Vitellogenin association and oocytic accumulation of thyroxine and 3, 5,3'-triiodothyronine in gravid Fundulus heteroclitus

Evolution of thyroid hormone distributor proteins in fish

In plasma, thyroid hormone (TH) is bound to several TH distributor proteins (THDPs), constituting a TH delivery/distribution network. Extensive studies of THDPs from tetrapods has proposed an evolutionary scenario concerning structural and functional changes in THDPs, especially for transthyretin (TTR). When assessing, in an evolutionary context, the roles of THDPs as a component constituting part of the vertebrate thyroid system, the data from fish THDPs are critical. In this review the phylogenetic distributions, spatiotemporal expression patterns and binding properties of THDPs in fish are described, and the question of whether the evolutionary hypotheses proposed in tetrapod THDPs can be applied to fish THDPs is assessed. The phylogenetic distributions of THDPs are highly variable among fish groups. Analysis in this review reveals that the evolutionary hypotheses proposed in tetrapod THDPs cannot be applied to fish THDPs, and that the role of plasma lipoproteins as THDPs grows in importance in fish groups. In primitive fish, zinc is an import factor in TH binding to TTR, and high zinc content may facilitate the acquisition of high TH binding activity during the early evolution of TTR. Finally, the possible roles of THDPs in the vertebrate thyroid system are discussed.

Insight into the transgenerational effect of benzo[a]pyrene on bone formation in a teleost fish (Oryzias latipes)

Recent cross-generational studies in teleost fish have raised the awareness that high levels of benzo[a]pyrene (BaP) could affect skeletal integrity in the directly exposed F0 and their F1-F2. However, no further details were provided about the causes for abnormalities on the molecular and cellular level and the persistence of such sub-organismal impairments at the transgenerational scale (beyond F2). Adult Oryzias latipes were exposed to 1μg/L BaP for 21days. The F1-F3 were examined for skeletal deformities, histopathological alterations of vertebral bodies and differential expression of key genes of bone metabolism. Significant increase of dorsal-ventral vertebral compression was evident in ancestrally exposed larvae. Histopathological analysis revealed abnormal loss of notochord sheath, a lack of notochord epithelial integrity, reduced bone tissue and decreased osteoblast abundance. A significant downregulation of ATF4 and/or osterix and a high biological variability of COL10, coupled with a significant deregulation of SOX9a/b in the F1-F3 suggest that ancestral BaP exposure most likely perturbed chordoblasts, chondroblast and osteoblast differentiation, resulting in defective notochord sheath repair and rendering the vertebral column more vulnerable to compression. The present findings provide novel molecular and cellular insights into BaP-induced transgenerational bone impairment in the unexposed F3. From the ecological risk assessment perspective, BaP needs to be regarded as a transgenerational skeletal toxicant, which exerts a far-reaching impact on fish survival and fitness. Given that basic mechanisms of cartilage/bone formation are conserved between medaka and mammals, the results may also shed light on the potential transgenerational effect of BaP on the genesis of skeletal diseases in humans.

Effects of glucose, insulin and triiodothyroxine on leptin and leptin receptor expression and the effects of leptin on activities of enzymes related to glucose metabolism in grass carp (Ctenopharyngodon idella) hepatocytes

Leptin is an important regulator of appetite and energy expenditure in mammals, but its role in fish metabolism control is poorly understood. Our previous studies demonstrated that leptin has an effect on the regulation of food intake and energy expenditure as well as lipid metabolism (stimulation of lipolysis and inhibition of adipogenesis) in the grass carp Ctenopharyngodon idella. To further investigate the role of leptin in fish, the effects of glucose, insulin and triiodothyroxine (T3) on the expression levels of leptin and leptin receptor (Lepr) and the effects of leptin on the activities of critical glucose metabolism enzymes in grass carp hepatocytes were evaluated in the present study. Our data indicated that leptin gene expression was induced by glucose in a dose-dependent manner, while Lepr gene expression exhibited a biphasic change. A high dose of insulin (100 ng/mL) significantly up-regulated the expression of leptin and Lepr. Leptin expression was markedly up-regulated by a low concentration of T3 but inhibited by a high concentration of T3. T3 up-regulated Lepr expression in a dose-dependent manner. Together, these data suggest that leptin had a close relationship with three factors (glucose, insulin and T3) and might participate in the regulation of glucose metabolism in grass carp. In addition, we also found that leptin affected the activities of key enzymes that are involved in glucose metabolism, which might be mediated by insulin receptor substrate-phosphoinositol 3-kinase signaling.

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

Long-term exposure of European flounder (Platichthys flesus) to the flame-retardants tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD)

Tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD) are widely used flame retardants that have increasingly been found as contaminants in the aquatic environment. In the present study, European flounder (Platichthys flesus) were chronically exposed to TBBPA; (105 days) and HBCD (78 days), in a wide range including environmentally relevant concentrations. TBBPA was administered via the water, whereas HBCD was administered in food and sediment, or in sediment alone. Chemical analysis of muscle showed an average increase in internal concentrations of approximately two orders of magnitude for both compounds tested. Animals exposed to HBCD via sediment alone (8000 microg/g total organic carbon, TOC) showed a proportional increase of alpha-HBCD in muscle compared to animals exposed via food and sediment. In both studies, exposure to the test compounds did not affect general health and toxicity parameters (behavior, survival, growth rate, relative liver and gonad weight). Hepatic microsomal enzyme activities (TBBPA: EROD; HBCD: EROD, PROD, and BROD) were not induced by any of the tested chemicals. Aromatase activity in male gonads showed a mild increase with rising TBBPA levels. There were no morphological and immunohistochemical indications for increased production of the yolk precursor protein vitellogenin (VTG) in animals exposed to TBBPA and HBCD; immunochemical analysis of plasma VTG levels showed no dose response in animals exposed to TBBPA. In animals exposed to TBBPA, levels of the thyroid hormone thyroxin (T(4)) increased with internal concentrations of the test compound, possibly indicating competition of TBBPA for plasma protein binding. Triiodothyronin (T(3)) levels were not affected and histology showed no signs of altered thyroid gland activity. Other organs investigated (liver, gills, kidney, skin, and gonads) revealed no histological changes related to TBBPA or HBCD exposure. Overall, the present results indicate limited endocrine effects of these widely used flame retardants in a test species representative of European estuaries at environmentally relevant exposure levels and at internal levels up to 4300 ng TBBPA/g wet weight, and 446 microg HBCD/g lipid weight in flounder muscle.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

Thyroid hormone profile during annual reproductive cycle of diploid and triploid catfish, Heteropneustes fossilis (Bloch)

Triploid fishes generally show sterility along with retarded gonadal development and aneuploid gametes. In teleosts, thyroid hormones influence seasonal adaptations and annual events such as reproduction. In addition, thyroid hormone deposition in matured ova is important for reproductive success as the role of thyroid hormones in early development and metamorphosis is well established. The present study deals with measurements of free and total thyroxine (T4) and triiodothyronine (T3) in the plasma of triploid and diploid catfish Heteropneustes fossilis (Bloch) in a complete reproductive cycle. Accumulations of total T4 and T3 within the oocytes have also been measured during the spawning period from fishes of both ploidy groups. No difference of plasma free hormones was noticed between the diploids and triploids of both the sexes in any period of reproductive cycle, although, seasonal variations were noted in both the groups. A significant decrease in the total thyroid hormone levels was noticed in plasma of the diploids in the spawning period compared to triploid fish. During the same period, accumulation of THs was significantly higher in the oocytes of diploids than that of the triploids. Thyroid gland structure also revealed a higher state of activity in the female diploids than the triploids during spawning period. Lower activity of thyroid tissue, higher levels of THs in plasma, and lower accumulation of maternally derived hormones in the oocytes of triploid females during spawning period may be associated with sterility of triploids.

Isolation and characterization of two cytochrome P450 aromatase forms in killifish (Fundulus heteroclitus): differential expression in fish from polluted and unpolluted environments

Populations of killifish (Fundulus heteroclitus) persist in many different highly polluted environment indicative of adaptation or tolerance. In this study, we sought to determine whether long term, multigenerational exposures to environmental contaminants has affected reproductively relevant genes and biological processes. A homology cloning strategy was used to isolate the killifish cytochrome P450 aromatase (P450arom, estrogen synthetase) cDNAs. Consistent with previous fish studies, killifish were found to have two P450arom forms, which segregated into A- and B-gene clades and were differentially expressed in brain (B >> A) and ovary (A >> B). Comparison of killifish from highly polluted (New Bedford Harbor, NBH) and unpolluted (Scorton Creek, SC) environments revealed no site-related differences in P450arom coding sequences or in overall tissue distribution patterns. As measured by real-time quantitative PCR (QPCR) analysis, however, P450arormB (a known marker of estrogen effect) was approximately two-fold higher in the brain of NBH than of SC fish, a difference seen in reproductively active and inactive males and females. Providing further evidence of exposure to estrogen-like pollutants or metabolites in NBH, vitellogenin (vtg) mRNA and protein were elevated in seasonally active and inactive males, and in reproductively inactive females, when compared to SC fish. By contrast, during the period of reproductive activity, NBH females had a lower gonadosomatic index, lower plasma estrogen, a decreased hepatosomatic index, and reduced vtg expression as compared to SC females, indicating that the female hypothalamic-pituitary-gonadal (HPG)-liver axis is impaired in the polluted environment. As measured by a decrease in plasma androgen (but not GSI), the male HPG axis was impaired in reproductively active NBH versus SC fish. In agreement with reports that NBH killifish are resistant to dioxin-like chemicals (DLC) that activate arylhydrocarbon receptor (AhR) signaling, ovarian P450aromA (a marker of dioxin-like effect in zebrafish embryos) did not differ in SC and NBH fish. In conclusion, the killifish population at the NBH Superfund site maintains a level of reproductive competence in the face of evidence of exposure to estrogen-like pollutants and endocrine disruption.

Differential effects of melatonin on plasma levels of thyroxine and triiodothyronine levels in the air-breathing fish, Clarias gariepinus, during breeding and quiescent periods

Effects of morning and evening injections of three doses of melatonin were studied on the plasma levels of thyroxine (T(4)) and triiodothyronine (T(3)) in an air-breathing fish Clarias gariepinus during its quiescent phase and breeding phase. Depending on seasons and time of administration, melatonin increased, decreased or had no effects on the plasma levels of T(4) and T(3). Present findings strongly suggest that melatonin is involved in the regulation of thyroid hormones in C. gariepinus, and differentially influences T(4) and T(3) level.

Entry of thyroid hormones into tilapia oocytes

The patterns of entry of thyroid hormones into live tilapia oocytes were examined by incubating ovarian follicles in L-15 medium containing 125I-labeled thyroxine (T4) or 3,5,3'-triiodothyronine (T3). As judged from HPLC profiles, radioactivity in extracts of follicles immersed in T3 was identified to reside in T3, while most of the radioactivity in the extract of T4 immersed follicle was not associated with T4. Radioactivity of T3 immersed follicles reached a constant level after 18 h of incubation. Entry of T3 into the oocytes was non-saturable within the range of 0.5-5000 ng/ml of T3 in the incubation medium, suggesting the absence of specific mechanisms for T3 entry into the oocyte. Presence of female plasma at a level of 20% of incubation medium inhibited the T3 entry into the oocytes by approximately 80%. When follicles were back-transferred to medium without T3, only 15% of T3 in the oocyte disappeared within the following 24 h. From our results, we conclude that free T3 seems to enter oocytes freely across the membranes by diffusion, and that T3 in the oocytes may bind to some molecules in the oocyte. However, during egg formation in vivo, contribution of free T3 entry into the oocytes did not seem to be significant when considering the free T3 ratio in female plasma.