The heart is one of the first organs to be formed in the embryo, and its function is critical for the circulation of nutrients and the removal of waste products to ensure proper development of the embryo subsequently.
The formation of the mammalian heart is a complex process, involving two distinct groups of mesodermal progenitor cells, the first and second heart fields (FHF and SHF, respectively). While the FHF primarily gives rise to the left ventricle and parts of atria, the SHF develops into the right ventricle, outflow tract (OFT) and parts of atria. Both FHF and SHF cardiac progenitors, marked by HCN4 and ISL1, respectively, have since been isolated and characterized.
At present, ISL1+ SHF cardiovascular progenitors (CVPs) can only be expanded and kept multipotent on WNT3A-over-expressing feeders. Owing to a lack of robust, defined long-term culture conditions that is capable of maintaining clonally expanded, multipotent ISL1+ CVP over prolong periods, researchers sought to identify novel factors that can support the long-term propagation of ISL1+ CVPs in vitro.
Authors differentiated human ESCs with ISL1+ reporter into CVPs, and adopted a comparative bioinformatics approach to identify candidate signalling pathways that may be active in uncommitted ISL1+ CVPs. This was achieved by performing microarray analysis on subregions of the fetal heart, followed by pairing the paracrine factors present in specific regions, with corresponding receptors that are highly expressed in uncommitted CVPs.
The rationale of this approach is that during early cardiogenesis, it is the surrounding mesenchyme that secretes factors to either maintain the proliferation of progenitor cells or provide signals to direct their differentiation. Therefore, examination of paracrine factors that are specific and highly expressed in both OFT and right ventricle would be useful for clonal expansion of ISL1+ CVPs that were previously shown to be specifically present in these two regions of the heart.
Researchers demonstrate the intrinsic ability of vascular progenitors to develop and self-organize into cardiac tissues by clonally isolating and expanding second heart field cardiovascular progenitors using WNT3A and endothelin-1 (EDN1) human recombinant proteins.
Progenitor clones undergo long-term expansion and differentiate primarily into endothelial and smooth muscle cell lineages in vitro, and contribute extensively to coronary-like vessels in vivo, forming a functional human–mouse chimeric circulatory system.
This study identifies EDN1 as a key factor towards the generation and clonal derivation of ISL1+ vascular intermediates, and demonstrates the intrinsic cell-autonomous nature of these progenitors to differentiate and self-organize into functional vasculatures in vivo.