Abnormal arterial-venous fusions and fate specification in mouse embryos lacking blood flow

Abstract

The functions of blood flow in the morphogenesis of mammalian arteries and veins are not well understood. We examined the development of the dorsal aorta (DA) and the cardinal vein (CV) in Ncx1 −/− mutants, which lack blood flow due to a deficiency in a sodium calcium ion exchanger expressed specifically in the heart. The mutant DA and CV were abnormally connected. The endothelium of the Ncx1 −/− mutant DA lacked normal expression of the arterial markers ephrin-B2 and Connexin-40. Notch1 activation, known to promote arterial specification, was decreased in mutant DA endothelial cells (ECs), which ectopically expressed the venous marker Coup-TFII. These findings suggest that flow has essential functions in the DA by promoting arterial and suppressing venous marker expression. In contrast, flow plays a lesser role in the CV, because expression of arterial-venous markers in CV ECs was not as dramatically affected in Ncx1 −/− mutants. We propose a molecular mechanism by which blood flow mediates DA and CV morphogenesis, by regulating arterial-venous specification of DA ECs to ensure proper separation of the developing DA and CV.

Molecular regulation of arteriovenous endothelial cell specification

“…Supporting this point, loss of systemic blood flow in chick 13, 14 and mice 15, 16 produces defects in arteriovenous specification and induces AVMs. For example, in Ncx1 −/− mouse embryos (which lack a heartbeat), blood flow is not required for initial formation of the dorsal aorta and the cardinal vein but is necessary to induce separation of these vessel structures and to maintain arterial marker expression and suppress venous identity genes in the dorsal aorta 15 . Thus, early endothelial cell fate acquisition is dynamic, and hemodynamic signaling is needed to sustain arteriovenous identity in the remodeling vasculature.…”

Location, Location, Location: How Vascular Specialization Influences Hematopoietic Fates During Development

“…In contrast to EMPs, these lymphoid progenitors arise from HE with arterial characteristics (Yzaguirre and Speck, 2016), suggesting important roles for arterial endothelial niche and hemodynamic forces in the specification of hematopoietic cells with lymphoid potential. Supporting this association, Notch1 signaling, which plays an essential role in arterial endothelial specification (Lawson et al, 2001;Corada et al, 2014) and stabilization of arterial transcriptional programs in response to blood flow (Lawson et al, 2001;le Noble et al, 2004;Hwa et al, 2017;Weijts et al, 2018), is required for multilineage hematopoiesis and HSC development in vivo (Kumano et al, 2003;Hadland et al, 2004) and has been shown to play a role in conferring lymphocyte potential to nascent stem/progenitor cells derived during the differentiation of pluripotent stem cells (Lu et al, 2016;Uenishi et al, 2018). The role of Notch signaling and blood flow hemodynamics in the emergence of multilineage hematopoiesis and HSCs will be discussed in more detail in the following section, "HSC Development in the Arterial Vascular Niche" of this review.…”

Influence of blood flow on cardiac development

“…In both the sih −/− zebrafish and the Ncx1 knockout, embryos lacked endocardial cushion development, while looping and chamber specification remained unaffected (Koushik et al, 2001; Sehnert et al, 2002; Bartman et al, 2004). Studies with sih −/− zebrafish and Ncx1 knockout mice support the idea that endocardial cushion development is exquisitely sensitive to changes in myocardial function (Koushik et al, 2001; Bartman et al, 2004; Bartman and Hove, 2005; Hwa et al, 2017).…”

“…Mouse and zebrafish models have been used extensively for genetic manipulations, and transgenic lines are widely available for both species (Koushik et al, 2001; Huang et al, 2003; Hwa et al, 2017). Transgenic lines that express fluorescent probes driven by a chosen promoter have been widely used both for visualization and lineage studies, and are often bred into other mutant lines to augment analysis (Auman et al, 2007; Zhang et al, 2014; Samsa et al, 2015; Lee et al, 2016; Battista et al, 2017).…”