Functional Characterization of the TBX/bHLH Transcription Factor MGA During Embryogenesis
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This thesis analyzes the transcription factor mga during development using the zebrafish model. The mga gene encodes a unique transcription factor containing both TBOX and bHLHzip DNA-binding domains, which are both conserved with the mammalian homologs. The mga gene is expressed maternally, and subsequently in the developing brain, heart, and gut, and its depletion causes morphogenetic defects in each of these organ systems. The heart and gut phenotypes are similar to those described previously for loss of gata4, and the mga morphant shows increased levels of gata4 transcripts in lateral mesoderm. Knockdown of gata4 rescues the severe cardiac morphogenetic defect (but not the gut defect), indicating that mga restricts the normal levels of gata4 required for heart tube looping, while both genes are important for gut development. Transcript profiling experiments show that mga functions early to influence key regulators of mesendoderm, including tbx6, cas, and sox17. We also found that mga is co-expressed in the mesendoderm during early embryogenesis with tbx5, another member of the T-box transcription factor family. The simultaneous knockdown of both mga and tbx5 using specific morpholinos generates phenotypes in the yolk syncytial layer and endoderm that are not seen when either gene product is individually depleted. Together, the two genes interact with the nodal pathway to regulate endoderm development. We show that mga and tbx5 regulate transcript levels of gata5, foxa2 and sox17 during endoderm specification, and are then necessary to restrict the levels of casanova and sox17 when endoderm normally differentiates. Therefore, the mga + tbx5 double morphants fail to form a gut tube or derivative organs. We show that tbx5 has a previously unrecognized role in endoderm development, which is only revealed in the context of depletion for the functionally redundant sister gene mga.
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