Expression of Nkx2.5 and Isl1 in the cardiopharyngeal mesoderm of developing Xenopus embryos

Abstract

The cardiopharyngeal field (CPF) is a mesodermal field that gives rise to the first (FHF) and second heart field (SHF)‐derived myocardium and the branchiomeric musculature. Isl1 and Nkx2.5 are transcription factors expressed in the CPF. Both genes are crucial for the specification of the myocardial cells derived from the SHF and for the further development of the heart in vertebrates. The importance of these genes is supported by the fact that perturbations in their expression lead to severe heart malformations and often result in embryonic death. In order to better understand the role of these two genes and their impact on the head and heart development, we analysed their expression in Xenopus laevis, which is potentially a good model to study the CPF and craniofacial defects in terms of evolutionary developmental medicine. In contrast to basal vertebrates, such as the urochordate Ciona, Xenopus' heart has an advanced morphology with several heart chambers and in‐ and outflow tracts. Concurrently, Xenopus is easy to rear in laboratory conditions. There are various genetic tools available to work with this species (e.g., TALENs, CRISPR/Cas9, ZFNs) and there are certain advantages for the study of embryology (e.g., external fertilization and large size of eggs) that favour this species over the mouse and chicken models.Whole‐mount in situ hybridization of embryos revealed that during early stages Isl1 and Nkx2.5 are co‐expressed in cardiac progenitor cells localized near the cement gland. Isl1 was also present in the stomodeum region and other cell populations; e.g., cephalic placodes and their associated ganglia or cranial paraxial mesoderm. At stage 25 (Nieuwkoop and Faber), Nkx2.5 was expressed in the FHF, SHF, and anterior endoderm. Later, it nearly separated from the Isl1 expression and became restricted to the heart tube, derived from the FHF. Isl1 was mainly expressed in the pharyngeal arch mesoderm adjacent to the heart and in the dorsolateral heart domain. At late stages, Nkx2.5 could be detected throughout the myocardium, while Isl1 was present in the foregut and cephalic ganglia. In spite of minor differences, our results conform to early studies in X. laevis. The comparison with expression patterns in mouse and chicken shows that Xenopus is indeed a good organism to study the CPF derivatives, like the myocardium and head musculature. Therefore, Xenopus can be further extensively used for developmental, evolutionary, and medical studies of the normal and abnormal head and heart myogenesis.Support or Funding InformationJMZ is supported by a Howard University College of Medicine pilot study award (BFPSAP: U100217).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.