MEF2 transcription factors are key regulators of sprouting angiogenesis

Abstract

Angiogenesis, the fundamental process by which new blood vessels form from existing ones, depends on precise spatial and temporal gene expression within specific compartments of the endothelium. However, the molecular links between proangiogenic signals and downstream gene expression remain unclear. During sprouting angiogenesis, the specification of endothelial cells into the tip cells that lead new blood vessel sprouts is coordinated by vascular endothelial growth factor A (VEGFA) and Delta-like ligand 4 (Dll4)/Notch signaling and requires high levels of Notch ligand DLL4. Here, we identify MEF2 transcription factors as crucial regulators of sprouting angiogenesis directly downstream from VEGFA. Through the characterization of a Dll4 enhancer directing expression to endothelial cells at the angiogenic front, we found that MEF2 factors directly transcriptionally activate the expression of Dll4 and many other key genes up-regulated during sprouting angiogenesis in both physiological and tumor vascularization. Unlike ETS-mediated regulation, MEF2-binding motifs are not ubiquitous to all endothelial gene enhancers and promoters but are instead overrepresented around genes associated with sprouting angiogenesis. MEF2 target gene activation is directly linked to VEGFA-induced release of repressive histone deacetylases and concurrent recruitment of the histone acetyltransferase EP300 to MEF2 target gene regulatory elements, thus establishing MEF2 factors as the transcriptional effectors of VEGFA signaling during angiogenesis.

The endothelial transcription factor ERG mediates Angiopoietin-1-dependent control of Notch signalling and vascular stability

“…This is in line with a recent study by the De Val group56, showing that ETS motifs within the Dll4 intron 3 enhancer are not required for its expression at the angiogenic front. The authors suggest a model in which MEF2 transcription factors cooperate with ETS factors, where ETS provide essential endothelial expression information and MEF2 contribute angiogenic sprout specificity56. Ang1 can also promote Dll4 expression, and here we show that this requires ERG.…”

Dynamic regulation of VEGF-inducible genes by an ERK-ERG-p300 transcriptional network

“…The kinetics of this repressive TEL/CTBP complex disassembly are comparable to the assembly of the activating ERG/p300 complex that we report here, suggesting that TEL and ERG dynamically control co-activator/co-repressor recruitment. Recently, VEGF has also been shown to stimulate dynamic exchange of co-repressors for co-activators bound to MEF2 transcription factors (Sacilotto et al, 2016), suggesting that several families of transcription factors may coordinate VEGFdependent sprouting angiogenesis. Indeed, ETS proteins interact with multiple transcription factor families (Carrere et al, 1998;De Val et al, 2008).…”

Endothelial Myocyte Enhancer Factor 2c Inhibits Migration of Smooth Muscle Cells Through Fenestrations in the Internal Elastic Lamina

“…They further revealed a protective role for Mef2c in inflammation in that LPS-induced leukocyte adhesion in retinal vessels is increased in Mef2c deleted endothelium 15 . While deletion of Mef2c alone did not affect angiogenesis, combined endothelial-specific deletion of Mef2a and -c revealed that they regulate angiogenic sprouting and are direct transcriptional regulators of Dll4 in angiogenic tip cells 29 .…”

“…In vivo experiments supporting a role for Mef2 in the endothelium have been restricted to the retinal vasculature and developing embryo 15,28,29 . These experiments show that Mef2c is important for protection to hyperoxia and pathological angiogenesis, partly through Hmox1 regulation 28 .…”