Induction of cardiomyocytes by GATA4 in Xenopus ectodermal explants

The earliest step in heart formation in vertebrates occurs during gastrulation, when cardiac tissue is specified. Dorsoanterior endoderm is thought to provide a signal that induces adjacent mesodermal cells to adopt a cardiac fate. However, the nature of this signalling and the precise role of endoderm are unknown because of the close proximity and interdependence of mesoderm and endoderm during gastrulation. To better define the molecular events that underlie cardiac induction, we have sought to develop a simple means of inducing cardiac tissue. We show that the transcription factor GATA4, which has been implicated in regulating cardiac gene expression, is sufficient to induce cardiac differentiation in Xenopus embryonic ectoderm (animal pole) explants, frequently resulting in beating tissue. Lineage labelling experiments demonstrate that GATA4 can trigger cardiac differentiation not only in cells in which it is present, but also in neighbouring cells. Surprisingly, cardiac differentiation can occur without any stable differentiation of anterior endoderm and is in fact enhanced under conditions in which endoderm formation is inhibited. Remarkably, cardiac tissue is formed even when GATA4 activity is delayed until long after explants have commenced differentiation into epidermal tissue. These findings provide a simple assay system for cardiac induction that may allow elucidation of pathways leading to cardiac differentiation. Better knowledge of the pathways governing this process may help develop procedures for efficient generation of cardiomyocytes from pluripotent stem cells.