Thyroid hormones are crucial mediators of many aspects of vertebrate life, including reproduction. The key player is the biologically active 3,5,3’-triiodothyronine (T₃), whose local bio-availability is strictly regulated by deiodinase enzymes. Deiodinase type 2 (Dio2) is present in many tissues and is the main enzyme for local T₃ production. To unravel its role in different physiological processes, we generated a mutant zebrafish line, completely lacking Dio2 activity. Here we focus on the reproductive phenotype studied at the level of offspring production, gametogenesis, functioning of the hypothalamic–pituitary–gonadal axis and sex steroid production. Homozygous Dio2-deficient zebrafish were hypothyroid, displayed a delay in sexual maturity and the duration of their reproductive period was substantially shortened. Fecundity and fertilization were also severely reduced. Gamete counts pointed to a delay in oogenesis at onset of sexual maturity and later on to an accumulation of oocytes in mutant ovaries due to inhibition of ovulation. Analysis of spermatogenesis showed a strongly decreased number of spermatogonia A at onset of sexual maturity. Investigation of the hypothalamic–pituitary–gonadal axis revealed that dysregulation was largely confined to the gonads with significant upregulation of igf3, and a strong decrease in sex steroid production concomitant with alterations in gene expression in steroidogenesis/steroid signaling pathways. Rescue of the phenotype by T₃ supplementation starting at 4 weeks resulted in normalization of reproductive activity in both sexes. The combined results show that reproductive function in mutants is severely hampered in both sexes, thereby linking the loss of Dio2 activity and the resulting hypothyroidism to reproductive dysfunction.

Iodothyronine deiodinases are important mediators of thyroid hormone (TH) action. They are present in tissues throughout the body where they catalyse 3,5,3′-triiodothyronine (T₃) production and degradation via, respectively, outer and inner ring deiodination. Three different types of iodothyronine deiodinases (D1, D2 and D3) have been identified in vertebrates from fish to mammals. They share several common characteristics, including a selenocysteine residue in their catalytic centre, but show also some type-specific differences. These specific characteristics seem very well conserved for D2 and D3, while D1 shows more evolutionary diversity related to its Km, 6-n-propyl-2-thiouracil sensitivity and dependence on dithiothreitol as a cofactor in vitro. The three deiodinase types have an impact on systemic T₃ levels and they all contribute directly or indirectly to intracellular T₃ availability in different tissues. The relative contribution of each of them, however, varies amongst species, developmental stages and tissues. This is especially true for amphibians, where the impact of D1 may be minimal. D2 and D3 expression and activity respond to thyroid status in an opposite and conserved way, while the response of D1 is variable, especially in fish. Recently, a number of deiodinases have been cloned from lower chordates. Both urochordates and cephalochordates possess selenodeiodinases, although they cannot be classified in one of the three vertebrate types. In addition, the cephalochordate amphioxus also expresses a non-selenodeiodinase. Finally, deiodinase-like sequences have been identified in the genome of non-deuterostome organisms, suggesting that deiodination of externally derived THs may even be functionally relevant in a wide variety of invertebrates.

The hatching process in the chicken is accompanied by dramatic changes in plasma thyroid hormones. The cause of these changes, though crucial for hatching and the onset of endothermy, is not known. One hypothesis is that the pituitary gland becomes more sensitive to hypothalamic stimulation during this period. We have tested whether the responsiveness of the thyrotropes to hypothalamic stimuli changes throughout the last week of embryonic development and hatching by studying the mRNA expression of receptors involved in the control of the secretory activity of this cell type. We used a real-time PCR set-up to quantify whole pituitary mRNA expression of the β subunit of thyrotrophin (TSH-β), type 1 thyrotrophin-releasing hormone receptor (TRH-R1), corticotrophin-releasing hormone receptors (CRH-R1 and CRH-R2) and somatostatin subtype receptor 2 (SSTR2) on every day of the last week of embryonic development, including the day of hatch and the first day of posthatch life. The thyrotrope-specific expression was investigated by a combination of in situ hybridization with immunohistochemistry at selected ages. Although TSH-β mRNA levels increased towards day 19 of incubation (E19), the expression of CRH-R2 and TRH-R1 mRNA by the thyrotropes tended to decrease during this period, suggesting a lower sensitivity of the thyrotropes to the stimulatory factors CRH and TRH. CRH-R1, which is not involved in the control of TSH secretion, increased steadily throughout the period tested. The expression of SSTR2 mRNA by the thyrotropes was low during embryonic development and increased just before hatching. We have concluded that the sensitivity of the pituitary thyrotropes to hypothalamic stimulation decreases throughout the last week of embryonic development, so that the higher expression of TSH-β mRNA around E16–E19, and hence the increasing plasma thyroxine level, is unlikely to be the result of an increased stimulation by either TRH or CRH.

In this study, we tried to elucidate the changes in thyroid hormone (TH) receptor β2 (TRβ2) expression at the different levels of the hypothalamo–pituitary–thyroidal (HPT) axis during the last week of chicken embryonic development and hatching, a period characterized by an augmented activity of the HPT axis. We quantified TRβ2 mRNA in retina, pineal gland, and the major control levels of the HPT axis – brain, pituitary, and thyroid gland – at day 18 of incubation, and found the most abundant mRNA content in retina and pituitary. Thyroidal TRβ2 mRNA content increased dramatically between embryonic day 14 and 1 day post-hatch. In pituitary and hypothalamus, TRβ2 mRNA expression rose gradually, in parallel with increases in plasma thyroxine concentrations. Using in situ hybridization, we have demonstrated the presence of TRβ2 mRNA throughout the diencephalon and confirmed the elevation in TRβ2 mRNA expression in the hypophyseal thyrotropes. In vitro incubation with THs caused a down-regulation of TRβ2 mRNA levels in embryonic but not in post-hatch pituitaries. The observed expression patterns in pituitary and diencephalon may point to substantial changes in TRβ2-mediated TH feedback active during the perinatal period. The strong rise in thyroidal TRβ2 mRNA content could be indicative of an augmented modulation of thyroid development and/or function by THs toward and after hatching. Finally, THs proved to exert an age-dependent effect on pituitary TRβ2 mRNA expression.

In this paper we report the cloning of the chicken preprothyrotropin-releasing hormone (TRH) cDNA and the study of its hypothalamic distribution. Chicken pre-proTRH contains five exact copies of the TRH progenitor sequence (Glu-His-Pro-Gly) of which only four are flanked by pairs of basic amino acids. In addition, the amino acid sequence contains three sequences that resemble the TRH progenitor sequence but seem to have lost their TRH-coding function during vertebrate evolution. The amino acid sequence homology of preproTRH between different species is very low. Nevertheless, when the tertiary structures are compared using hydrophobicity plots, the resemblance between chicken and rat prepro-TRH is striking. The cloning results also showed that the chicken preproTRH sequence includes neither a rat peptide spacer 4 (Ps4) nor a Ps5 connecting peptide. Comparison of the cDNA sequence with the chicken genome database revealed the presence of two introns, one in the 5′ untranslated region, and another downstream from the translation start site. This means that the gene structure of chicken preproTRH resembles the gene stucture of this precursor in mammals. Based on the cDNA sequence, digoxigenin-labelled probes were produced to study the distribution of preproTRH in the chicken brain. By means of in situ hybridization, preproTRH mRNA was detected in the chicken paraventricular nucleus (PVN) and in the lateral hypothalamus (LHy).

Inactivating mutations in the human SLC16A2 gene encoding the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in the Allan–Herndon–Dudley syndrome accompanied by severe locomotor deficits. The underlying mechanisms of the associated cerebellar maldevelopment were studied using the chicken as a model. Electroporation of an MCT8-RNAi vector into the cerebellar anlage of a 3-day-old embryo allowed knockdown of MCT8 in Purkinje cell precursors. This resulted in the downregulation of the thyroid hormone-responsive gene RORα and the Purkinje cell-specific differentiation marker LHX1/5 at day 6. MCT8 knockdown also results in a smaller and less complex dendritic tree at day 18 suggesting a pivotal role of MCT8 for cell-autonomous Purkinje cell maturation. Early administration of the thyroid hormone analogue 3,5,3′-triiodothyroacetic acid partially rescued early Purkinje cell differentiation. MCT8-deficient Purkinje cells also induced non-autonomous effects as they led to a reduced granule cell precursor proliferation, a thinner external germinal layer and a loss of PAX6 expression. By contrast, at day 18, the external germinal layer thickness was increased, with an increase in presence of Axonin-1-positive post-mitotic granule cells in the initial stage of radial migration. The concomitant accumulation of presumptive migrating granule cells in the molecular layer, suggests that inward radial migration to the internal granular layer is stalled. In conclusion, early MCT8 deficiency in Purkinje cells results in both cell-autonomous and non-autonomous effects on cerebellar development and indicates that MCT8 expression is essential from very early stages of development, providing a novel insight into the ontogenesis of the Allan–Herndon–Dudley syndrome.

This paper reports the results of in vivo and in vitro experiments on the feedback effects of corticosterone on the hypothalamo–pituitary–adrenal axis in embryos at day 18 of incubation and in 9-day-old chickens. In vivo, a significant negative feedback was detected on the levels of corticotropin-releasing factor (CRF) precursor (proCRF) mRNA and on the plasma concentration of corticosterone, two hours after a single intravenous injection with 40 μg corticosterone. In contrast, the levels of CRF peptide in the hypothalamic area, the CRF receptor type 1 (CRF-R1) mRNA and pro-opiomelanocortin (POMC) mRNA levels in the pituitary were not affected by the in vivo administration of corticosterone. In vitro, incubation with 1 μM corticosterone did not affect the CRF-R1 mRNA levels in the pituitary, but significant feedback inhibition was observed on the POMC mRNA levels. These in vitro effects were the same at the two ages studied. The in vitro feedback effect on the proCRF gene expression, however, differed with age. In 9-day-old animals a decrease in gene expression was observed which was not detectable in embryonic tissue at day 18 of the ontogeny.

Methimazole (MMI) is an anti-thyroid drug used in the treatment of chronic hyperthyroidism. There is, however, some debate about its use during pregnancy as MMI is known to cross the mammalian placenta and reach the developing foetus. A similar problem occurs in birds, where MMI is deposited in the egg and taken up by the developing embryo. To investigate whether maternally derived MMI can have detrimental effects on embryonic development, we treated laying hens with MMI (0.03% in drinking water) and measured total and reduced MMI contents in the tissues of hens and embryos at different stages of development. In hens, MMI was selectively increased in the thyroid gland, while its levels in the liver and especially brain remained relatively low. Long-term MMI treatment induced a pronounced goitre with a decrease in thyroxine (T₄) content but an increase in thyroidal 3,5,3′-triiodothyronine (T₃) content. This resulted in normal T₃ levels in tissues except in the brain. In chicken embryos, MMI levels were similar in the liver and brain. They gradually decreased during development but always remained above those in the corresponding maternal tissues. Contrary to the situation in hens, T₄ availability was only moderately affected in embryos. Peripheral T₃ levels were reduced in 14-day-old embryos but normal in 18-day-old embryos, while brain T₃ content was decreased at all embryonic stages tested. We conclude that all embryonic tissues are exposed to relatively high doses of MMI and its oxidised metabolites. The effect of maternal MMI treatment on embryonic thyroid hormone availability is most pronounced for brain T₃ content, which is reduced throughout the embryonic development period.