Isolation and characterisation of thyroid hormone-responsive genes of amphibian tail
Amphibian metamorphosis is a post-embryonic process that systematically transforms different tissues in a tadpole. This transformation requires extensive remodelling of almost every tissue in the animal. Thyroid hormone plays a causative role in this complex process by inducing a cascade of gene regulation. One of the more dramatic effects of thyroid hormone (triiodothyronine T 3) is to induce a complete regression of tadpole tail in culture in a simple chemically defined medium. The technique of differential display proposed by Liang and Pardee in 1992, has been applied in an attempt to isolate and then characterise responsive genes induced by thyroid hormone triiodothyronine (T3). Library screens using the PCR fragment xL52 as a probe allowed the isolation of -2.5kb clone termed xth-2. Sequence analysis and database searches at the amino acid level revealed that this clone (xth-2) showed approximately 91% identity to some of the members of a recently discovered family of tissue-specific transmembrane proteins called Hem proteins. Temporal expression of xth-2 using RT-PCR technique revealed that this gene is developmentally regulated. Whole mount in situ hybridisation used for detecting the location of this mRNA in Xenopus laevis embryos at different developmental stages indicated that xth-2 protein was highly expressed in the brain and the pattern of expression has been extended along the central nervous system (CNS) and the caudal region (tail bud). Expression of xth-2 protein in Xenopus embryos, did not show any significant effect on the phenotypic features of the embryos examined. The Nap1 protein, a member of Hem family proteins has recently been found to associate with the SH3 domain of Nck protein, and is thought to play an important role in signalling transduction. We could therefore, speculate that protein xth-2 will have the same function as does the Nap1 on the basis of their sequence similarity, tissue distribution and also the expression pattern.