Abstract
In vivo, the chemokine CCL19 and its receptor CCR7 control T cell retention in lymph nodes, while the lipid chemoattractant spingosine-1-phosphate (S1P) drives T cell egress from lymphoid organs. CCL19 is known to activate actin polymerization at the leading edge of migrating cells, generating a mode of motility driven by lamellipodial protrusions. In contrast, we showed recently that S1P induces a transient lamellipodial response, followed by pressure-driven bleb-based motility. Here, we elucidate the mechanisms controlling S1P responses in naïve T cells. We show that S1P signals through S1PR1, with coupling through Gai. In contrast to CCR7, which signals through Gai to induce sustained Rac1 activation, S1PR1 engagement yields only weak and transient Rac1 activation; the dominant response is sustained activation of RhoA. This pathway, together with a pathway involving phospholipase C and myosin light chain kinase, results in phosphorylation of myosin regulatory light chain (MLC) and enhanced myosin contractility. Inhibition of mTORC2 blocks MLC phosphorylation, consistent with evidence that tension sensing by mTORC2 can couple Rac1 and RhoA signaling during leukocyte migration. Surprisingly, although RhoA pathway inhibitors blocked S1P-induced MLC phosphorylation and blebbing, they failed to block S1P-dependent chemotaxis. This led to the identification of a second arm of the S1P response: WNK1-dependent phosphorylation of SPAK1 and OSXR1, proteins that regulate ion channels and water influx. Partial WNK1 inhibition, together with inhibition of myosin contractility, was sufficient to block S1P-induced blebbing and chemotaxis, indicating that S1P-driven T cell migration involves coordinate activation of myosin contractility and water influx.