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Thyroid Hormone Metabolism Defects in a Mouse Model of SBP2 Deficiency.

ABSTRACT: Selenocysteine insertion sequence binding protein 2 (SBP2) is an essential factor in selenoprotein synthesis. Patients with SBP2 defects have a characteristic thyroid phenotype and additional manifestations such as growth delay, male infertility, impaired motor coordination, and developmental delay. The thyroid phenotype has become pathognomonic for this defect, and putative deficiencies in the iodothyronine deiodinases selenoenzymes have been implicated. To investigate the role of SBP2 and selenoproteins in thyroid physiology and answer questions raised by the human syndrome, we generated a tamoxifen-inducible Sbp2 conditional knockout (iCKO) mouse model. These Sbp2-deficient mice have high serum thyroxine (T4), thyrotropin, and reverse triiodothyronine (T3), similar to the human phenotype of SBP2 deficiency, whereas serum T3 is normal. Their liver T4 and T3 content reflect the serum levels, and deiodinase 1 expression and enzymatic activity were decreased. In contrast, brain T3 content is decreased, indicative of local hypothyroidism, confirmed by the decreased expression of the thyroid hormone (TH) positively regulated gene hairless. Interestingly, the cerebrum T4 content did not parallel the high serum T4 levels, and the expression of TH transporters was decreased. Deiodinase 2 enzymatic activity and deiodinase 3 expression were decreased in cerebrum. The expression and/or activity of other selenoproteins were decreased in brain, liver, and serum, thus demonstrating a global deficiency in selenoprotein synthesis. Sbp2 iCKO mice replicate the thyroid phenotype of SBP2 deficiency and represent an important tool to advance our understanding of the role of SBP2 in thyroid homeostasis and for investigating selenoprotein biology relevant to human disease.

SUBMITTER: Fu J

PROVIDER: S-EPMC5711384 | biostudies-literature | 2017 Dec

REPOSITORIES: biostudies-literature

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Thyroid Hormone Metabolism Defects in a Mouse Model of SBP2 Deficiency.

Fu Jiao J Fujisawa Haruki H Follman Benjamin B Liao Xiao-Hui XH Dumitrescu Alexandra M AM

Endocrinology 20171201 12

Selenocysteine insertion sequence binding protein 2 (SBP2) is an essential factor in selenoprotein synthesis. Patients with SBP2 defects have a characteristic thyroid phenotype and additional manifestations such as growth delay, male infertility, impaired motor coordination, and developmental delay. The thyroid phenotype has become pathognomonic for this defect, and putative deficiencies in the iodothyronine deiodinases selenoenzymes have been implicated. To investigate the role of SBP2 and sele ...[more]

PMID: 29029094

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Project description:Thyroid cancers are the most common malignancy of the endocrine system in humans. To understand the molecular genetic events underlying thyroid carcinogenesis, we have generated a mouse model that spontaneously develops follicular thyroid carcinoma similar to human thyroid cancer (Thrb(PV/PV) mouse). This mutant mouse harbors a dominant-negative mutated thyroid hormone receptor ? (denoted PV). The PV mutation was identified in a patient with resistance to thyroid hormone (TH). Thrb(PV/PV) mice exhibit highly elevated serum thyroid-stimulating hormone levels and increased TH. We have previously shown that thyroid-stimulating hormone is required, but not sufficient to induce metastatic follicular thyroid cancer in Thrb(PV/PV) mice. However, whether the elevated TH also contributes to the thyroid carcinogenesis of Thrb(PV/PV) mice was not elucidated. To understand the role of TH in thyroid carcinogenesis, we blocked the production of TH by treating Thrb(PV/PV) mice with propylthiouracil (Thrb(PV/PV)-PTU mice) and compared the development of thyroid cancer in Thrb(PV/PV)-PTU and untreated Thrb(PV/PV) mice. We found that thyroid tumor growth was reduced by ?42% in Thrb(PV/PV)-PTU mice as compared with Thrb(PV/PV) mice. Analysis by bromodeoxyuridine-nuclear labeling showed decreased incorporation of bromodeoxyuridine in thyroid tumor cells of Thrb(PV/PV)-PTU mice, indicative of decreased tumor cell proliferation. However, cleaved-caspase 3 staining showed no apparent changes in apoptosis of tumor cells in Thrb(PV/PV)-PTU mice. Molecular studies identified a marked attenuation of the PI3K-AKT-?-catenin signaling pathway that led to decreased protein levels of cyclin D2, thereby decreasing tumor cell proliferation in Thrb(PV/PV)-PTU mice. Furthermore, matrix metalloproteinase-2, a downstream target of ?-catenin and a key regulator during tumor invasion and metastasis, was also decreased. Thus, the present study uncovers a critical role of TH in promoting the thyroid carcinogenesis of Thrb(PV/PV) mice via membrane signaling events. Importantly, these findings suggest that anti-thyroid drugs could be considered as possible therapeutic agents of thyroid cancer.

Project description:Background: Mutations of the thyroid hormone receptor α (THRA) gene cause resistance to thyroid hormone (RTHα). RTHα patients exhibit very mild abnormal thyroid function test results (serum triiodothyronine can be high-normal to high; thyroxine normal to low; thyrotropin is normal or mildly raised) but manifest hypothyroid symptoms with growth retardation, delayed bone development, and anemia. Much has been learned about the in vivo molecular actions in TRα1 mutants affecting abnormal growth, bone development, and anemia by using a mouse model of RTHα (Thra1PV/+ mice). However, it is not clear whether TRα1 mutants affect lymphopoiesis in RTHα patients. The present study addressed the question of whether TRα1 mutants could cause defective lymphopoiesis. Methods: We assessed lymphocyte abundance in the peripheral circulation and in the lymphoid organs of Thra1PV/+ mice. We evaluated the effect of thyroid hormone on B cell development in the bone and spleen of these mice. We identified key transcription factors that are directly regulated by TRα1 in the regulation of B cell development. Results: Compared with wild-type mice, a significant reduction in B cells, but not in T cells, was detected in the peripheral circulation, bone marrow, and spleen of Thra1PV/+ mice. The expression of key transcription regulators of B cell development, such as Ebf1, Tcf3, and Pax5, was significantly decreased in the bone marrow and spleen of Thra1PV/+ mice. We further elucidated that the Ebf1 gene, essential for lineage specification in the early B cell development, was directly regulated by TRα1. Thus, mutations of TRα1 could impair B cell development in the bone marrow via suppression of key regulators of B lymphopoiesis. Conclusions: Analysis of lymphopoiesis in a mouse model of RTHα showed that B cell lymphopoiesis was suppressed by TRα1 mutations. The suppressed development of B cells was, at least in part, via inhibition of the expression of key regulators, Ebf1, Tcf3, and Pax5, by TRα1 mutations. These findings suggest that the mutations of the THRA gene in patients could lead to B cell deficiency.

Project description:Background: Mutations of thyroid hormone receptor α1 (TRα1) cause resistance to thyroid hormone (RTHα). Patients exhibit growth retardation, delayed bone development, anemia, and bradycardia. By using mouse models of RTHα, much has been learned about the molecular actions of TRα1 mutants that underlie these abnormalities in adults. Using zebrafish models of RTHα that we have recently created, we aimed to understand how TRα1 mutants affect the heart function during this period. Methods: In contrast to human and mice, the thra gene is duplicated, thraa and thrab, in zebrafish. Using CRISPR/Cas9-mediated targeted mutagenesis, we created C-terminal mutations in each of two duplicated thra genes in zebrafish (thraa 8-bp insertion or thrab 1-bp insertion mutations). We recently showed that these mutant fish faithfully recapitulated growth retardation as found in patients and thra mutant mice. In the present study, we used histological analysis, gene expression profiles, confocal fluorescence, and transmission electron microscopy (TEM) to comprehensively analyze the phenotypic characteristics of mutant fish heart during development. Results: We found both a dilated atrium and an abnormally shaped ventricle in adult mutant fish. The retention of red blood cells in the two abnormal heart chambers, and the decreased circulating blood speed and reduced expression of contractile genes indicated weakened contractility in the heart of mutant fish. These abnormalities were detected in mutant fish as early as 35 days postfertilization (juveniles). Furthermore, the expression of genes associated with the sarcomere assembly was suppressed in the heart of mutant fish, resulting in abnormalities of sarcomere organization as revealed by TEM, suggesting that the abnormal sarcomere organization could underlie the bradycardia exhibited in mutant fish. Conclusions: Using a zebrafish model of RTHα, the present study demonstrated for the first time that TRα1 mutants could act to cause abnormal heart structure, weaken contractility, and disrupt sarcomere organization that affect heart functions. These findings provide new insights into the bradycardia found in RTHα patients.

Dataset Information

Thyroid Hormone Metabolism Defects in a Mouse Model of SBP2 Deficiency.

Publications

Thyroid Hormone Metabolism Defects in a Mouse Model of SBP2 Deficiency.

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