The adult epidermis of the frog, Xenopus laevis, is formed at metamorphosis. During this period the tadpole's two cell layered epidermis is replaced by a stratified keratinized epithelium typically found in the adults of most vertebrates. The formation of the adult skin, including the appearance of adult-specific keratins, is controlled by the thyroid gland hormone T3. We have found that the activation of keratin gene expression during Xenopus development proceeds by a two step process. The first step occurs at stage 48 and results in a low level of 63kd keratin synthesis. This step does not require T3 nor can it be precociously induced by T3 in vivo or in vitro. The second step is the high level induction of the 63kd keratin genes by T3. We have proposed that T3 exerts its control of 63kd keratin gene expression indirectly, possibly via the production of a regulatory factor, rather than interacting directly with cis-acting control elements of the 63kd keratin gene. The major goals of the experiments described here are to test the validity of these proposals and to investigate the physiological and molecular mechanisms involved in the conversion of the larval program of differentiation to the adult program of differentiation. We will study the role of the dermis in establishing the competence of the epidermis to respond to T3 and the initial events which occur in the epidermis after T3 treatment. Then we will isolate genomic clones coding for a 63kd keratin gene and use them for transfection experiments which will allow us to identify cis-acting regulatory elements which mediate the two inducible steps involved in the determination and differentiation of the epidermis. Our studies are important because they will provide insights for the mechanism of action of embryonic inducers and the developmental role of T3 in human development. Furthermore they will help us understand the mechanism of gene action during development, the mechanism of T3-action, the control of keratin gene expression and the role of regulatory genes is the development of the epidermis. Future studies can then be designed to clone these regulatory genes and study their regulation. Since these factors may be similar to human regulatory proteins our studies may be important for understanding the mechanism by which the human embryonic epidermis acquires its adult program of terminal differentiation. Thus, our studies may provide insights for understanding abnormalities seen in various congenital human skin diseases.
