Thyroid hormones (THs), thyroxine (T4), and triiodothyronine (T3), regulate growth, metabolism, and neurodevelopment. THs secretion is controlled by the pituitary thyroid-stimulating hormone (TSH) and the hypothalamic–pituitary–thyroid (HPT) axis. The organic anion-transporting polypeptide 1C1 (OATP1C1/SLCO1C1) and the monocarboxylate transporter 8 (MCT8/SLC16A2) actively transport THs, which bind to their nuclear receptors and induce gene expression. A mutation in OATP1C1 is associated with brain hypometabolism, gradual neurodegeneration, and impaired cognitive and motor functioning in adolescent patients. To understand the role of Oatp1c1 and the mechanisms of the disease, we profiled the transcriptome of oatp1c1 mutant (oatp1c1-/-) and mct8-/-xoatp1c1-/- adult male and female zebrafish brains. Among dozens of differentially expressed genes, agouti-related neuropeptide 1 (agrp1) expression increased in oatp1c1-/- adult brains. Imaging in the hypothalamus revealed enhanced proliferation of Agrp1 neurons in oatp1c1-/- larvae and adults, and increased food consumption in oatp1c1-/- larvae. Similarly, feeding and the number of Agrp1 neurons increased in thyroid gland-ablated zebrafish. Pharmacological treatments showed that the T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid), but not T4, normalized the number of Agrp1 neurons in oatp1c1-/- zebrafish. Since the HPT axis is hyperactive in the oatp1c1-/- brain, we used the CRISPR-Cas9 system to knockdown tsh in oatp1c1-/- larvae, and inducibly enhanced the HPT axis in wild-type larvae. These manipulations showed that Tsh promotes proliferation of Agrp1 neurons and increases food consumption in zebrafish. The results revealed upregulation of both the HPT axis–Agrp1 circuitry and feeding in a zebrafish model for OATP1C1 deficiency.
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