Degradation of ceramide to sphingosine by ceramidase and subsequent phosphorylation by sphingosine kinase 1 or sphingosine kinase 2 produces S1P. S1P can be reversibly degraded to sphingosine by S1P phosphatase and lipid phosphate phosphatases or irreversibly degraded by S1P lyase to 2-trans hexadecenal and phosphoethanolamine^,. Notably, FTY720 and some of its analogues are preferentially phosphorylated by sphingosine kinase 2 (REF. ) and were shown to be degraded by lipid phosphate phosphatase 3 in vitro.

S1P is present at high nanomolar concentrations in plasma; 98.5% is bound to albumin and other transporters such as high density lipoproteins (HDLs). S1P is released to the blood from various sources. It was originally thought to be mainly produced by platelets, but recent evidence suggests that red blood cells are also a source of S1P in mice^– and that secreted sphingosine kinase 1 (SphK1) could also help regulate plasma S1P levels. S1P can act intracellularly (not shown) or be transported to the extracellular space by ATP-binding-cassette transporters. Another S1P transporter was recently identified in the zebrafish. Once in the extracellular space, S1P is susceptible to degradation to sphingosine by different classes of lipid phosphate phosphatases (LPPs). In turn, sphingosine might also be phosphorylated by secreted SphK1 in the extracellular space. S1P1 and S1P3 are known to couple to Gi/o, which is associated with endothelial barrier enhancement. Signalling through Gi/o leads to Rac activation, which is coupled to the translocation of cortactin to the periphery of the cell, where it co-localizes with myosin light chain kinase (MLCK) and leads to the formation of the cortical polymerized actin ring. Moreover, cortactin is also thought to contribute to the organization of tight junctions. The cortical actin ring is stabilized by the assembly of focal adhesions (FAs) at the periphery of the cell, leading to tightening of the endothelial barrier. In an apposite manner, S1P3 and S1P2 couple to G12 and G13. G12 and G13 activation leads to Rho activation, stress fibre formation and disruption of the endothelial barrier. This figure was inspired by various working models^,,,.

S1P receptors are involved in the regulation of various physiological and pathophysiological phenomena, including hearing^,, vasodilation and vasoconstriction, heart rate, airway hyper-responsiveness and lymphoid tissue function. Accumulating literature supports the use of small molecules that target the S1P immunoregulatory pathway to modulate barrier activity in different organs. For example, S1P receptor activation was shown to favour pulmonary barrier integrity in models of acute lung injury and acute respiratory distress syndrome^,,, and in the kidneys and myocardium after ischaemia–reperfusion stresses^,, and to favour blood–brain barrier protection during experimental autoimmune encephalomyelitis. Moreover, the S1P pathway was shown to enhance the lymphatic endothelial barrier integrity in lymph nodes, leading to sequestration of T cells in the lymph nodes.

a | During homeostasis, blood-borne S1P can activate S1P1, which induces Rac activation. This leads to the arrangement of the cortical actin ring and tightening of adherens junctions, favouring the maintenance of barrier integrity. b | The essential requirement of tonic S1P1 activation for the maintenance of endothelial barrier integrity can be shown by in vivo blockade of S1P1 by a chemical probe, such as W146, which leads to disruption of the endothelial barrier and plasma leakage into the interstitium.

a | After tissue insult, tissue-homing cells, such as macrophages or dendritic cells, release proinflammatory and chemotactic factors, which leads to activation of the endothelium. The endothelium expresses adhesion molecules, such as selectins and integrins, at its surface, thereby inducing tethering, rolling, firm adhesion and infiltration of leukocytes. Pro-inflammatory mediators, vasodilatory agents and infiltrating cells induce barrier disruption and injury, resulting in leakage of plasma into the interstitium and infiltration of inflammatory cells that can release toxic molecules and induce cell death. b | Sphingosine-1-phosphate (S1P) and sphingosine analogues were shown to impair the release of pro-inflammatory factors in various models, such as ischaemia–reperfusion injuries or viral infections^,,. Although S1P3 activation can disrupt barrier integrity, it seems that broad S1P receptor agonists, such as FTY720, favour barrier integrity. S1P1 activation was also shown to reduce expression of adhesion molecules, to impair interactions between leukocytes and the endothelium and to reduce inflammatory-cell infiltration^,–,. Moreover, S1P2 and S1P3 activation might favour cell survival by activating the pro-survival Akt (also known as protein kinase B) pathway.

a | The blood–brain barrier (BBB) is central to the pathogenesis of various CNS diseases, as it controls the passage of molecules and cells, and can be damaged in the course of disease. During autoimmune disease, autoreactive lymphocytes access the brain through the bloodstream. Like the endothelium of acutely injured tissues, the BBB expresses adhesion molecules and becomes disrupted, leading to accumulation of fluids, macrophages and autoreactive T cells in the CNS. b | The sphingosine-1-phosphate (S1P) analogue FTY720 (grey rectangles) and S1P receptor 1-specific agonists (shown in green) induce sequestration of lymphocytes in secondary lymphoid organs owing to lymphatic barrier tightening, which might decrease the capacity of these lymphocytes to reach the CNS. In addition to reducing leukocyte infiltration, BBB enhancement with S1P1-specific agonists could lead to decreased fluid accumulation in the CNS, which is an aggravating factor in brain cancer and might interfere with drug delivery. Decreased accumulation of fluids and activated lymphocytes in the CNS could translate into alleviation of tissue damage. Blood-borne small, lipophilic molecules, such as FTY720, can accumulate in the CNS (indicated by a red arrow). FTY720 is then phosphorylated (FTY720-P) in the CNS, where it can act on widely distributed S1P receptors.