Title OVERLAPPING EXPRESSION OF AMPHIOXUS HOMOLOGS OF THE THYROID TRANSCRIPTION FACTOR-1 GENE AND THYROID PEROXIDASE GENE IN THE ENDOSTYLE: INSIGHT INTO EVOLUTION OF THE THYROID

The endostyle is a pharyngeal organ of urochordates, cephalochordates and primitive vertebrates. This organ has iodine concentrating and iodinemetabolism activities, and therefore the endostyle is considered to be homologous to the follicle of the thyroid gland. In higher vertebrates, both the TTF-1 (thyroid transcription factor-1) and TPO (thyroid peroxidase) genes are expressed in the thyroid gland follicle. TTF-1 regulates the expression of TPO, which encodes an iodinating enzyme associated with thyroid hormone synthesis. A recent study showed that the ascidian TTF-1 and TPO genes were specifically expressed in the endostyle, but that the expression domains of these genes were not overlapping, suggesting that ascidian TPO is not regulated by TTF-1. To investigate the molecular mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the follicle of the thyroid gland, I isolated and characterized cDNA clones for the amphioxus homologs of the TTF-1 gene (BbTTF-1) and TPO gene (BbTPO) from Branchiostoma belcheri. RT-PCR/Southern blotting revealed that both amphioxus TTF-1 and TPO were mainly expressed in the adult endostyle. Spatial and temporal expression patterns assessed by in situ hybridization revealed that expression of BbTTF-1 occurred in the endodermal cells during early embryogenesis, and was maintained in all zones of the adult endostyle: On the other hand, expression of BbTPO was chiefly in zones 5 and 6 of the adult endostyle where it overlapped with that of BbTTF-1, and to a lesser extent in zones 1 and 3. This restriction of the expression of BbTTF-1 and BbTPO to the endostyle strongly suggests that the endostyle is homologous to the follicle of the thyroid gland. Moreover, the spatial and temporal expression patterns of these genes suggest that TTF-1 may regulate TPO expression. The co-expression of these genes in amphioxus suggests that regulation of TPO by TTF-1 was present in the common ancestor of cephalochordates (acraniates) and craniates.