Early activation of FGF and nodal pathways mediates cardiac specification independently of Wnt/beta-catenin signaling.
BACKGROUND
Cardiac induction, the first step in heart development in vertebrate embryos, is thought to be initiated by anterior endoderm during gastrulation, but what the signals are and how they act is unknown. Several signaling pathways, including FGF, Nodal, BMP and Wnt have been implicated in cardiac specification, in both gain- and loss-of-function experiments. However, as these pathways regulate germ layer formation and patterning, their specific roles in cardiac induction have been difficult to define.


RESULTS
To investigate the mechanisms of cardiac induction directly we devised an assay based on conjugates of anterior endoderm from early gastrula stage Xenopus embryos as the inducing tissue and pluripotent ectodermal explants as the responding tissue. We show that the anterior endoderm produces a specific signal, as skeletal muscle is not induced. Cardiac inducing signal needs up to two hours of interaction with the responding tissue to produce an effect. While we found that the BMP pathway was not necessary, our results demonstrate that the FGF and Nodal pathways are essential for cardiogenesis. They were required only during the first hour of cardiogenesis, while sustained activation of ERK was required for at least four hours. Our results also show that transient early activation of the Wnt/beta-catenin pathway has no effect on cardiogenesis, while later activation of the pathway antagonizes cardiac differentiation.


CONCLUSIONS
We have described an assay for investigating the mechanisms of cardiac induction by anterior endoderm. The assay was used to provide evidence for a direct, early and transient requirement of FGF and Nodal pathways. In addition, we demonstrate that Wnt/beta-catenin pathway plays no direct role in vertebrate cardiac specification, but needs to be suppressed just prior to differentiation.
Samuel
Lee J
LJ
School of Biosciences, Cardiff University, Cardiff, United Kingdom.
Latinkić
Branko V
BV
D000818:Animals; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D002454:Cell Differentiation; D019070:Cell Lineage; D004707:Endoderm / Q000378:metabolism; D005346:Fibroblast Growth Factors / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D009206:Myocardium / Q000378:metabolism*; D055457:Nodal Protein / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D051153:Wnt Proteins / Q000378:metabolism*; D014981:Xenopus; D051176:beta Catenin / Q000378:metabolism*
D019485:Bone Morphogenetic Proteins; D055457:Nodal Protein; D051153:Wnt Proteins; D051176:beta Catenin; D005346:Fibroblast Growth Factors