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FIG. 6. Based on the data presented, we hypothesize that the levels or boundaries of GATA-4/5/6 expression delineate the region of the heart field that will express Nkx2 genes and be competent for cardiomyocyte development. When GATA factors are overexpressed, cardiomyocyte differentiation can be repressed. Inhibition of GATA-dependent gene expression results in an expanded pattern of Nkx2.5 transcripts, perhaps providing competence to a larger part of the heart field to contribute to the cardiomyocyte population. The model is consistent with additional reports in the literature demonstrating the ability of enhanced levels of GATA-6 (Gove et al., 1997) or Nkx2.5 (Cleaver et al., 1996) to inhibit or expand, respectively, the population of differentiated cardiomyocytes (although note that this is a relatively late effect and neither GATA-4 or GATA-6 overexpression alone is reported to alter Nkx2.5 transcript patterns). The domain of Nkx2 expression in the pharyngeal endoderm is not indicated in the schematic, although we note that RA does not activate GATA expression in the pharyngeal endoderm and also does inhibit Nkx2.5 transcription in this region. Image published in: Jiang Y et al. (1999) Copyright © 1999. Image reproduced with permission of the Publisher, Elsevier B. V. Larger Image Printer Friendly View

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