Thyrotropic activity of recombinant human glycoprotein hormone analogs and pituitary mammalian gonadotropins in goldfish (Carassius auratus): insights into the evolution of thyrotropin receptor specificity

Response of the thyroid axis and appetite-regulating peptides to fasting and overfeeding in goldfish (Carassius auratus)

The thyroid axis is a major regulator of metabolism and energy homeostasis in vertebrates. There is conclusive evidence in mammals for the involvement of the thyroid axis in the regulation of food intake, but in fish, this link is unclear. In order to assess the effects of nutritional status on the thyroid axis in goldfish, Carassius auratus, we examined brain and peripheral transcripts of genes associated with the thyroid axis [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptors (TRH-R type 1 and 2), thyroid stimulating hormone beta (TSHβ), deiodinase enzymes (DIO2, DIO3) and UDP-glucoronsyltransferase (UGT)] and appetite regulators [neuropeptide Y (NPY), proopiomelanocortin (POMC), agouti-related peptide (AgRP) and cholecystokinin (CCK)] in fasted and overfed fish for 7 and 14 day periods. We show that the thyroid axis responds to overfeeding, with an increase of brain TRH and TSHβ mRNA expression after 14 days, suggesting that overfeeding might activate the thyroid axis. In fasted fish, hepatic DIO3 and UGT transcripts were downregulated from 7 to 14 days, suggesting a time-dependent inhibition of thyroid hormone degradation pathways. Nutritional status had no effect on circulating levels of thyroid hormone. Central appetite-regulating peptides exhibited temporal changes in mRNA expression, with decreased expression of the appetite-inhibiting peptide POMC from 7 to 14 days for both fasted and overfed fish, with no change in central NPY or AgRP, or intestinal CCK transcript expression. Compared to control fish, fasting increased AgRP mRNA expression at both 7 and 14 days, and POMC expression was higher than controls only at 7 days. Our results indicate that nutritional status time-dependently affects the thyroid axis and appetite regulators, although no clear correlation between thyroid physiology and appetite regulators could be established. Our study helps to fill a knowledge gap in current fish endocrinological research on the effects of energy balance on thyroid metabolism and function.

Biologically active recombinant carp LH as a spawning-inducing agent for carp

Currently, spawning is induced in carp species by carp pituitary extract (CPE) and a combination of synthetic agonist of GnRH combined with a dopamine antagonist. The main goal of this study was the production of recombinant gonadotropins (GtHs) on a large scale to serve as an alternative to currently used agents. We produced carp (c) recombinant (r) Lh as a single chain in the methylotrophic yeast Pichia pastoris Lha subunit was joined with Lhb subunit with a flexible linker of three glycine-serine repeats and six Histidines to form a mature protein, the β-subunit formed the N-terminal part and the α-subunit formed the C-terminal part. The ability of the rcLh to elicit biological response was tested by in vivo stimulation of estradiol (E2) and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and by its in vivo potency to induce ovulation and spawning induction. rcLh tested in this work significantly enhanced both E2 and DHP secretion in a dose-dependent manner similar to the results obtained with CPE. E2 levels showed a moderate rise following the priming injection and a subsequent decrease during the rest of the trial. DHP levels were only increased after the resolving injection, approximately 5 h before spawning. At the highest dose of rcLh (350 µg/kg BW), the recombinant protein was more efficient than CPE in terms of both spawning success and fertilization rate. It is shown here that rcLh can elicit the secretion of DHP in vivo and actually trigger spawning. These novel findings introduce the potential of utilizing recombinant gonadotropins in aquaculture.

New Frontier in Glycoprotein Hormones and Their Receptors Structure-Function

Last two decades of structure-function studies performed in numerous laboratories provided substantial progress in understanding basic science, physiological, pathophysiological, pharmacological, and comparative aspects of glycoprotein hormones (GPHs) and their cognate receptors. Multiple concepts and models developed based on experimental data in the past stood the test of time and have been, at least in part, confirmed and/or remained compatible with the new structures resolved at the atomic level. Major advances in understanding of the ligand-receptor relationships are heralding the dawn of a new era for GPHs and their receptors, although many basic questions still remain unanswered. This article examines retrospectively several basic science aspects of GPH super-agonists and related "biosuperiors" in a broader context of the advances in the ligand-receptor structure-function relationships and new mechanistic models generated based on the structure elucidation. Due to selective focus of my comments and perspectives in certain parts, the reader is directed to the most relevant publications and reviews in the field for more comprehensive analyses.

Cyclic mRNA expression of thyrotropin subunits and deiodinases in red drum, Sciaenops ocellatus

The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T₄), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T₄ cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T₄ feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T₄ cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6-12 h out of phase with, the T₄ cycle, consistent with the hypothesis that TSH secretion drives the T₄ cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T₄ providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.