Deiodinase activity is present in Xenopus laevis during early embryogenesis

Thyroid hormones orchestrate amphibian metamorphosis. The type 2 and type 3 deiodinases make vital contributions to this process by controlling levels of the thyroid hormones T-4 and T-3 available to different tissues. Because the tadpole thyroid gland is not functional until stage NF44, it has been widely assumed that thyroid signaling is absent during amphibian early development, thyroid hormone only becoming a major regulator during premetamorphic stages. Similarly, in mammals, thyroid function is known to be essential to neuronal development, especially during the perinatal stages, but again little is known about early stages of development. Here we demonstrate that key elements of thyroid hormone signaling are present during early development of Xenopus. In particular, we find functional thyroid hormone-activating deiodinases and significant levels of their substrates, T-4 and T-3, during early embryogenesis. Furthermore, we have further characterized a recently identified deiodinase in amphibians, homologous to mammalian type 1 deiodinase (D1). This enzyme is expressed in marked, spatially defined patterns during embryogenesis. The patterns of expression of type 1 deiodinase are distinct from those of type 2 and type 3 deiodinases. Deiodinase expression is found in neurogenic areas from stage NF30 onward, both in the central and peripheral nervous systems. We conclude that both activating and inactivating deiodinases show dynamic patterns of expression during early embryogenesis in amphibians, particularly in neurogenic areas. These findings suggest that thyroid hormone signaling is a key component of early neuronal development in vertebrates.