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Divergent Expression of Type 2 Deiodinase and the Putative Thyroxine-Binding Protein p29, in Rat Brain, Suggests that They Are Functionally Unrelated Proteins

Instituto de Investigaciones Biomédicas "Alberto Sols," Consejo Superior de Investigaciones Científicas-Universidad Autónoma de Madrid, 28029 Madrid, Spain

Address all correspondence and requests for reprints to: Dr. Juan Bernal, Instituto de Investigaciones Biomédicas, Arturo Duperier 4, 28029 Madrid, Spain. E-mail: jbernal{at}iib.uam.es.

Deiodinases (D1, D2, and D3) are selenoproteins involved in thyroid hormone metabolism. Generation of the active hormone T₃, from T₄, is carried out by D1 and D2, whereas D3 degrades both hormones. The identity of the cloned D2 as a selenoprotein is well supported by biochemical and physiological data. However, an alternative view has proposed that type 2 deiodinase is a nonselenoprotein complex containing a putative T₄ binding subunit called p29, with an almost identity in sequence with the Dickkopf protein Dkk3.

To explore a possible functional relationship between p29 and D2, we have compared their mRNA expression patterns in the rat brain. In brain, parenchyma p29 was expressed in neurons. High expression levels were found in all the regions of the blood-cerebrospinal fluid (CSF) barrier. p29 was present in different types of cells than D2, with the exception of the tanycytes. Our data do not support that p29 has a functional relationship with D2. On the other hand, expression of p29 in the blood-CSF barrier suggests that it might be involved in T₄ transport to and from the CSF, but further studies are needed to substantiate this hypothesis.

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