Functions of the Thyroid-Stimulating Hormone on Key Developmental Features Revealed in a Series of Zebrafish Dyshormonogenesis Models

Abstract

The hypothalamic–pituitary–thyroid (HPT) axis regulates many critical features in vertebrates. Utilizing TALENs and CRISPR/Cas9 techniques, thyroid-stimulating hormone subunit beta a (tshba), thyroglobulin (tg), and solute carrier family 16 member 2 (slc16a2) mutant zebrafish lines were generated. Among the three mutants, the earliest time point for the significantly altered T3 contents was observed in tshba mutants, which resulted in the most severe defects, including typical defects such as the retardation of inflated anterior swimming bladder (aSB), proper formation of fin ray and posterior squamation (SP), the larval-to-juvenile transition (LTJT) process, juvenile growth retardation, and mating failure. In tg mutants, which are actually compensated with an alternative splicing form, growth retardation was observed in the juvenile stage without LTJT and reproductive defects. The evident goiter phenotype was only observed in tg- and slc16a2 mutants, but not in tshba mutants. Other than goiters being observed, no other significant developmental defects were found in the slc16a2 mutants. Regarding the reproductive defects observed in tshba mutants, the defective formation of the secondary sex characteristics (SSCs) was observed, while no obvious alterations during gonad development were found. Based on our analyses, zebrafish at the 6–12 mm standard length or 16–35 days post-fertilization (dpf) should be considered to be in their LTJT phase. Using a series of zebrafish dyshormonogenesis models, this study demonstrated that the TSH function is critical for the proper promotion of zebrafish LTJT and SSC formation. In addition, the elevation of TSH levels appears to be essential for goiter appearance in zebrafish.

Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

“…Reduced tshb expression in maturing fish is very interesting in context with observations in zebrafish, showing that thyroid hormone potentiated Fsh-induced androgen production ( 68 ). Recent genetic evidence supported this thyroid-testis connection, considering that tshba mutant males neither showed secondary sex characters nor reproductive behaviour when tested with wild-type females ( 69 ). Assuming the existence of a similar thyroid-testis connection in salmon, it seems possible for example that androgens exert a negative feedback on pituitary thyrotropes.…”

Thyroid Hormones Deficiency Impairs Male Germ Cell Development: A Cross Talk Between Hypothalamic-Pituitary-Thyroid, and—Gonadal Axes in Zebrafish

“…Song and collaborators (2021) reported reproductive performance defects, but normal gonadal histology. However, no histomorphometry data (e.g., germ cell cyst composition) was performed in their study, and detailed analysis of the gonadal histology ( Song et al, 2021 ) showed a reduced testicular lumen and apparently less spermatozoa in the tshba−/− as compared to wild type ( tshba+/+ ) . Histomorphometric analysis of zebrafish tshba−/− testis should be investigated to address whether thyroid hormones are necessary for zebrafish spermatogenesis.…”

“…These data also reveal that normal thyroid hormone levels are fundamental to normal reproduction in zebrafish. On the other hand, Song et al (2021) have recently reported that knockout of tshba in zebrafish resulted in defective development of secondary sex characteristics, but did not affect the spermatogenesis, indicating that thyroid hormones may be not necessary to male germ cell development. According to our knowledge, we cannot compare the methimazole exposure with the knockout of tshba in zebrafish.…”

Characterization of the Interrenal Gland and Sexual Traits Development in cyp17a2-Deficient Zebrafish

“…The similar compensatory upregulation of pituitary gonadotropins, lhβ and fshβ , were observed in the cyp17a1 -/- and cyp19a1a -/- fish, as well as upregulated pituitary hormone, tshβ , in thyroglobulin -/- and monocarboxylate transporter 8 -/- fish. These suggest that the feedback mechanism exists in pituitary-gonad/interrenal/thyroid axis: when the effector organ is dysregulated in biosynthesis of sex steroids/corticosteroid/thyroxine, the pituitary hormones corresponding to the catalysate-deficiency would be upregulated due to feedback mechanism removal and usually accompanied with the enlargement of the gland ( 19 , 36 , 37 ).…”