Endocardial differentiation in zebrafish occurs during early somitogenesis and is dependent on BMP and etv2 signalling

The endocardium and adjacent vascular endothelial network share a number of molecular markers however there are distinct physiological functions of these tissues. What distinguishes these lineages on a molecular level remains an important, unanswered question in cardiovascular biology. We have identified the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of early endocardial development and used this line to examine endocardial differentiation. Our results show that the endocardium emerges from the anterior lateral plate mesoderm at the 8-somite stage (13 hpf). Analysis in a number of loss-of-function models showed that whilst nkx2.5, hand2 and tal1 loss-of-function have no effect on the endocardial progenitor domain, both etv2 loss-of-function and inhibition of BMP signalling reduce the endocardial domain. Furthermore, manipulating BMP signalling alters etv2 expression. Together, these results describe the onset of endocardial molecular identity and suggest a signalling cascade whereby BMP signalling acts upstream of etv2 to direct differentiation of endocardial progenitors.

Hippo signaling determines the number of venous pole cells that originate from the anterior lateral plate mesoderm in zebrafish

The differentiation of the lateral plate mesoderm cells into heart field cells constitutes a critical step in the development of cardiac tissue and the genesis of functional cardiomyocytes. Hippo signaling controls cardiomyocyte proliferation, but the role of Hippo signaling during early cardiogenesis remains unclear. Here, we show that Hippo signaling regulates atrial cell number by specifying the developmental potential of cells within the anterior lateral plate mesoderm (ALPM), which are incorporated into the venous pole of the heart tube and ultimately into the atrium of the heart. We demonstrate that Hippo signaling acts through large tumor suppressor kinase 1/2 to modulate BMP signaling and the expression of hand2, a key transcription factor that is involved in the differentiation of atrial cardiomyocytes. Collectively, these results demonstrate that Hippo signaling defines venous pole cardiomyocyte number by modulating both the number and the identity of the ALPM cells that will populate the atrium of the heart.