THE EXPRESSION PATTERN OF THYROID-HORMONE RESPONSE GENES IN REMODELING TADPOLE TISSUES DEFINES DISTINCT GROWTH AND RESORPTION GENE-EXPRESSION PROGRAMS

Thyroid hormone (TH) induces dramatic skeletal and tissue remodeling o f the anuran head and body at metamorphosis. The expression pattern of TH up-regulated genes has been correlated with tissues that either gr ow or resorb at metamorphosis. Whereas the expression of the thyroid h ormone receptors in Xenopus laevis tadpoles is ubiquitous, the locatio ns where many of the TH up-regulated genes are activated fall into dis tinct classes. Genes in the early response class are expressed predomi nantly in cartilage and brain regions undergoing cell proliferation an d at a higher level in the remodeling and growing body than in the res orbing tail. In contrast, expression of genes in the delayed response class is highest in resorbing tissues and higher in the tail than in t he body within the subepidermal fibroblast layer, further indicating t hat this single cell layer is involved in tissue resorption. The expre ssion boundary of delayed response class genes in the subepidermal fib roblasts in the body correlates with epidermal lamella invasion and su bsequent adult skin differentiation. Differences in the expression pat terns of stromelysin-3 and the delayed response proteinases in the hea d delineate separate programs of tissue resorption, one for the loss o f epithelial structures, and one for the loss of cartilages. Expressio n of the type III deiodinase is up-regulated in growing tissues nearin g completion of their metamorphic changes, suggesting a role for the d eiodinase in modulating the influence of TH on these tissues. (C) 1998 Academic Press.