Regulation of steroidogenic acute regulatory protein transcription in largemouth bass by orphan nuclear receptor signaling pathways

Star1 gene mutation reveals the essentiality of 11-ketotestosterone and glucocorticoids for male fertility in Nile Tilapia (Oreochromis niloticus)

Steroidogenic acute regulatory protein (Star) plays an essential role in the biosynthesis of corticosteroids and sex steroids by mediating the transport of cholesterol from the outer to the inner membrane of mitochondria. Two duplicated Star genes, namely star1 and star2, have been identified in non-mammalian vertebrates. To investigate the roles of star genes in fish steriodogenesis, we generated two mutation lines of star1-/- and star1-/-/star2-/- in Nile tilapia (Oreochromis niloticus). Previous studies revealed that deficiency of star2 gene caused delayed spermatogenesis, sperm apoptosis and sterility in male tilapia. Our present data revealed that mutation of star genes impaired male fertility. Disordered seminiferous lobules and spermatic duct obstruction were found in the testis of both types of mutants. Moreover, significant decline in semen volume, sperm abnormality and impaired fertility were also detected in star1-/- and star1-/-/star2-/- males. In star1-/- male fish, lipid accumulation, up-regulation of steroidogenic enzymes, and significant decline of androgens were found. Additionally, hyperplasic interrenal cells, elevated steroidogenic gene expression level and decline of serum glucocorticoids were detected in star1 mutants. Intriguingly, either 11-KT or cortisol supplementation successfully rescued the impaired fertility of the star1-/- mutants. Taken together, these results further indicate that Star1 might play critical roles in the production of both 11-KT and glucocorticoids, which are indispensable for the maintenance of male fertility in fish.

REVERBA couples the circadian clock to Leydig cell steroidogenesis

The involvement of the molecular clock in regulating cell physiological processes on a specific time scale is a recognized concept, yet its specific impact on optimizing androgen production in Leydig cells has been unclear. This study aimed to confirm the role of the REVERBA (NR1D1) gene in controlling the transcription of key genes related to Leydig cell steroid production. We investigated daily variations by collecting Leydig cells from rats at various times within a 24-h period. Chromatin immunoprecipitation study showed a time-dependent pattern for genes linked to steroid production (Nur77, Star, Cyp11a1, and Cyp17a1), which closely matched the 24-h REVERBA levels in Leydig cells, peaking between zeitgeber time (ZT) 7-11. To understand the physiological significance of REVERBA's interaction with promoters of steroidogenesis-related genes, Leydig cells from rats at two different times (ZT7 and ZT16; chosen based on REVERBA expression levels), were treated with either an agonist (GSK4112) or an antagonist (SR8278). The results revealed that the REVERBA agonist stimulated gene transcription, while the antagonist inhibited it, but only when REVERBA was sufficiently present, indicating a reliance on REVERBA's circadian fluctuation. Moreover, this REVERBA-dependent stimulation had a clear impact on testosterone production in the culture medium, underscoring REVERBA's involvement in the circadian regulation of testosterone. This study indicates that REVERBA, in addition to being a core component of the cellular clock, plays a key role in regulating androgen production in Leydig cells by influencing the transcription of critical steroidogenesis-related genes.

Hypoxia activation attenuates progesterone synthesis in goat trophoblast cells via NR1D1 inhibition of StAR expression†

Trophoblast plays a crucial role in gestation maintenance and embryo implantation, partly due to the synthesis of progesterone. It has been demonstrated that hypoxia regulates invasion, proliferation, and differentiation of trophoblast cells. Additionally, human trophoblasts display rhythmic expression of circadian clock genes. However, it remains unclear if the circadian clock system is present in goat trophoblast cells (GTCs), and its involvement in hypoxia regulation of steroid hormone synthesis remains elusive. In this study, immunofluorescence staining revealed that both BMAL1 and NR1D1 (two circadian clock components) were highly expressed in GTCs. Quantitative real-time PCR analysis showed that several circadian clock genes were rhythmically expressed in forskolin-synchronized GTCs. To mimic hypoxia, GTCs were treated with hypoxia-inducing reagents (CoCl2 or DMOG). Quantitative real-time PCR results demonstrated that hypoxia perturbed the mRNA expression of circadian clock genes and StAR. Notably, the increased expression of NR1D1 and the reduction of StAR expression in hypoxic GTCs were also detected by western blotting. In addition, progesterone secretion exhibited a notable decline in hypoxic GTCs. SR9009, an NR1D1 agonist, significantly decreased StAR expression at both the mRNA and protein levels and markedly inhibited progesterone secretion in GTCs. Moreover, SR8278, an NR1D1 antagonist, partially reversed the inhibitory effect of CoCl2 on mRNA and protein expression levels of StAR and progesterone synthesis in GTCs. Our results demonstrate that hypoxia reduces StAR expression via the activation of NR1D1 signaling in GTCs, thus inhibiting progesterone synthesis. These findings provide new insights into the NR1D1 regulation of progesterone synthesis in GTCs under hypoxic conditions.

cAMP signaling in ovarian physiology in teleosts: A review

Ovarian function in teleosts, like in other vertebrates, is regulated by two distinct gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin effects are mediated by membrane-bound G protein-coupled receptors localized on the surface of follicle cells. Gonadotropin receptor activation results in increased intracellular cAMP, the most important second cellular signaling molecule. FSH stimulation induces the production of 17β-estradiol in the cells of growing follicles to promote vitellogenesis in oocytes. In contrast, in response to LH, fully grown post-vitellogenic follicles gain the ability to synthesize maturation-inducing steroids, which induce meiotic resumption and ovulation. All these events were induced downstream of cAMP. In this review, we summarize studies addressing the role of the cAMP pathway in gonadotropin-induced processes in teleost ovarian follicles. Furthermore, we discuss future problems concerning cAMP signaling in relation to teleost ovarian function and the differences and similarities in the gonadotropin-induced cAMP signaling pathways between mammals and teleosts.

Expression and Transcript Localization of star, sf-1, and dax-1 in the Early Brain of the Orange-Spotted Grouper Epinephelus coioides

We investigated the developmental expression and localization of sf-1 and dax-1 transcripts in the brain of the juvenile orange-spotted grouper in response to steroidogenic enzyme gene at various developmental ages in relation to gonadal sex differentiation. The sf-1 transcripts were significantly higher from 110-dah (day after hatching) and gradually increased up to 150-dah. The dax-1 mRNA, on the other hand, showed a decreased expression during this period, in contrast to sf-1 expression. At the same time, the early brain had increased levels of steroidogenic gene (star). sf-1 and star hybridization signals were found to be increased in the ventromedial hypothalamus at 110-dah; however, dax-1 mRNA signals decreased in the early brain toward 150-dah. Furthermore, the exogenous estradiol upregulated star and sf-1 transcripts in the early brain of the grouper. These findings suggest that sf-1 and dax-1 may have an antagonistic expression pattern in the early brain during gonadal sex differentiation. Increased expression of steroidogenic gene together with sf-1 during gonadal differentiation strongly suggests that sf-1 may play an important role in the juvenile grouper brain steroidogenesis and brain development.

Role of the cytoskeleton in steroidogenesis

Steroidogenesis in the adrenal cortex or gonads is a complicated process, modulated by various elements either at the tissue or molecular level. The substrate-cholesterol is first delivered to the outer membrane of mitochondria, undergoing a series of enzymatic reactions along with the material exchange between the mitochondria and the ER (endoplasmic reticulum) and ultimately yield various steroids: aldosterone, cortisol, testosterone and estrone. Several valves are set to adjust the amount of production to the needs. e.g. StAR(steroidogenic acute regulator) is in charge of the rate-limiting step-traffic of cholesterol from outer membrane to inner membrane of mitochondria. And the "needs" is partly reflected by trophic signals like ACTH、LH and downstream pathways-- intracellular cAMP pathway, which represents the endocrinal regulation of steroid synthesis, too. The coordinated activities of these related factors are all associated with another crucial cellular constituent-the cytoskeleton, which plays a crucial role in the cellular architecture and substrate trafficking. Though considerable studies have been performed regarding steroid synthesis, details about the upstream signaling pathways and mechanisms of the regulation by cytoskeleton network still remain unclear. The metabolism and interplays of the pivotal cellular organelles with cytoskeleton are worth exploring as well. In this review, we summarize research of different time span, describing the roles of specific cytoskeleton elements in steroidogenesis and related signaling pathways involved in the steroid synthesis. In addition, we discussed the inner cytoskeletal network involved in steroidogenic processes such as mitochondrial movement, organelle interactions and cholesterol trafficking.

Carotenoid coloration and coloration-linked gene expression in red tilapia (Oreochromis sp.) tissues

Background

Production, marketability and consumer preference of red tilapia often depends upon the intensity of coloration. Hence, new approaches to develop coloration are now geared to improve market acceptability and profit. This study evaluated the effects of carotenoid-rich diets on the phenotypic coloration, carotenoid level, weight gain and expression of coloration-linked genes in skin, fin and muscle tissues. Carotenoids were extracted from dried Daucus carota peel, Ipomoea aquatica leaves, and Moringa oleifera leaves. Eighty (80) size-14 fish were fed with carotenoid-rich treatments twice a day for 120 days. The phenotypic effect of the carotenoid extracts was measured through a color chart. Skin carotenoid level was measured through UV-vis spectrophotometer. csf1ra, Bcdo2 and StAR expression analysis was done using qRT-PCR.

Results

Treatments with carotenoid extracts yielded higher overall scores on phenotypic coloration and tissue carotenoid levels. Differential expression of carotenoid-linked genes such as the elevated expression in csf1ra and lower expression in Bcdo2b following supplementation of the enhanced diet supports the phenotypic redness and increased carotenoid values in red tilapia fed with D. carota peel and I. aquatica leaves.

Conclusions

Overall improvement in the redness of the tilapia was achieved through the supplementation of carotenoid-rich diet derived from readily available plants. Differential expression of coloration-linked genes supports the increase in the intensity of phenotypic coloration and level of carotenoids in the tissues. The study emphasizes the importance of carotenoids in the commercial tilapia industry and highlights the potential of the plant extracts for integration and development of feeds for color enhancement in red tilapia.

Glyphosate exposure attenuates testosterone synthesis via NR1D1 inhibition of StAR expression in mouse Leydig cells

Glyphosate is a broad-spectrum herbicide that impairs testosterone synthesis in mammals. Leydig cells (LCs), the primary producers of testosterone, demonstrate rhythmic expression of circadian clock genes both in vivo and in vitro. The nuclear receptor NR1D1 is an important clock component that constitutes the subsidiary transcriptional/translational loop in the circadian clock system. Nr1d1 deficiency resulted in diminished fertility in both male and female mice. However, whether NR1D1 is involved in the glyphosate-mediated inhibition of testosterone synthesis in LCs remains unclear. Here, the involvement of NR1D1 in glyphosate-mediated inhibition of testosterone synthesis was investigated both in vitro and in vivo. Glyphosate exposure of TM3 cells significantly increased Nr1d1 mRNA levels, but decreased Bmal1, Per2, StAR, Cyp11a1, and Cyp17a1 mRNA levels. Western blotting confirmed elevated NR1D1 and reduced StAR protein levels following glyphosate exposure. Glyphosate exposure also reduced testosterone production in TM3 cells. In primary LCs, glyphosate exposure also upregulated Nr1d1 mRNA levels and downregulated the mRNA levels of other clock genes (Bmal1 and Per2) and steroidogenic genes (StAR, Cyp17a1, Cyp11a1, and Hsd3b2), and inhibited testosterone synthesis. Moreover, glyphosate exposure significantly reduced the amplitude and shortened the period of PER2::LUCIFERASE oscillations in primary LCs isolated from mPer2Luciferase knock-in mice. Four weeks of oral glyphosate upregulated NR1D1 at both the mRNA and protein levels in mouse testes, and this was accompanied by a reduction in StAR expression. Notably, serum testosterone levels were also drastically reduced in mice treated with glyphosate. Moreover, dual-luciferase reporter and EMSA assays revealed that in TM3 cells NR1D1 inhibits the expression of StAR by binding to a canonical RORE element present within its promoter. Together, these data demonstrate that glyphosate perturbs testosterone synthesis via NR1D1 mediated inhibition of StAR expression in mouse LCs. These findings extend our understanding of how glyphosate impairs male fertility.

Prebiotics and phytogenics functional additives in low fish meal and fish oil based diets for European sea bass (Dicentrarchus labrax): Effects on stress and immune responses

The use of terrestrial raw materials to replace fish meal (FM) and fish oil (FO) in marine fish diets may affect fish growth performance and health. In the last years functional additives have been profiled as good candidates to reduce the effects on health and disease resistance derived from this replacement, via reinforcement of the fish immune system. In the present study, three isoenergetic and isonitrogenous diets with low FM and FO (10% and 6% respectively) were tested based on supplementation either with 0.5% galactomannanoligosaccharides (GMOS diet) or 0.02% of a mixture of essential oils (PHYTO diet), a non-supplemented diet was defined as a control diet. Fish were fed the experimental diets in triplicate for 9 weeks and then they were subjected to a stress by confinement as a single challenge (C treatment) or combined with an experimental intestinal infection with Vibrio anguillarum (CI treatment). Along the challenge test, selected stress and immunological parameters were evaluated at 2, 24 and 168h after C or CI challenges. As stress indicators, circulating plasma cortisol and glucose concentrations were analyzed as well as the relative gene expression of cyp11b hydroxylase, hypoxia inducible factor, steroidogenic acute regulatory protein, heat shock protein 70 and heat shock protein 90 (cyp11b, hif-1α, StAR, hsp70 and hsp90). As immune markers, serum and skin mucus lysozyme, bactericidal and peroxidase activities were measured, as well as gene expression of Caspase-3 (casp-3) and interleukin 1β (il-1ß). The use of functional additives induced a significant (p < 0.05) reduction of circulating plasma cortisol concentration when confinement was the unique challenge test applied. Supplementation of PHYTO induced a down-regulation of cyp11b, hif-1α, casp-3 and il-1β gene expression 2h after stress test, whereas StAR expression was significantly (p < 0.05) up-regulated. However, when combination of confinement stress and infection was applied (CI treatment), the use of PHYTO significantly (p < 0.05) down-regulated StAR and casp-3 gene expression 2h after challenge test, denoting that PHYTO diet reinforced fish capacity of stress response via protection of head kidney leucocytes from stress-related apoptotic processes, with lower caspase-3 gene expression and a higher il-1β gene expression when an infection occurs. Additionally, dietary supplementation with GMOS and PHYTO compounds increased fish serum lysozyme after infection. Both functional additives entailed a better capability of the animals to cope with infection in European sea bass when fed low FM and FO diets.

Steroidogenic acute regulatory protein transcription is regulated by estrogen receptor signaling in largemouth bass ovary

Estrogenic contaminants in the environment are linked to the occurrence of reproductive abnormalities in many aquatic species, including largemouth bass (Micropterus salmoides; LMB). Previous work has shown that many different types of xenoestrogens regulate expression of the Steroidogenic Acute Regulatory protein (StAR), a cholesterol-transporting protein vital to steroid hormone biosynthesis; however, the regulatory mechanisms of StAR are incompletely characterized in fish. To learn more about endogenous expression patterns of StAR in the ovary, LMB were collected from the St. John's River (Florida, USA) over an entire breeding season to investigate StAR expression. Plasma 17β-estradiol (E2) and StAR mRNA levels were positively correlated in females, and StAR mRNA levels displayed ~ 100-fold increase between primary oocyte growth stages and final maturation. To further study the regulation of StAR, female LMB in the laboratory were fed at ≃2% of their weight on a diet laden with 17α-ethinylestradiol (EE2, 70 or 200 ng EE2 per gram feed). Diets were designed to achieve a physiologically-relevant exposure to EE2, and StAR expression was assessed in vivo. We observed a dose-dependent suppression of StAR mRNA levels, however both diets led to high, pharmacological levels in the blood and do not represent normal physiological ranges of estrogens. In the 200 ng EE2/gm feed group, ovarian StAR mRNA levels were suppressed to approximately 5% of that of the LMB control group. These investigations suggest that LMB StAR increases in expression during oocyte maturation and that it is suppressed by E2 feedback when estrogen levels are high, through the HPG axis. A 2.9 kb segment of the LMB StAR promoter was examined for putative E2 response elements using in silico software, and a putative estrogen receptor binding element (ERE/-1745) was predicted in the promoter. The functionality of the ERE was examined using MA-10 mouse Leydig cells transfected with the LMB StAR promoter. Estrogen receptor (ER) interaction with ERE/-1745 was evaluated under basal and human chorionic gonadotropin (hCG)-treated conditions in the presence and absence of E2. Chromatin immunoprecipitation (ChIP) experiments revealed that ESR1 binding to the promoter was enriched under basal conditions and E2 exposure elicited an increase in enrichment (4-fold) above that observed under basal conditions. ESR2 was not strongly enriched at the ERE/-1745 site, suggesting that StAR may be preferentially regulated by LMB estrogen receptor 1 (esr1). Taken together, these different experiments provide evidence that LMB StAR is under the control of estrogens and that ESR1 binds directly to the LMB StAR promoter in an E2-responsive manner.

Parabens Accelerate Ovarian Dysfunction in a 4-Vinylcyclohexene Diepoxide-Induced Ovarian Failure Model

Parabens are widely used preservatives in basic necessities such as cosmetic and pharmaceutical products. In previous studies, xenoestrogenic actions of parabens were reported in an immature rat model and a rat pituitary cell line (GH3 cells). The relationship between parabens and ovarian failure has not been described. In the present study, the influence of parabens on ovarian folliculogenesis and steroidogenesis was investigated. A disruptor of ovarian small pre-antral follicles, 4-vinylcyclohexene diepoxide (VCD, 40 mg/kg), was used to induce premature ovarian failure (POF). Methylparaben (MP, 100 mg/kg), propylparaben (PP, 100 mg/kg), and butylparaben (BP, 100 mg/kg) dissolved in corn oil were treated in female 8-week-old Sprague-Dawley rat for 5 weeks. Estrus cycle status was checked daily by vaginal smear test. Ovarian follicle development and steroid synthesis were investigated through real-time PCR and histological analyses. Diestrus phases in the VCD, PP, and BP groups were longer than that in the vehicle group. VCD significantly decreased mRNA level of folliculogenesis-related genes (Foxl2, Kitl and Amh). All parabens significantly increased the Amh mRNA level but unchanged Foxl2 and Kitlg acting in primordial follicles. VCD and MP slightly increased Star and Cyp11a1 levels, which are related to an initial step in steroidogenesis. VCD and parabens induced an increase in FSH levels in serum and significantly decreased the total number of follicles. Increased FSH implies impairment in ovarian function due to VCD or parabens. These results suggest that VCD may suppress both formation and development of follicles. In particular, combined administration of VCD and parabens accelerated inhibition of the follicle-developmental process through elevated AMH level in small antral follicles.

Molecular identification of StAR and 3βHSD1 and characterization in response to GnIH stimulation in protogynous hermaphroditic grouper (Epinephelus coioides)

Gonadal steroids are critical factors in reproduction and sex reverse process. StAR (steroidogenic acute regulatory protein), transferring the cholesterol from the outer mitochondrial membrane to the inner membrane, is the rate-limiting factor of steroidogenesis. 3βHSD (3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase), converting Δ5-steroids into Δ4-steroids, is an important oxidoreductase in steroidogenesis. In the present study, StAR and 3βHSD1 were cloned and characterized from protogynous orange-spotted grouper. StAR cDNA contains an 861bp open reading frame (ORF), encoding a predicted protein of 286 amino acids, and the ORF of 3βHSD1 was 1125bp, encoding a predicted protein of 374 amino acids. The transcript of StAR was mainly expressed in gonad, while 3βHSD1 mRNA was predominantly detected in brain and gonad. In the previous study, we found the expression of GnIH mRNA level in male, as well as in 17 alpha-methyltestosterone (MT)-induced male fish was significantly higher than in female fish, this indicating that GnIH/GnIHR signaling might be involved in the regulation of sex reversal and male maintenance. In order to figure out the function of GnIH in steroidogenesis, the expression of StAR and 3βHSD1 regulated by GnIH was examined. In vitro study showed that treatment of cultured ovary fragments with gGnIH peptides significantly stimulated the expression of StAR and 3βHSD1. In addition, the mRNA levels of StAR and 3βHSD1 were significantly increased after intraperitoneal injection (i.p.) with gGnIH peptides. Moreover, during MT-induced sex change from female to male, the levels of StAR mRNA significantly increased by 5.2, 24.8 and 353.5 folds, and that of 3βHSD1 mRNA by 3.5, 32.5 and 55.4 folds at the 2nd, 4th and 6th week after MT implantation, respectively. Collectively, our results indicate that GnIH may be involved in the regulation of sex reversal or male maintenance by stimulating the expression of StAR and 3βHSD1 in protogynous grouper.

Characterization and Developmental Expression Profile of the Steroidogenic Acute Regulatory Protein (StAR) in the Gonad-Mesonephros Complex of Lithobates sylvaticus

The steroidogenic acute regulatory (StAR) protein is responsible for the movement of cholesterol across mitochondrial membranes and is therefore a key factor in regulating the timing and rate of steroidogenesis. In this study, we characterized the coding region of the star gene in the ranid Lithobates sylvaticus and studied star mRNA levels in steroidogenic tissues during development and under natural conditions. Our results support previous research showing that the StAR sequence is well conserved. We determined that star is expressed in both the interrenal and gonadal tissues of adults and in the tadpole gonad-mesonephros complex (GMC). The mRNA levels of star in the GMC were found to increase during tadpole development, reaching a maximum between Gosner stages (Gs) 32 and 38. We observed a significant drop in star mRNA levels at the end of prometamorphosis (Gs40-41), just before the start of the metamorphic climax. Significant differences in star levels between females and males, with males presenting higher levels than females, were detected at Gs36-38. To our knowledge, this is the first study that reports transitory star sex differences in tadpoles' developing GMC. Our results suggest an involvement of StAR in anuran late male GMC formation and development that requires further investigation.

Gene networks and toxicity pathways induced by acute cadmium exposure in adult largemouth bass (Micropterus salmoides)

Cadmium is a heavy metal that can accumulate to toxic levels in the environment leading to detrimental effects in animals and humans including kidney, liver and lung injuries. Using a transcriptomics approach, genes and cellular pathways affected by a low dose of cadmium were investigated. Adult largemouth bass were intraperitoneally injected with 20μg/kg of cadmium chloride (mean exposure level - 2.6μg of cadmium per fish) and microarray analyses were conducted in the liver and testis 48h after injection. Transcriptomic profiles identified in response to cadmium exposure were tissue-specific with the most differential expression changes found in the liver tissues, which also contained much higher levels of cadmium than the testis. Acute exposure to a low dose of cadmium induced oxidative stress response and oxidative damage pathways in the liver. The mRNA levels of antioxidants such as catalase increased and numerous transcripts related to DNA damage and DNA repair were significantly altered. Hepatic mRNA levels of metallothionein, a molecular marker of metal exposure, did not increase significantly after 48h exposure. Carbohydrate metabolic pathways were also disrupted with hepatic transcripts such as UDP-glucose, pyrophosphorylase 2, and sorbitol dehydrogenase highly induced. Both tissues exhibited a disruption of steroid signaling pathways. In the testis, estrogen receptor beta and transcripts linked to cholesterol metabolism were suppressed. On the contrary, genes involved in cholesterol metabolism were highly increased in the liver including genes encoding for the rate limiting steroidogenic acute regulatory protein and the catalytic enzyme 7-dehydrocholesterol reductase. Integration of the transcriptomic data using functional enrichment analyses revealed a number of enriched gene networks associated with previously reported adverse outcomes of cadmium exposure such as liver toxicity and impaired reproduction.

The Interplay between Circadian System, Cholesterol Synthesis, and Steroidogenesis Affects Various Aspects of Female Reproduction

Circadian aspect of reproduction has gained much attention in recent years. In mammals, it is very important that the timing of greatest sexual motivation is in line with the highest fertility. Peripheral clocks have been found to reside also in reproductive organs, such as the uterus and ovary. The timing signal from the suprachiasmatic nucleus is suggested to be transmitted via hormonal and neural mechanisms, and could thus mediate circadian expression of target genes in these organs. In turn, estrogens from the ovary have been found to signal back to the hypothalamus, completing the feedback loop. In this review we will focus on the interplay between clock and estrogens. Estradiol has been directly linked with expression of Per1 and Per2 in the uterus. CLOCK, on the other hand, has been shown to alter estradiol signaling. We also present the idea that cholesterol could play a vital role in the regulation of reproduction. Cholesterol synthesis itself is circadially regulated and has been found to interfere with steroidogenesis in the ovary on the molecular level. This review presents a systems view on how the interplay between circadian clock, steroidogenesis, and cholesterol synthesis affect various aspects of mammalian reproduction.

Gene expression networks underlying ovarian development in wild largemouth bass (Micropterus salmoides)

Background

Oocyte maturation in fish involves numerous cell signaling cascades that are activated or inhibited during specific stages of oocyte development. The objectives of this study were to characterize molecular pathways and temporal gene expression patterns throughout a complete breeding cycle in wild female largemouth bass to improve understanding of the molecular sequence of events underlying oocyte maturation.

Methods

Transcriptomic analysis was performed on eight morphologically diverse stages of the ovary, including primary and secondary stages of oocyte growth, ovulation, and atresia. Ovary histology, plasma vitellogenin, 17β-estradiol, and testosterone were also measured to correlate with gene networks.

Results

Global expression patterns revealed dramatic differences across ovarian development, with 552 and 2070 genes being differentially expressed during both ovulation and atresia respectively. Gene set enrichment analysis (GSEA) revealed that early primary stages of oocyte growth involved increases in expression of genes involved in pathways of B-cell and T-cell receptor-mediated signaling cascades and fibronectin regulation. These pathways as well as pathways that included adrenergic receptor signaling, sphingolipid metabolism and natural killer cell activation were down-regulated at ovulation. At atresia, down-regulated pathways included gap junction and actin cytoskeleton regulation, gonadotrope and mast cell activation, and vasopressin receptor signaling and up-regulated pathways included oxidative phosphorylation and reactive oxygen species metabolism. Expression targets for luteinizing hormone signaling were low during vitellogenesis but increased 150% at ovulation. Other networks found to play a significant role in oocyte maturation included those with genes regulated by members of the TGF-beta superfamily (activins, inhibins, bone morphogenic protein 7 and growth differentiation factor 9), neuregulin 1, retinoid X receptor, and nerve growth factor family.

Conclusions

This study offers novel insight into the gene networks underlying vitellogenesis, ovulation and atresia and generates new hypotheses about the cellular pathways regulating oocyte maturation.

Identification and transcriptional modulation of the largemouth bass, Micropterus salmoides, vitellogenin receptor during oocyte development by insulin and sex steroids

Fish vitellogenin synthesized and released from the liver of oviparous animals is taken up into oocytes by the vitellogenin receptor. This is an essential process in providing nutrient yolk to developing embryos to ensure successful reproduction. Here we disclose the full length vtgr cDNA sequence for largemouth bass (LMB) that reveals greater than 90% sequence homology with other fish vtgr sequences. We classify LMB Vtgr as a member of the low density lipoprotein receptor superfamily based on conserved domains and categorize as the short variant that is devoid of the O-glycan segment. Phylogenetic analysis places LMB Vtgr sequence into a well-supported monophyletic group of fish Vtgr. Real-time PCR showed that the greatest levels of LMB vtgr mRNA expression occurred in previtellogenic ovarian tissues. In addition, we reveal the effects of insulin, 17beta-estradiol (E(2)), and 11-ketotestosterone (11-KT) in modulation of vtgr, esr, and ar mRNAs in previtellogenic oocytes. Insulin increased vtgr expression levels in follicles ex vivo while exposure to E(2) or 11-KT did not result in modulation of expression. However, both steroids were able to repress insulin-induced vtgr transcript levels. Coexposure with insulin and E(2) or of insulin and 11-KT increased ovarian esr2b and ar mRNA levels, respectively, which suggest a role for these nuclear receptors in insulin-mediated signaling pathways. These data provide the first evidence for the ordered stage-specific expression of LMB vtgr during the normal reproductive process and the hormonal influence of insulin and sex steroids on controlling vtgr transcript levels in ovarian tissues.

Cloning and expression of the translocator protein (18 kDa), voltage-dependent anion channel, and diazepam binding inhibitor in the gonad of largemouth bass (Micropterus salmoides) across the reproductive cycle

Cholesterol transport across the mitochondrial membrane is rate-limiting for steroidogenesis in vertebrates. Previous studies in fish have characterized expression of the steroidogenic acute regulatory protein, however the function and regulation of other genes and proteins involved in piscine cholesterol transport have not been evaluated. In the current study, mRNA sequences of the 18 kDa translocator protein (tspo; formerly peripheral benzodiazepine receptor), voltage-dependent anion channel (vdac), and diazepam binding inhibitor (dbi; also acyl-CoA binding protein) were cloned from largemouth bass. Gonadal expression was examined across reproductive stages to determine if expression is correlated with changes in steroid levels and with indicators of reproductive maturation. In testis, transcript abundance of tspo and dbi increased with reproductive maturation (6- and 23-fold maximal increase, respectively) and expression of tspo and dbi was positively correlated with reproductive stage, gonadosomatic index (GSI), and circulating levels of testosterone. Testis vdac expression was positively correlated with reproductive stage and GSI. In females, gonadal tspo and vdac expression was negatively correlated with GSI and levels of plasma testosterone and 17β-estradiol. Ovarian dbi expression was not correlated with indicators of reproductive maturation. These studies represent the first investigation of the steroidogenic role of tspo, vdac, and dbi in fish. Findings suggest that cholesterol transport in largemouth bass testis, but not in ovary, may be transcriptionally-regulated, however further investigation will be necessary to fully elucidate the role of these genes in largemouth bass steroidogenesis.

Cadmium-mediated disruption of cortisol biosynthesis involves suppression of corticosteroidogenic genes in rainbow trout

Cadmium is widely distributed in the aquatic environment and is toxic to fish even at sublethal concentrations. This metal is an endocrine disruptor, and one well established role in teleosts is the suppression of adrenocorticotrophic hormone (ACTH)-stimulated cortisol biosynthesis by the interrenal tissue. However the mechanism(s) leading to this steroid suppression is poorly understood. We tested the hypothesis that cadmium targets genes encoding proteins critical for corticosteroid biosynthesis, including melanocortin 2 receptor (MC2R), steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc), in rainbow trout (Oncorhynchus mykiss). To test this, head kidney slices (containing the interrenal tissues) were incubated in vitro with cadmium chloride (0, 10, 100 and 1000nM) for 4h either in the presence or absence of ACTH (0.5IU/mL). In the unstimulated head kidney slices, cadmium exposure did not affect basal cortisol secretion and the mRNA levels of MC2R and P450scc, while StAR gene expression was significantly reduced. Cadmium exposure significantly suppressed ACTH-stimulated cortisol production in a dose-related fashion. This cadmium-mediated suppression in corticosteroidogenesis corresponded with a significant reduction in MC2R, StAR and P450scc mRNA levels in trout head kidney slices. The inhibition of ACTH-stimulated cortisol production and suppression of genes involved in corticosteroidogenesis by cadmium were completely abolished in the presence of 8-Bromo-cAMP (a cAMP analog). Overall, cadmium disrupts the expression of genes critical for corticosteroid biosynthesis in rainbow trout head kidney slices. However, the rescue of cortisol production as well as StAR and P450scc gene expressions by cAMP analog suggests that cadmium impact occurs upstream of cAMP production. We propose that MC2R signaling, the primary step in ACTH-induced cortocosteroidogenesis, is a key target for cadmium-mediated disruption of cortisol production in trout.