Expression of Nkx2.5 in wild type, cardiac mutant, and thyroxine-induced metamorphosed hearts of the Mexican axolotl.

Nkx2.5, a homeodomain-containing transcription factor, is known to be necessary for normal heart development in vertebrates. It is one of the earliest lineage-restricted genes expressed in cardiovascular progenitor cells and knowledge of its expression patterns has important therapeutic implications for damaged cardiomyocytes. Mexican axolotl is a unique system to study heart development for two reasons: the presence of a mutant phenotype lacking organized myofibrils due to sarcomeric tropomyosin deficiency and the ability to induce metamorphosis by administration of exogenous thyroid hormone. In this study, we cloned and sequenced the as yet uncharacterized Nkx2.5 cDNA from normal and cardiac mutant axolotl heart RNA. Comparison of cDNA sequences of Nkx2.5 from normal and mutant axolotl hearts did not show differences suggesting that loss of function mutation in Nkx2.5 is not responsible for the mutant phenotype. However, quantitative studies show higher expression of Nkx2.5 in mutant hearts raising the possibility that increased expression of Nkx2.5 may contribute to the mutant phenotype. We also evaluated quantitative changes in expression of Nkx2.5 in axolotl hearts during embryonic and postembryonic heart development induced by exogenous thyroid hormone. There is an apparent increase in Nkx2.5 transcript levels in metamorphosed hearts.