Possible functional origin of TPO-like genes in chordates revealed by their pleiotropic expression and disturbed phenomena in protochordates.

Mammalian thyroid peroxidase (TPO), a thyroid-specific peroxidase involved in thyroid hormone synthesis, has a characteristic hydrophobic region in the C-terminus (HRC) that anchors peroxidase to the apical membrane of epithelial follicle cells. Protochordates (lancelets and ascidians) have TPO-like (TPOL) peroxidases with or without HRC, and their expression has been reported in the endostyle. We herein investigated the molecular structures of TPOL in protochordates and TPO relatives in vertebrates, including eosinophil peroxidase, myeloperoxidase, and lactoperoxidase, with a focus on the existence of HRC. Our molecular phylogenetic analysis implied that ancestral chordates evolved HRC-containing peroxidases, and mammalian TPO maintained HRC despite many TPO relatives losing it. Gene expression profiles revealed by in situ hybridization and an RNA-seq analysis showed that transcripts of TPOL genes in protochordates were distributed to pharyngeal epithelia in addition to the endostyle. Furthermore, disturbances in TPO activity by a thiourea treatment and gene knockdown experiments resulted in a deficiency of pharyngeal mucus-sheets. Collectively, these results suggest that HRC-containing TPOL was originally involved in pharyngeal mucus-sheet formation for suspension feeding in ancestral chordates, and HRC may have evolved to maintain peroxidase anchoring to the apical surface of pharyngeal epithelia.