Endothelial differentiation: molecular mechanisms of specification and heterogeneity

Abstract

A complex and diverse vascular system is requisite for the survival of higher organisms. The process of vascular development is highly regulated, involving the de novo formation of vessels (vasculogenesis), followed by expansion and remodeling of the primitive vasculature (angiogenesis), culminating in differentiation of endothelial phenotypes, as found in the mature vascular system. Over the last decade, significant advances have been made in understanding the molecular regulation of endothelial cell development and differentiation. Endothelial development, in particular the mechanisms in play during vasculogenesis and angiogenesis, is discussed in a sister review to this article. This review highlights the key pathways governing in endothelial differentiation, with a focus on the major molecular mechanisms of endothelial specification and heterogeneity.

Figure 1

A model for the proposed molecular pathways in arterial, venous and lymphatic specification in a developing embryo. Schematics on the left portion of the figure illustrate anatomic correlates of the molecular pathways described in the central flow diagram. The bars on the right portion of the figure highlight developmental correlates to the molecular pathways. Shh expression in the notchochord triggers VEGF expression by the somites and creation of a VEGF gradient. High levels of VEGF activate PLCγ and induce arterial specification by activating the MEK/ERK pathway through the VEGFR1-NRP1 receptor complex, and the PI3K/AKT pathway in cells residing further ventral. Arterial specification proceeds with activation of Foxc1/2 and Notch signaling, inducing ephrinB2 and Nrp-1expression. Nrp-1 creates a positive feedback loop by acting as co-receptor for VEGF. Venous specification occurs when ERK is suppressed, and COUP-TFII induced, by AKT. COUP-TFII inhibits arterial specification by down-regulating Notch and Nrp-1, allowing expression of venous-specific markers including Nrp-2. A subpopulation of venous EC subsequently express Sox18 and Prox1, resulting in lymphatic EC specification. The upper schematic highlights the migration of angioblasts and subsequent development, directed by VEGF concentrations, of the DA and PCV; the middle schematic the importance of activation of the Notch pathway by cleavage and nuclear translocation of the cytoplasmic domain of the receptor; and the lower schematic the requirement of cell-to-cell communication via ephrin-Eph receptor kinases in maintaining arterial/venous discrimination. T, neural tube; M, lateral mesoderm; S, somite; N, notochord; ICM, inner cell mass. Full details are found in the text.

Figure 2

Endothelial Cell Heterogeneity. A, Schematic diagram of the three main endothelial structural classifications (modified from ). TEC= transendothelial channel, VVO=vesiculo-vacuolar organelles. B, Schematic diagram of the molecular mechanisms of blood brain barriergenesis (modified from ). C, Heterogeneity of the aortic endothelium. Turbulent flow occurring in the lesser curvature of the aortic arch or at branch points in the vascular tree inhibits KLF2/4 expression resulting in a more pro-inflammatory endothelial phenotype. Whereas laminar flow occurring in linear segments upregulates endothelial KLF2/4 expression resulting in a more anti-inflammatory phenotype.