Transthyretin and thyroid hormone transport in fish and the effect of endocrine disruptors on this process

Transthyretin (TTR) is a homotetrameric protein that transports thyroid hormones (THs) thyroxine (T4) and triiodothyronine (T3) in vertebrate’s blood. TTR features raise important questions: understanding its structural/functional evolution in vertebrates; its ability to form amyloid fibrils causing human disease and TTR’s high affinity for endocrine disruptor chemicals (EDCs). In fish, TTR features are poorly explored. In this study, sea bream recombinant TTR (sbrTTR), specific anti-sbrTTR antisera and two sbrTTR N-terminal mutants were produced. A [ 125 I-T3] competitive binding assay demonstrated that sbrTTR binds T3 (Kd=10.6nM) and T4 (Kd=9.8nM) with a similar affinity and the N-terminus was found to influence binding to sbrTTR but also the formation of fibrils. TTR plasma levels, measured (3.8μg/ml) for the first time in fish using a specific ELISA, increased upon administration of THs, although hepatic gene expression was not modified suggesting THs regulate TTR secretion but not synthesis. Putative EDCs strongly displace in vitro sbrTTRTHs binding indicating the sbrTTR binding assay could be a useful risk assessment tool for thyroid axis disruption in fish. In vivo, TTR-binders ioxynil and diethylstilbestrol altered the thyroid axis in sea bream; brain TSH and deiodinases expression was downregulated although TH plasma levels and TTR hepatic expression remained unaltered. Key-words: Transthyretin, thyroid hormone binding proteins, thyroid hormones, teleost fish, thyroid system, endocrine disrupting chemicals. List of Publications and Communications Articles in refereed journals Morgado, I. , Santos, C.R. A. , Jacinto, R. , Power, D. M. 2007. Regulation of Transthyretin by thyroid hormones in fish. Gen Comp Endocrinol. 152, 189-197. Morgado, I. , Hamers, T. , Van der Ven, L. , Power, D.M. 2007. Disruption of thyroid hormone binding to sea bream recombinant Transthyretin by ioxynil and polybrominated diphenyl ethers. Chemosphere. 69, 155-163. Conference proceedings Morgado, I. , Anjos, L. , Power, D. M. 2005. Transthyretin and thyroid hormone transport in fish. Avances en Endocrinologia Comparada vol.II, 277-79. Morgado I. , Sauer-Eriksson, E. , Power, D. M. 2006. Thyroid hormone binding by recombinant sea bream transthyretin: The role of the N-terminal region Journal of experimental zoology. Part Acomparative experimental biology 305A (2): 158158 FEB 1 2006 Communications in conferences Morgado, I. , Power, D. M. Thyroid hormone binding by recombinant sea bream transthyretin: the role of the N-terminal region; Oral communication. 15th International Congress of Comparative Endocrinology, May 2005,Boston, USA. Morgado, I. , Power, D. M. Transthyretin and thyroid hormone transport in fish; Oral communication. V Mini-Symposium on Comparative and Molecular Endocrinology, Junho 2004, Universidade do Algarve. Morgado, I. , Anjos, L. , Power, D. M. Transthyretin (TTR) and thyroid hormone transport in fish-expression and purification of sea bream TTR; Poster communication 4o Congreso de la Asociacion Iberica de Endocrinologia Comparada, Setembro 2003, Universidad de Cordoba, Espanha. Morgado, I. , Power, D. M. Production, purification and thyroid hormone binding affinity of recombinant sea bream transthyretin; Poster communication. 5th International Symposium on Fish Endocrinology, September 2004, Castellon, Spain, Morgado, I. , Power, D. M. Endocrine disruption of thyroid hormone binding to recombinant sea bream transthyretin; Poster communication. V Congresso da Associacao Iberica de Endocrinologia Comparativa, September 2005, Faro, Portugal.