Mesodermal Nkx2.5 is necessary and sufficient for early second heart field development

The vertebrate heart develops from mesoderm and requires inductive signals secreted from early endoderm. During embryogenesis, Nkx2.5 acts as a key transcription factor and plays essential roles for heart formation from Drosophila to human. In mice, Nkx2.5 is expressed in the early first heart field, second heart field pharyngeal mesoderm, as well as pharyngeal endodermal cells underlying the second heart field. Currently, the specific requirements for Nkx2.5 in the endoderm versus mesoderm with regard to early heart formation are incompletely understood. Here, we performed tissue-specific deletion in mice to dissect the roles of Nkx2.5 in the pharyngeal endoderm and mesoderm. We found that heart development appeared normal after endodermal deletion of Nkx2.5 whereas mesodermal deletion engendered cardiac defects almost identical to those observed on Nkx2.5 null embryos (Nkx2.5−/−). Furthermore, re-expression of Nkx2.5 in the mesoderm rescued Nkx2.5−/− heart defects. Our findings reveal that Nkx2.5 in the mesoderm is essential while endodermal expression is dispensable for early heart formation in mammals. •Nkx2.5 is a key cardiac transcription factor expressed in the early FHF, SHF and pharyngeal endoderm.•Mouse heart development appears normal after endodermal deletion of Nkx2.5.•Mesodermal disruption of Nkx2.5 engenders almost identical cardiac defects to Nkx2.5 null mice.•Specific re-expression of Nkx2.5 in the mesoderm rescues Nkx2.5 null cardiac defects.•Nkx2.5 expression in the mesoderm is essential but its endodermal expression is dispensable for early SHF development.