Myocardium and BMP signaling are required for endocardial differentiation

Abstract

Endocardial and myocardial progenitors originate in distinct regions of the anterior lateral plate mesoderm and migrate to the midline where they coalesce to form the cardiac tube. Endocardial progenitors acquire a molecular identity distinct from other vascular endothelial cells and initiate expression of specific genes such as nfatc1. Yet the molecular pathways and tissue interactions involved in establishing endocardial identity are poorly understood. The endocardium develops in tight association with cardiomyocytes. To test for a potential role of the myocardium in endocardial morphogenesis, we used two different zebrafish models deficient in cardiomyocytes: the hand2 mutant and a myocardial-specific genetic ablation method. We show that in hand2 mutants endocardial progenitors migrate to the midline but fail to assemble into a cardiac cone and do not express markers of differentiated endocardium. Endocardial differentiation defects were rescued by myocardial but not endocardial-specific expression of hand2. In metronidazole-treated myl7:nitroreductase embryos, myocardial cells were targeted for apoptosis, which resulted in the loss of endocardial nfatc1 expression. However, endocardial cells were present and retained expression of general vascular endothelial markers. We further identified bone morphogenetic protein (BMP) as a candidate myocardium-derived signal required for endocardial differentiation. Chemical and genetic inhibition of BMP signaling at the tailbud stage resulted in severe inhibition of endocardial differentiation while there was little effect on myocardial development. Heat-shock-induced bmp2b expression rescued endocardial nfatc1 expression in hand2 mutants and in myocardiumdepleted embryos. Our results indicate that the myocardium is crucial for endocardial morphogenesis and differentiation, and identify BMP as a signal involved in endocardial differentiation.

Myocardial differentiation is dependent upon endocardial signaling during early cardiogenesis in vitro

“…Data from this study also imply that interactions between endocardial and myocardial cells during early cardiomyogenesis involve Bmp signaling. However, our results differ from those of Palencia-Desai et al who demonstrated that chemical and genetic ablation of Bmp signaling in the tailbud-stage zebrafish embryo resulted in severe inhibition of endocardial differentiation with little effect on myocardial development (Palencia-Desai et al, 2015). We cannot explain this discrepancy other than to suggest this might reflect species variability in dependence on Bmp signaling during early cardiogenesis as chicken (Schlange et al, 2000), mice and humans (Kattman et al, 2011) depend on Bmp signaling for early myocardial differentiation.…”

Single-Cell Resolution of Temporal Gene Expression during Heart Development

“…As recent evidence indicates developmental cross-talk between CMs and ECs lineages (Palencia-Desai et al, 2015), we also assessed the maturation of ECs from Nkx2.5 +/− mice ( Fig S3D ). Transcriptional profiles of Nkx2.5 +/− ECs were comparable to WT ECs at E14.5, but significantly delayed at P0 ( Nkx2.5 +/− 12/14 immature ECs versus WT 4/19; p=0.038, Fisher's exact test).…”

Genome editing of factor X in zebrafish reveals unexpected tolerance of severe defects in the common pathway

“…Dysregulated apoptosis has been shown to cause loss of vascular integrity, 29 and F10 has been implicated in apoptosis through PAR-1 signaling. 50 To detect apoptotic cells, we stained larvae with acridine orange 51,52 or probed with an antibody to caspase 3, 53 and this was followed by blinded phenotypic observation and subsequent genotyping. We did not observe any differences in f10 2/2 mutants versus their siblings (supplemental Figure 4), suggesting that deficiency of F10 does not affect cellular apoptosis.…”