Revisiting available knowledge on teleostean thyroid hormone receptors

Molecular characterization and expression analysis of thyroid hormone receptors in protogynous rice field eel, Monopterus albus

Thyroid hormones (THs) play important roles in growth, development, morphogenesis, reproduction, and so on. They are mainly meditated by binding to thyroid hormone receptors (TRs) in vertebrates. As important members of the nuclear receptor superfamily, TRs and their ligands are involved in many biological processes. To investigate the potential roles of TRs in the gonadal differentiation and sex change, we cloned and characterized the TRs genes in protogynous rice field eel (Monopterus albus). In this study, three types of TRs were obtained, which were TRαA, TRαB and TRβ, encoding preproproteins of 336-, 409- and 415-amino acids, respectively. Multiple alignments of the three putative TRs protein sequences showed they had a higher similarity. Tissue expression analysis showed that TRαA mainly expressed in the gonad, while TRαB and TRβ in the brain. During female-to-male sex reversal, the expression levels of all the three TRs showed a similar trend of increase followed by a decrease in the gonad. Intraperitoneal injection of triiodothyronine (T3) stimulated the expression of TRαA and TRαB, while it had no significant change on the expression of TRβ in the ovary. Gonadotropin-releasing hormone analogue (GnRHa) injection also significantly upregulated the expression levels of TRαA and TRαB after 6 h, while it had no significant effect on TRβ. These results demonstrated that TRs were involved in the gonadal differentiation and sex reversal, and TRα may play more important roles than TRβ in reproduction by the regulation of GnRHa in rice field eel.

A comprehensive coding and microRNA transcriptome of vertebral bone in postlarvae and juveniles of Senegalese sole (Solea senegalensis)

Understanding vertebral bone development is essential to prevent skeletal malformations in farmed fish related to genetic and environmental factors. This is an important issue in Solea senegalensis, with special impact of spinal anomalies in postlarval and juvenile stages. Vertebral bone transcriptomics in farmed fish mainly comes from coding genes, and barely on miRNA expression. Here, we used RNA-seq of spinal samples to obtain the first comprehensive coding and miRNA transcriptomic repertoire for postlarval and juvenile vertebral bone, covering different vertebral phenotypes and egg-incubation temperatures related to skeleton health in S. senegalensis. Coding genes, miRNA and pathways regulating bone development and growth were identified. Differential transcriptomic profiles and suggestive mRNA-miRNA interactions were found between postlarvae and juveniles. Bone-related genes and functions were associated with the extracellular matrix, development and regulatory processes, calcium binding, retinol and lipid metabolism or response to stimulus, including those revealed by the miRNA targets related to signaling, cellular and metabolic processes, growth, cell proliferation and biological adhesion. Pathway enrichment associated with fish skeleton were identified when comparing postlarvae and juveniles: growth and bone development functions in postlarvae, while actin cytoskeleton, focal adhesion and proteasome related to bone remodeling in juveniles. The transcriptome data disclosed candidate coding and miRNA gene markers related to bone cell processes, references for functional studies of the anosteocytic bone of S. senegalensis. This study establishes a broad transcriptomic foundation to study healthy and anomalous spines under early thermal conditions across life-stages in S. senegalensis, and for comparative analysis of skeleton homeostasis and pathology in fish and vertebrates.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Ocean acidification affects the expression of neuroplasticity and neuromodulation markers in seabream

A possible explanation for acidification-induced changes in fish behaviour is that acidification interferes with neurogenesis and modifies the plasticity of neuronal circuitry in the brain. We tested the effects on the olfactory system and brain of gilthead seabream (Sparus aurata) to 4 weeks' exposure to ocean acidification (OA). Olfactory epithelium (OE) morphology changed shortly after OA exposure and persisted over the 4 weeks. Expression of genes related to olfactory transduction, neuronal excitability, synaptic plasticity, GABAergic innervation, and cell proliferation were unchanged in the OE and olfactory bulb (OB) after 4 weeks' exposure. Short-term changes in the ionic content of plasma and extradural fluid (EDF) returned to control levels after 4 weeks' exposure, except for [Cl⁻], which remained elevated. This suggests that, in general, there is an early physiological response to OA and by 4 weeks a new homeostatic status is achieved. However, expression of genes involved in proliferation, differentiation and survival of undifferentiated neurons were modified in the brain. In the same brain areas, expression of thyroid hormone signalling genes was altered suggesting modifications in the thyroid-system may be linked to the changes in neuroplasticity and neurogenesis. Overall, the results of the current study are consistent with and effect of OA on neuroplasticity.

Summary: Ocean acidification alters fish behaviour. We show altered expression of genes involved in neuroplasticity and neuromodulation in fish exposed to high PCO₂, highlighting their possible roles in such behavioural alterations.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Novel potent natural peroxidases inhibitors with in vitro assays, inhibition mechanism and molecular docking of phenolic compounds and alkaloids

Peroxidase inhibition produced by phenolic compounds as hispidin and gallic acid, alkaloids as harmine and natural extracts of Inonotus hispidus, and Marrubium vulgare were investigated in this study. No further studies have been found in this context. Thus, the results show that the phenolic and the alkaloidal extracts with the three molecules are potent inhibitors of horseradish peroxidase. Uric acid is used as a substrate reaction to finding the enzymatic inhibition for the first time. The results show that the best inhibitor is hispidin with a value of IC50 = 23 µg/ml. Moreover, Molecular docking has been carried out using the AutoDock Vina program to discuss the nature of interactions and the mechanism of inhibition between both peroxidases (horseradish and thyroid) which is performed with and without heme group for the first time. The three studied compounds were further subjected to ADEMT and Lipinski filtering analyses for drug-likeness prediction analysis. However, the results show that all the docked molecules are competitive inhibitors confirming that no further studies have been published before. Thus, hispidin is a more potent irreversible TPO inhibitor then propylthiouracil anti-thyroid drug. Its inhibition mechanism is well described through this work for the first time; which suggests is used as an anti-thyroid drug to treat hyperthyroidism. Furthermore, the studied phenolic compounds (Hispidin and Gallic acid) and one alkaloid (Harmine) are non-toxic, that bind to the receptor-binding site and catalytic dyad of peroxidases were identified from the predictive ADMET and Lipinski filter analysis.Communicated by Ramaswamy H. Sarma.

ERGO: Breaking Down the Wall between Human Health and Environmental Testing of Endocrine Disrupters

ERGO (EndocRine Guideline Optimization) is the acronym of a European Union-funded research and innovation action, that aims to break down the wall between mammalian and non-mammalian vertebrate regulatory testing of endocrine disruptors (EDs), by identifying, developing and aligning thyroid-related biomarkers and endpoints (B/E) for the linkage of effects between vertebrate classes. To achieve this, an adverse outcome pathway (AOP) network covering various modes of thyroid hormone disruption (THD) in multiple vertebrate classes will be developed. The AOP development will be based on existing and new data from in vitro and in vivo experiments with fish, amphibians and mammals, using a battery of different THDs. This will provide the scientifically plausible and evidence-based foundation for the selection of B/E and assays in lower vertebrates, predictive of human health outcomes. These assays will be prioritized for validation at OECD (Organization for Economic Cooperation and Development) level. ERGO will re-think ED testing strategies from in silico methods to in vivo testing and develop, optimize and validate existing in vivo and early life-stage OECD guidelines, as well as new in vitro protocols for THD. This strategy will reduce requirements for animal testing by preventing duplication of testing in mammals and non-mammalian vertebrates and increase the screening capacity to enable more chemicals to be tested for ED properties.

Knock-Down of Specific Thyroid Hormone Receptor Isoforms Impairs Body Plan Development in Zebrafish

The role of thyroid hormones (THs) in development has been extensively studied, however, the specific molecular mechanisms involved are far from being clear. THs act by binding to TH nuclear receptors (TR) that act as ligand-dependent transcription factors to regulate TH-dependent gene expression. Like vertebrates, zebrafish express different isoforms of functional Tr alpha and beta, some of which can bind alternative ligands like 3,5-T2. In this study, we first analyzed the effects of exogenous T3 and 3,5-T2 exposure during embryogenesis. The percentage of affected embryos was similar to those vehicle-injected, suggesting that the early exposure to low TH levels is not sufficient to elicit effects upon the phenotype of the embryo. We then generated crispants for four isoforms of thr to learn more about the role of these receptors in early development. We found that crispant larvae from thraa and a newly identified l-thrb+, but not thrab and canonical thrb1 showed profound deleterious effects upon symmetry and laterality, suggesting early novel roles for these Tr isoforms in the body plan developmental program. Since critical events that determine cell fate start in the late gastrula, we tested if some genes that are expressed during early developmental stages could indeed be TH targets. We identify early development genes, like sox10 and eve, that were specifically over-expressed in thraa and l-thrb+ crispants, suggesting that these specific thr isoforms function as transcription repressors for these genes, while transcription of zic and ets appear to be thraa and l-thrb+-mediated, respectively. Overall, present results show that TH signaling participates in early zebrafish development and identify Tr isoform-specific mediated regulation of early gene expression.