Endothelin Receptor Type A-Expressing Cell Population in the Inflow Tract Contributes to Chamber Formation

Abstract

The mammals and birds have four chambered hearts. The current models show the majority of myocardial cells derive from the first and second heart field, however, little is known regarding how heart field subpopulations contribute to specific regions in the heart. In this study, we revealed that the early Ednra-positive population in the inflow tract region contributes to the chamber myocardium by mouse-chick chimera and CreERT2/loxP lineage tracing system.

The avian and mammalian heart mainly originates from two distinct embryonic regions: an early differentiating first heart field and a dorsomedially located second heart field. It remains largely unknown when and how these subpopulations of the heart field are established as regions with different fates.

Endothelin-1 (Edn-1) acts on cardiac neural crest cells through endothelin receptor type A (Ednra) and is involved in the normal formation of pharyngeal artery-derived great vessels and ventricular septum [1]. Previously, we identified a distinct cell population defined by the expression of Ednra in the mouse inflow region [2]. These cells are derived from a part of the first heart field, and largely confined to the inflow region at E8.25.

From the expression patterns of β-gal in the Ednra-lacZ knock-in mice, we thought a possibility that this Ednra-positive cell population might move into cardiac chambers from the inflow region. By dye injection and transplantation experiments, we showed that the Ednra-positive cell population moved toward not only the left and right atria but also the left ventricle.

Then, to perform lineage analysis, we have generated an Ednra-CreERT2 mouse line (unpublished). We activated Cre recombinase by tamoxifen at E7.25 and E8.25 and analyzed the distribution of β-gal-labeled cells in the Ednra^CreERT2/+;R26R embryo hearts. As a result, the β-gal-labeled cells were found to contribute to the left ventricle and both atria.

To facilitate this lineage analysis, we established a mouse-chick chimera model. When we transplanted the inflow region of the Ednra^CreERT2/+;R26R mouse embryos (tamoxifen i.p. at E7.25) to orthotopically into chick embryos, β-gal-positive cells were detected in the right atrium and left ventricles 7 days after transplantation.

From these results, we conclude that the Ednra-positive cell lineage in the early inflow tract certainly contribute to chamber myocardial formation.