Meeting Abstract

S5.2  Monday, Jan. 5  Thyroid hormones and iodine in the evolutionary history of cell-cell signalling CROCKFORD, S.J.; University of Victoria, BC scrock@uvic.ca

In vertebrates, thyroid hormones (THs, thyroxine and triiodothyronine) are critical cell signalling molecules. THs regulate and coordinate physiology within and between cells, tissues and whole organisms, in addition to controlling embryonic growth and development, via dose-dependent regulatory effects on essential genes. While invertebrates and plants do not have thyroid glands, many utilize THs for development, while others store iodine as TH precursor molecules (mono- and di-iodotyrosine, MIT and DIT) or produce similar hormones that act in analogous ways. Such common roles for THs and iodotyrosines across kingdoms suggest that a common endocrine signalling mechanism may account for coordinated evolutionary change in all multi-cellular organisms. Here I expand my earlier hypothesis for the role of THs in vertebrate evolution (Crockford 2006) by proposing a critical evolutionary role for iodine, the essential ingredient in all THs. Iodine is crucial to life for virtually all unicellular organisms (including evolutionarily-ancient cyanobacteria), in part because it acts as a powerful antioxidant that protects cells from the chemically disruptive effects of oxygen. I propose that during the evolution of early cells, the ease with which iodine reacts with simple biological compounds explains why iodine became the antioxidant of choice. Transferred within cells as a consequence of reactions at the cell wall, iodine became incorporated into basic internal biochemical reactions, including those involved in metabolism and mtDNA replication. The coupling of iodine with the amino acid tyrosine was a critical step in the evolution of complex cell-cell signalling, as MIT and DIT eventually became components of ubiquitous signalling molecules for communicating within and between cells, tissues and organs, and for coordinating whole body physiology.