Sphingosine-1-phosphate cross-talks to Notch via a S1PR1-Dll4-MPDZ complex to regulate endothelial barrier function

Sphingosine-1-phosphate (S1P) - a key bioactive component of high-density lipoproteins (HDL) - is instrumental in mediating HDL's cardiovascular benefits, largely by enhancing endothelial barrier integrity ^{1, 2} . Here, we discovered that S1P induces Notch1 activation, and this Notch activation is required to enhance Rac1 activity and adherens junction assembly, which in turn stimulates endothelial barrier integrity. S1P rapidly activates Notch1 by stimulating the G-coupled protein receptor, S1P Receptor 1 (S1PR1) to drive internalization of the Notch ligand Delta-like protein 4 (Dll4). Notably, this internalization of Dll4 and subsequent activation of Notch does not involve traditional G-protein signaling; instead, S1P-bound S1PR1 forms a complex with Dll4 via the scaffolding protein MPDZ, and the undergoes co-endocytosis. Importantly, the loss or inhibition of Notch, Dll4, S1PR1, or MPDZ results in barrier defects. These findings elucidate a novel S1PR1-Dll4-MPDZ-Notch1 signaling axis that coordinates S1P and Notch signaling to regulate of endothelial cell signaling and barrier function.