Thiourea-induced alteration in the expression patterns of some steroidogenic enzymes in the air-breathing catfish Clarias gariepinus

Disruption of Thyroid Hormone Receptor Thrab Leads to Female Infertility in Zebrafish

Thyroid hormones (THs) T4 and T3 are vital for development, growth, and metabolism. Thyroid dysfunction can also cause problems in fertility, suggesting involvement of THs in reproduction. In zebrafish, there exist 2 forms of TH receptor alpha gene (thraa and thrab). Disruption of these genes by CRISPR/Cas9 showed no reproductive irregularities in the thraa mutant; however, inactivation of the thrab gene resulted in female infertility. Although young female mutants (thrabm/m) showed normal ovarian development and folliculogenesis before sexual maturation, they failed to release eggs during oviposition after sexual maturation. This spawning failure was due to oviductal blockage at the genital papilla. The obstruction of the oviduct subsequently caused an accumulation of the eggs in the ovary, resulting in severe ovarian hypertrophy, abdominal distention, and disruption of folliculogenesis. Gene expression analysis showed expression of both TH receptors and estrogen receptors in the genital papilla, suggesting a direct TH action and potential interactions between thyroid and estrogen signaling pathways in controlling genital papilla development and function. In addition to their actions in the reproductive tracts, THs may also have direct effects in the ovary, as suggested by follicle atresia and cessation of folliculogenesis in the heterozygous mutant (thrab+/m), which was normal in all aspects of female reproduction in young and sexually mature fish but exhibited premature ovarian failure in aged females. In summary, this study provides substantial evidence for roles of THs in controlling the development and functions of both reproductive tract and ovary.

Effects of bisphenols on lipid metabolism and neuro-cardiovascular toxicity in marine medaka larvae

Bisphenols are environmental endocrine disruptors that have detrimental effects on aquatic organisms. Using marine medaka larvae, this study explored the effects of bisphenol compounds [bisphenol A (BPA), bisphenol S (BPS), bisphenol F (BPF), and bisphenol AF (BPAF)] on the early growth and development of aquatic organisms. Marine medaka larvae were exposed to bisphenol compounds at concentrations of 0.05, 0.5, and 5 μM for 72 h, and changes in heartbeat rate, behavior, hormone levels, and gene expression were determined. Bisphenols were shown to have a toxic effect on the cardiovascular system of larvae and can cause neurotoxicity and endocrine disruption, such as changes to thyroid-related hormones. Functional enrichment showed that bisphenols mainly affect lipid metabolism and cardiac muscle contraction of larvae, which implied that the main toxic effects of bisphenols on marine medaka larvae targeted the liver and heart. This study provides a theoretical foundation for evaluating the toxicological effects of bisphenols on the early development of aquatic organisms.

The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Lifecycle exposure to perchlorate differentially alters morphology, biochemistry, and transcription as well as sperm motility in Silurana tropicalis frogs

Perchlorate (ClO₄^-) contamination has been reported in ground and surface waters across North America. However, few studies have examined the effects of prolonged exposure to this thyroid hormone disrupting chemical, particularly at environmentally relevant concentrations in lower vertebrates, such as amphibians. The aim of this study was to examine the effects of a yearlong chronic exposure to ClO₄^- in adult male and female Western clawed frogs (Silurana tropicalis). Frogs were spawned and raised from fertilized embryo until sexual maturity in potassium perchlorate (KClO₄)-treated water at different concentrations (0, 20, 53, and 107 μg/L). Developmental and reproductive indices - including adult morphology, androgen plasma levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility - were evaluated in male and female adult frogs. Female growth (e.g., body mass, snout-vent length, and hind limb length) was significantly reduced following chronic exposure to environmentally relevant concentrations of KClO₄ resulting in females with morphometric indices similar to those of control males - indicating potential sex-specific sensitivities to KClO₄. Changes to reproductive indices (i.e., plasma androgen levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility) were also observed in both sexes and suggest that KClO₄ exposure may also have indirect secondary effects on the reproductive axes in male and female adult frogs. These effects were observed at concentrations at or below those reported in surface waters contaminated with ClO₄^- suggesting that this contaminant may have developmental and reproductive effects post-metamorphosis in natural amphibian populations.

Knotting nets: Molecular junctions of interconnecting endocrine axes identified by application of the adverse outcome pathway concept

To be defined as an endocrine disruptor, a substance has to meet several criteria, including the induction of specific adverse effects, a specific endocrine mode of action, and a plausible link between both. The latter criterion in particular might not always be unequivocally determined, especially because the endocrine system consists of diverse endocrine axes. The axes closely interact with each other, and manipulation of one triggers effects on the other. The present review aimed to identify some of the many interconnections between these axes. The focus was on fish, but data obtained in studies on amphibians and mammals were considered if they assisted in closing data gaps, because most of the endocrine mechanisms are evolutionarily conserved. The review includes data both from ecotoxicological studies and on physiological processes and gives information on hormone/hormone receptor interactions or gene transcription regulation. The key events and key event relationships identified provide explanations for unexpected effects on one axis, exerted by substances suspected to act specifically on another axis. Based on these data, several adverse outcome pathway (AOP) segments are identified, describing connections between the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, the HPG and hypothalamic-pituitary-adrenal/interrenal (HPA/I) axes, and the HPT and HPA/I axes. Central key events identified across axes were altered aromatase activity as well as altered expression and function of the proteins 11β-hydroxysteroid dehydrogenase (11β-HSD) and steroidogenic acute regulatory (StAR) protein. Substance classes that act on more than one endocrine axis were, for example, goitrogens or aromatase inhibitors. Despite the wealth of information gathered, the present review only provides a few insights into the molecular nets of endocrine axes, demonstrating the complexity of their interconnections. Environ Toxicol Chem 2018;37:318-328. © 2017 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

An on-line spectrophotometric determination of trace amounts of thiourea in tap water, orange juice, and orange peel samples using multi-channel flow injection analysis

In this work, a flow injection analysis (FIA) method was introduced for the determination of trace amounts of thiourea in tap water. This method is based upon the inhibition effect of thiourea on the reaction between meta-cresol purple (MCP) and potassium bromate catalyzed by bromide ions in a sulfuric acid medium. In the presence of thiourea, an induction period appears in the reaction system, and as a result, the absorbance of MCP increases at 525 nm in the FIA manifold. The chemical and FIA variables are studied and optimized using the univariate and Simplex optimization methods. Under the optimum conditions, thiourea can be determined in the range of 0.100-13.0 μg mL(-1). The limit of detection (3σ) for thiourea was found to be 0.0310 μg mL(-1). The relative standard deviations (RSDs) for six replicate determinations of 0.500, 5.00, and 12.0 μg mL(-1) of thiourea were 4.0%, 1.8%, and 1.2%, respectively. The proposed method was also applied for the determination of thiourea in orange juice and orange peel samples with recoveries in the range of 98.0-101%. The analytical speed of the method was calculated to be about 120 sample per hour.

Crossover of the hypothalamic pituitary-adrenal/interrenal, -thyroid, and -gonadal axes in testicular development

Besides the well-known function of thyroid hormones (THs) for regulating metabolism, it has recently been discovered that THs are also involved in testicular development in mammalian and non-mammalian species. THs, in combination with follicle stimulating hormone, lead to androgen synthesis in Danio rerio, which results in the onset of spermatogenesis in the testis, potentially relating the hypothalamic-pituitary-thyroid (HPT) gland to the hypothalamic-pituitary-gonadal (HPG) axes. Furthermore, studies in non-mammalian species have suggested that by stimulating the thyroid-stimulating hormone (TSH), THs can be induced by corticotropin-releasing hormone. This suggests that the hypothalamic-pituitary-adrenal/interrenal gland (HPA) axis might influence the HPT axis. Additionally, it was shown that hormones pertaining to both HPT and HPA could also influence the HPG endocrine axis. For example, high levels of androgens were observed in the testis in Odonthestes bonariensis during a period of stress-induced sex-determination, which suggests that stress hormones influence the gonadal fate toward masculinization. Thus, this review highlights the hormonal interactions observed between the HPT, HPA, and HPG axes using a comparative approach in order to better understand how these endocrine systems could interact with each other to influence the development of testes.

Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones

Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

Regulation of gonadal sex ratios and pubertal development by the thyroid endocrine system in zebrafish (Danio rerio)

We examined associations between thyroid condition, gonadal sex and pubertal development in zebrafish. Seventy-two-hour postfertilization larvae were reared in untreated medium or in the presence of goitrogens (sodium perchlorate, 0.82 mM; methimazole, 0.15 and 0.3 mM) or thyroxine (1 and 10 nM) for 30 days. Thyrocyte height, gonadal sex and gonadal development were histologically determined at 45 and 60 days postfertilization (dpf). Thyrocyte hypertrophy, an index of hypothyroidism, was observed at 45 and 60 dpf in perchlorate-treated but only at 45 dpf in methimazole-treated fish. Similarly, gonadal sex ratios were biased toward ovaries relative to control animals at 45 and 60 dpf in perchlorate-treated fish but only at 45 dpf in methimazole-treated fish. Gonadal sex ratios were biased toward testes at 45 and 60 dpf in thyroxine-treated fish. Spermatogenesis was delayed in testes from goitrogen-treated fish at 60 dpf relative to control values, but was unaffected in testes from thyroxine-treated individuals. Oogenesis seemed to be nonspecifically delayed in all treatments relative to control at 60 dpf. This study confirmed the previously reported association between hypothyroid condition and ovarian-skewed ratios, and hyperthyroid condition and testicular-skewed ratios, and also showed that male pubertal development is specifically delayed by experimental hypothyroidism. The simultaneous recovery from the hypothyroid and ovary-inducing effects of methimazole by 60 dpf (27 days post-treatment) suggests that the ovary-skewing effect of goitrogens is reversible when thyroid conditions return to basal levels before developmental commitment of gonadal sex. Conversely, the masculinizing effect of hyperthyroidism seems to be stable and perhaps permanent.

Cloning, expression and enzyme activity analysis of testicular 11beta-hydroxysteroid dehydrogenase during seasonal cycle and after hCG induction in air-breathing catfish Clarias gariepinus

A full-length cDNA encoding 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) was cloned from testis of air-breathing catfish, Clarias gariepinus which showed high sequence homology to zebrafish and eel. The open reading frame of 11beta-HSD2 was then transfected to COS-7 cells, which converted 11beta-hydroxytestosterone (11-OHT) to 11-ketotestosterone (11-KT). Using NAD(+), 11beta-HSD2 from testicular microsomes oxidized 11-OHT with apparent K(m) 56+/-4nM and V(max) 55+/-6pmol/h/mgprotein values. Tissue distribution analysis revealed prominent expression in testis, anterior kidney, liver and gills. Expression of 11beta-HSD2 in testis and serum levels of 11-KT were high in the prespawning phase. Administration of human chorionic gonadotropin (hCG) during prespawning and resting phases revealed initial rise in 11beta-HSD2 transcript at 4h followed by gradual increase at 8h, 12h and peaking at 24h, only in testis of prespawning phase. Rate of conversion of 11-OHT to 11-KT by testicular microsomes during different testicular phases and after hCG administration corroborated well with the expression of 11beta-HSD2. Ontogeny study indicated that this enzyme is expressed during testicular development. Thus the spatio-temporal expression supported with putative dehydrogenase activity and circulating 11-KT levels clearly suggest a major role for 11beta-HSD2 during testicular differentiation and seasonal testicular cycle in catfish.

Molecular cloning of two isoforms of 11beta-hydroxylase and their expressions in the Nile tilapia, Oreochromis niloticus

P450 11beta-hydroxylase, encoded by P450(11beta) gene, is a key mitochondrial enzyme to produce 11beta-hydroxy testosterone, substrate for the production of 11-ketotestosterone (11-KT), which has been shown to be potent androgen in several fish species. In the present work, two alternative splicing isoforms i.e. P450(11beta)-1 and P450(11beta)-2 cDNAs were cloned from the Nile tilapia, Oreochromis niloticus. They were 1614 and 1227bp in length with open reading frames encoding proteins of 537 and 408 amino acids, respectively. In contrast to P450(11beta)-1, which derived from 9 exons of the P450(11beta) gene, the 7th and 8th exons were absent in P450(11beta)-2. Tilapia P450(11beta)-1 shares the highest homology with that of medaka, Oryzias latipes. Expressions of P450(11beta)-1 and -2 were detected in the kidney and head kidney of both sexes, and in the testis but not in the ovary, with P450(11beta)-2 lower than P450(11beta)-1. Ontogenic expressions of both isoforms were detected in testis from 50dah onwards. P450(11beta)-1 and -2 were strongly expressed in sex reversed XX testis after fadrozole and tamoxifen treatment, but completely inhibited in 17beta-estradiol induced XY ovary. The existence of two alternatively spliced isoforms and the sexual dimorphic expression of P450(11beta)s were further confirmed by Northern blot. Strong expression signals in Leydig cells and weak signals in spermatogonia were detected by in situ hybridization and immunohistochemistry. Taken together, our data suggest a role for P450(11beta) in the spermatogenesis of tilapia through the production of 11-KT in testis, in addition to cortisol production in head kidney.

Novel 3beta-hydroxysteroid dehydrogenases from gonads of the Nile tilapia: phylogenetic significance and expression during reproductive cycle

Multiple forms of 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) and their differential tissue expression pattern have not been shown in any lower vertebrates. In the present study, we report cloning of two novel 3beta-HSDs and two variants from gonads of the Nile tilapia. 3beta-HSD cDNAs encode two peptides of 375 (3beta-HSD type-I/variant 1) and 367 (3beta-HSD type-II/variant 1) amino acid residues that share 31.9% homology. 3beta-HSD type-I/variant 1 shared high homology with other piscine counterparts while 3beta-HSD type-II/variant 1 exhibited homology to mammalian DeltaC27-3beta-HSD and multifunctional viral 3beta-HSD. The latter seems to be ancient form among vertebrates. Transiently transfected 3beta-HSDs' open reading frames in COS-7 cells converted exogenous pregnenolone/androsta-5-ene-3beta-17beta-diol to progesterone/testosterone. Tissue distribution pattern of 3beta-HSDs by RT-PCR revealed varied expression pattern. Northern blot analysis of 3beta-HSDs demonstrated steady or gradual rise in transcripts level at different gonadal stages. These data revealed the importance of novel 3beta-HSDs in teleosts and also provided phylogenetic significance.