Cardiac expression of the Drosophila Sulphonylurea receptor gene is regulated by an intron enhancer dependent upon the NK homeodomain factor Tinman

Cardiac development proceeds via the activation of a complex network of regulatory factors which both directly and indirectly impact downstream cardiac structural genes. In Drosophila, the NK homeodomain transcription factor Tinman is critical to cardiac specification and development via the activation of a number of key regulatory genes which mediate heart development. In this manuscript, we demonstrate that Tinman also functions in Drosophila to directly activate transcription of the ATP binding cassette gene Sulphonylurea receptor (Sur). Cardiac expression of Sur is regulated by Tinman via an intron enhancer which first becomes active at stage 12 of embryogenesis, and whose function is restricted to the Tin cardial cells by the end of embryogenesis. Cardiac Sur enhancer activity subsequently persists through larval and adult development, but interestingly becomes modulated in several unique subsets of Tin-expressing cardial cells. The cardiac enhancer contains four binding sites for Tinman protein; mutation of two of these sites significantly reduces enhancer activity at all stages of development, and activation of the wild-type enhancer by ectopic Tinman protein confirms Sur is a direct target of Tinman transcriptional activation. These findings delineate at the molecular level specific sub-types of Tin cardial cells, and define an important regulatory pathway between two Drosophila genes for which mutations in human homologs have been shown to result in cardiac disease.