The vascular network is arguably the most important organ in a human body. It connects all the other tissues and carries oxygen and nutrients to support their growth and maintenance. Vascular morphogenesis is a multi-step process that occurs in both embryogenesis and adults during pregnancy, wound healing and other pathological conditions. It is tightly regulated by a complex network of transcription factors and signaling pathways in endothelial cell (EC) intrinsic and extrinsic manners.
Slug (Snai2) is a member of the well-conserved Snail family of transcription factors and has roles in multiple developmental processes, including epithelial-to-mesenchymal transitions (EMT). Our lab has been previously demonstrated that Slug expression is up-regulated in angiogenic EC in vitro and is critical for sprouting, partly by controlling EC migration. Preliminary evidence suggests that Slug might play other roles in this process; however, a comprehensive investigation on the mechanisms of Slug-regulated vessel formation is lacking. In addition, although it has been suspected that Slug-mediated endothelial-to-mesenchymaltransition (EndoMT), a process similar to EMT, might have a role in angiogenesis, there is little direct experimental evidence supporting this hypothesis. Moreover, it is unclear whether the role of Slug in angiogenesis observed in our in vitro system also applies to in vivo models.
In this study, using various in vitro 3D culture models, as well as several in vivo models, we confirmed the critical role of Slug during both developmental and pathological vascular morphogenesis. Using RNA-sequencing analysis, we also demonstrated that Slug regulates an
EMT-like suite of target genes. The level of Slug expression determines whether EC perform normal sprouting angiogenesis or excessive pathological angiogenesis by directly suppressing Dll4-activated Notch signaling to upregulate VEGFR2. In vivo, reduced Notch signaling
completely rescues delayed retinal angiogenesis in SlugKO mice. Unlike Snail, Slug is absolutely required for pathological angiogenesis necessary for sustained tumor growth while it plays a transient role in developmental angiogenesis in mice. Slug, but not Snail, is activated in EC by the pro-angiogenic factor SDF-1 via its canonical receptor CXCR4 and subsequent ERK5 phosphorylation. These data are consistent with Slug regulating a partial endothelial-to mesenchymal transition in angiogenic EC.