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 gland

The endostyle is a pharyngeal organ of urochordates, cephalochordates, and primitive vertebrates. This organ has iodine-concentrati ng and iodine-meta bolism activities, and therefore the endostyle is considered to be homologo us to the follicle of the thyroid gland. In higher vertebrates the genes fo r both thyroid transcription factor-1 (TTF-1) and thyroid peroxidase (TPO) 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 are specifically expressed in the endostyle, but that the expression domains o f these genes are not overlapping, suggesting that ascidian TPO is not regu lated by TTF-1. To investigate the molecular mechanisms involved in the for mation and function of the endostyle, with special reference to the evoluti on 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. Reverse transcriptase-polymerase chain reaction/Southern blotting revealed that both amphioxus TTF-1 and TPO gene s are expressed mainly in the adult endostyle. Spatial and temporal express ion patterns assessed by in situ hybridization revealed that BbTTF-1 is exp ressed in the endodermal cells during early embryogenesis and is maintained in all zones of the adult endostyle. On the other hand, expression of BbTP O is chiefly in zones 5 and 6 of the adult endostyle where it overlaps with that of BbTTF-1, and to a lesser extent in zones and 3. This restriction o f the expression of BbTTF-1 and BbTPO to the endostyle strongly suggests th at the endostyle is homologous to the follicle of the thyroid gland. Moreov er, the spatial and temporal expression patterns of these genes suggest tha t TTF-1 regulates TPO expression. The coexpression of these genes in amphio xus suggests that regulation of TPO by TTF-1 was present in the common ance stor of cephalochordates (acraniates) and craniates.