The precise regulation of blood-brain barrier (BBB) permeability for immune cells and blood-borne substances is essential to maintain brain homeostasis. Sphingosine-1-phosphate (S1P), a lipid signaling molecule enriched in plasma, is known to affect BBB permeability. Previous studies focussed on endothelial S1P receptors 1 and 2, reporting a barrier-protective effect of S1P1 and a barrier-disruptive effect of S1P2. Here we present novel data characterizing the expression, localization and function of the S1P receptor 4 (S1P4) on primary brain microvascular endothelial cells (BMECs). Hitherto, the receptor was deemed to be exclusively immune cell-associated. We detected a robust expression of S1P4 in homeostatic murine, bovine and porcine BMECs and pinpointed its localization to abluminal endothelial membranes via immunoblotting of fractionated brain endothelial membrane fragments. Apical S1P treatment of BMECs tightened the endothelial barrier in vitro , whereas basolateral S1P treatment led to an increase...