Hormonas Tiroideas y Cerebro. Notas Sobre La Relación Bocio y ...

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7. Bianco AC and Kim BW. 2006. Deiodinases: implications of the local control of thyroid hormone action. J. Clin. Invest. 116:2571–2579. 8. Schimmel M. and Utiger R.D. 1977. Thyroidal and peripheral production of thyroid hormones. Review of recent findings and their clinical implications. Ann. Intern. Med. 87:760–768. 9. Zavacki A.M., et al. 2005. Type 1 iodothyronine deiodinase is a sensitive marker of peripheral thyroid status in the mouse. Endocrinology. 146:1568–1575. 10. Bianco A.C. and <strong>La</strong>rsen P.R. 2005. Cellular and structural biology of the deiodinases. Thyroid. 15:777–786. 11. Galton V.A. 2005. The roles of the iodothyronine deiodinases in mammalian development. Thyroid. 15:823– 834. 12. St. Germain D.L., Hernandez A., Schneider M.J., Galton V.A. 2005. Insights into the role of deiodinases from studies of genetically modified animals. Thyroid. 15:905–916. 13. Hernandez A., Martinez M.E., Fiering S., Galton V.A., St. Germain D. 2006. Type 3 deiodinase is critical for the maturation and function of the thyroid axis. J. Clin. Invest. 116:476–484. 20
The Type 3 Deiodinase (D3) and the Central Nervous System Arturo Hernandez Department of Medicine, Dartmouth Medical School, Lebanon, New Hampshire, USA Importance of Thyroid Hormone in Brain Development Thyroid hormones (TH) have important biological roles in mammals, as they affect many physiological functions including growth, development and metabolism. In particular, it is in the central nervous system (CNS) where their developmental impact is most dramatic (1). A lack of TH during development produces in humans the syndrome of cretinism, which is characterized by severe abnormalities in brain function including ataxia and poor motor skills, severe mental retardation and sensory deficits (1). Similar consequences of congenital hypothyroidism have been observed in rodents. Studies in animal models have shown that TH regulate cell proliferation, differentiation and migration in the developing brain, as well as affect important neural processes as myelinization and synaptic plasticity (2). Although the molecular mechanisms underlying the effects of TH in the CNS have not been fully delineated, a number of TH-regulated genes have been identified in the developing and adult brain (3, 4) and it is likely that the abnormal expression of some of these genes or others yet to be discovered mediates the detrimental effects of the lack of TH. Local Regulation of Thyroid Hormone Availability: the Type 3 Deiodinase (D3) The availability of TH to brain cells is regulated by several factors including i) the concentrations of TH in the serum; ii) the MCT8 TH transporter; and iii) the activities of the deiodinase enzymes. Serum TH levels depend primarily on the function of the thyroid axis. Thus, TH are secreted to the plasma by the thyroid gland in response to stimulation by thyrotropin stimulating hormone (TSH). TSH is produced and secreted by the anterior pituitary in response to thyrotropin releasing hormone (TRH) produced in the hypothalamus. To close the regulatory loop, serum TH have a negative feedback effect on the production of TRH and TSH at the pituitary and hypothalamic levels. The MCT8 transporter is highly expressed in the brain, in which it can specifically and efficiently transport both TH into neurons. The lack of a functional MCT8 protein in humans has been shown to have extremely severe consequences for the CNS (5). Finally, the three types of deioidinase enzymes,termed D1, D2 and D3, are able to remove iodine atoms from different positions of the chemical structure of TH resulting in either the activation or inactivation of TH action (6). In particular the type 3 deiodinase (D3) is exclusively an inactivating enzyme (7), as it converts both the pro-hormone T4 and the active hormone T3 into inactive metabolites (Fig. 1). 21
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