase inhibitor

BACKGROUND

Several chemicals in the environment have the potential to inhibit aromatase, an enzyme critical to estrogen synthesis.

OBJECTIVES

The objective of this study was to provide a detailed characterization of molecular and biochemical responses of female fathead minnows to a model aromatase inhibitor, fadrozole (FAD).

METHODS

Fish were exposed via water to 0, 3, or 30 microg FAD/L for 8 days and then held in clean water for 8 days, with samples collected at four time points during each 8-day period. We quantified ex vivo steroid production, plasma steroids, and plasma vitellogenin (Vtg) concentrations and analyzed relative transcript abundance of 10 key regulatory genes in ovaries and 3 in pituitary tissue by real-time polymerase chain reaction.

RESULTS

Ex vivo 17beta-estradiol (E2) production and plasma E2 and Vtg concentrations were significantly reduced after a single day of exposure to 3 microg or 30 microg FAD/L. However, plasma E2 concentrations recovered by the eighth day of exposure in the 3-microg/L group and within 1 day of cessation of exposure in the 30-microg/L group, indicating concentration- and time-dependent physiologic compensation and recovery. Concentration-dependent increases in transcripts coding for aromatase (A isoform), cytochrome P450 side-chain cleavage, steroidogenic acute regulatory protein, and follicle-stimulating hormone receptor all coincided with increased E2 production and recovery of plasma E2 concentrations.

CONCLUSIONS

Results of this research highlight the need to consider compensation/adaptation and recovery when developing and interpreting short-term bioassays or biomarkers or when trying to predict the effects of chemical exposures based on mode of action.

Regulation of body mass growth through activin type IIB receptor in teleost fish

Myostatin is a TGF-beta family member that plays a key role in regulating skeletal muscle growth. Previous studies in mammals have demonstrated that myostatin is capable of binding the two activin type II receptors. Additionally, activin type II receptors have been shown to be capable of binding a number of other TGF-beta family members besides myostatin. An injection of a soluble form of activin type IIB receptor obtained from CHO cells into wild-type mice generated up to a 60% increase in muscle mass in 2 weeks. The knowledge on the role of activin receptors in fish is limited. In the present study, we examined the growth effect of administering a recombinant, soluble form of goldfish activin type IIB receptor extracellular domain to juvenile and larval goldfish (Carassius auratus), African catfish (Clarias gariepinus) larvae and tilapia (Oreochromis aureus) larvae. We have expressed the goldfish activin type IIB receptor extracellular domain in the yeast Pichia pastoris and we have demonstrated for the first time that this recombinant molecule stimulates growth in teleost fish in a dose-dependent manner. We provide evidence that this body weight increase is achieved by an increase in muscle mass and protein content. Histological analysis of the goldfish muscle revealed that treated fish exhibited hyperplasia as compared to controls. These findings contribute to the understanding of the mechanisms that regulate growth in non-mammalian vertebrates and suggest a powerful biotechnology approach to improving fish growth in aquaculture.

Role of inhibin and activin in the modulation of gonadotropin- and steroid-induced oocyte maturation in the teleost Fundulus heteroclitus

BACKGROUND

Activin and inhibin are glycoproteins structurally related to the transforming growth factor-beta superfamily. These peptides were first described as factors that regulate the follicle-stimulating hormone (FSH) at the pituitary level. The possible role of inhibin and activin, at the ovarian level, in mediating the stimulatory actions of a Fundulus pituitary extract (FPE) and 17alpha,20beta-dihydroprogesterone (DHP) on oocyte maturation was investigated in this study.

METHODS

In vitro culture of ovarian follicles and induction of oocyte maturation were carried out in 75% Leibovitz L-15 medium. Follicles or denuded oocytes were exposed to FPE, inhibin, activin, ethanol vehicle (control group), or DHP. The competence of the follicles or denuded oocytes to respond to the hormones was assessed by scoring germinal vesicle breakdown (GVBD) used as an indication of the reinitiation of meiosis or oocyte maturation. DHP level was measured by radioimmunoassay.

RESULTS

Addition of FPE promoted the synthesis of DHP by the granulose cells of fully grown ovarian follicles and thus stimulated GVBD in the oocyte. Presence of porcine inhibin did not hinder the synthesis of DHP stimulated by FPE, although it did inhibit the subsequent GVBD in a dose-dependent manner, suggesting that the action of inhibin was at the oocyte level. Similarly to the findings with FPE, inhibin also blocked the DHP-induced GVBD in intact follicles, as well as the spontaneous and steroid-induced GVBD of denuded oocyte. Inhibin straightforwardly blocked the response to a low dose of DHP throughout the culture period, while higher doses of the steroid appeared to overcome the inhibitory effect especially at later times. In contrast to inhibin, recombinant human activin A significantly enhanced DHP-induced GVBD in a dose-dependent manner after 48 hr, although activin alone was not able to induce GVBD without the presence of the steroid.

CONCLUSION

Taking together with our previous studies that demonstrate the presence of activin/inhibin subunits in the ovary of F. heteroclitus, these in vitro findings indicate that inhibin and activin are local regulators in the teleost ovary and have opposing effects in modulating oocyte maturation.

Developmental Profiles of Activin βA, βB, and Follistatin Expression in the Zebrafish Ovary: Evidence for Their Differential Roles During Sexual Maturation and Ovulatory Cycle1

Our recent experiments showed that gonadotropin(s) stimulated activin betaA and follistatin expression through the cAMP-PKA pathway but suppressed betaB via a cAMP-dependent but PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the differential expression of activin betaA and betaB as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of activin betaA, betaB, and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with nearly full-grown follicles. Activin betaA expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In contrast, the expression of activin betaB could be easily detected in the ovary of all stages; however, it did not exhibit an obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin betaA, betaB, and follistatin at the follicle level in the adult mature zebrafish. The expression of activin betaA was again low in the follicles during the primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at midvitellogenic stage; in contrast, activin betaB mRNA could be easily detected at all stages with a slight increase during follicle growth. The expression of follistatin, on the other hand, also increased significantly during vitellogenesis; however, its level dropped sharply after reaching the peak at the midvitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin betaA, betaB, and follistatin expression during the daily ovulatory cycle. The expression of activin betaA and follistatin gradually increased from 1800 h onward and reached the peak level around 0400 h when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin betaB expression steadily declined, although not statistically significant, during the same period, but increased sharply at 0700 h when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin betaA and betaB exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin betaA is involved in promoting ovary and follicle growth, whereas activin betaB may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.

Cloning of epidermal growth factor (EGF) and EGF receptor from the zebrafish ovary: evidence for EGF as a potential paracrine factor from the oocyte to regulate activin/follistatin system in the follicle cells

In the present study, we cloned full-length cDNAs for epidermal growth factor (EGF), EGF receptor (EGFR), and three truncated forms of EGFR (EGFR15, 12, and 8) from the zebrafish ovary. Zebrafish EGF was predominantly expressed in the ovary and testis, while EGFR and its truncated forms were highly expressed in all tissues examined except the liver. In the ovary, the expression of EGF seemed to be more abundant in the follicles of early stages, while EGFR had much higher expression levels at later stages. Interestingly, although EGF was expressed in both the follicle cells and oocytes, its expression level was significantly higher in the oocytes. However, the expression of EGFR was mainly restricted to the follicle cells with little expression in the oocytes. The unique spatial patterns of EGF and EGFR expression within the follicle suggest that EGF may serve as a messenger from the oocyte to signal the follicle cells. EGF strongly stimulated the expression of both activin beta A and beta B, while it suppressed basal and hCG-induced follistatin expression in cultured follicle cells. These results, together with the evidence that EGF was predominantly expressed in the oocytes whereas EGFR was expressed in the follicle cells, strongly suggest that EGF is likely a potential paracrine/juxtacrine factor from the oocytes to regulate the function of the follicle cells.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations

The zebrafish (Danio rerio) is now the pre-eminent vertebrate model system for clarification of the roles of specific genes and signaling pathways in development. The zebrafish genome will be completely sequenced within the next 1-2 years. Together with the substantial historical database regarding basic developmental biology, toxicology, and gene transfer, the rich foundation of molecular genetic and genomic data makes zebrafish a powerful model system for clarifying mechanisms in toxicity. In contrast to the highly advanced knowledge base on molecular developmental genetics in zebrafish, our database regarding infectious and noninfectious diseases and pathologic lesions in zebrafish lags far behind the information available on most other domestic mammalian and avian species, particularly rodents. Currently, minimal data are available regarding spontaneous neoplasm rates or spontaneous aging lesions in any of the commonly used wild-type or mutant lines of zebrafish. Therefore, to fully utilize the potential of zebrafish as an animal model for understanding human development, disease, and toxicology we must greatly advance our knowledge on zebrafish diseases and pathology.