Arterial specification and differentiation are influenced by a number of regulatory pathways. While it is known that the Vegfa-Notch cascade plays a central role in this biological process, the transcriptional hierarchy controlling arterial specification has not been fully delineated. To elucidate the direct transcriptional regulators of Notch receptor expression in arterial endothelial cells, we used histone signatures, DNaseI hypersensitivity and ChIP-seq data to identify enhancers for the human NOTCH1 and zebrafish notch1b genes. These enhancers were able to direct arterial endothelial cell-restricted expression in transgenic models. Genetic disruption of SOXF binding sites clearly established a requirement for members of the SOXF group of transcription factors (SOX7,-17 and-18) to drive these enhancers activity in vivo. Further, endogenous deletion of the notch1b enhancer led to a significant augmentation of arterio-venous defects in notch-pathway deficient zebrafish. Loss of SoxF function revealed that these factors are necessary for the activity of NOTCH1 and notch1b enhancers, and for correct endogenous Notch1 gene transcription. These findings therefore position SOXF transcription factors directly upstream of Notch receptor expression during the acquisition of arterial identity in vertebrates.

Despite a growing catalog of secreted factors critical for lymphatic network assembly, little is known about the mechanisms that modulate the expression level of these molecular cues in blood vascular endothelial cells (BECs). Here, we show that a BEC‐specific transcription factor, SOX7, plays a crucial role in a non‐cell‐autonomous manner by modulating the transcription of angiocrine signals to pattern lymphatic vessels. While SOX7 is not expressed in lymphatic endothelial cells (LECs), the conditional loss of SOX7 function in mouse embryos causes a dysmorphic dermal lymphatic phenotype. We identify novel distant regulatory regions in mice and humans that contribute to directly repressing the transcription of a major lymphangiogenic growth factor (Vegfc) in a SOX7‐dependent manner. Further, we show that SOX7 directly binds HEY1, a canonical repressor of the Notch pathway, suggesting that transcriptional repression may also be modulated by the recruitment of this protein partner at Vegfc genomic regulatory regions. Our work unveils a role for SOX7 in modulating downstream signaling events crucial for lymphatic patterning, at least in part via the transcriptional repression of VEGFC levels in the blood vascular endothelium.

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