TheDrosophilacardiogenic transcription factors Myocyte enhancer factor-2 and Tinman contribute to heart lumen enlargement through direct activation of the collagen geneMultiplexin

During embryogenesis, the Drosophila heart forms a lumen, the posterior region of which is increased in diameter and corresponds to the heart proper. To identify the transcriptional control of this morphogenetic process, we analyzed the formation and enlargement of the heart lumen in mutants for the myogenic transcription factor gene Myocyte enhancer factor-2 ( Mef2 ). We found that Mef2 contributes to both lumen formation and lumen expansion, the latter through a requirement for both Mef2 and the cardiogenic gene tinman ( tin ) to activate the collagen gene Multiplexin ( Mp ). To determine if Tin and MEF2 act directly upon the Mp gene, we identified an enhancer whose activity recapitulates the cardiac expression of Mp . This enhancer contains binding sites for both Tin and MEF2 and is activated in tissue culture by MEF2 but not Tin. We did not observe synergistic activation of the enhancer when the factors were in combination, despite documenting a direct physical interaction between Tin and MEF2 in vitro. In vivo, the Tin sites are required for normal enhancer activity, whereas mutation of the MEF2 sites results in expanded expression of an enhancer- lacZ reporter, suggesting that transcriptional repression may also contribute to regulation of Mp . Our studies underline how transcription factors must utilize combinatorial interactions to achieve organ-specific and region-specific patterns of gene expression and cell morphogenesis.