Plasma membrane order in T cell signalling

Abstract

Plasma membrane nanodomains, referred to as lipid rafts, more ordered than the bulk membrane play an important role in T cell signalling by forming signalling platforms in activated T cells. However, the existence of lipid rafts in resting T cells is contentious. Using laurdan, a membrane probe whose peak emission wavelength depends on the lipid environment, evidence is presented for the existence of ordered nanodomains in resting T cells.T cell signalling can be initiated by stimulating the T cell receptor (TCR), crosslinking the lipid raft markers GM1 (sphingolipid) or glycosylphosphatidylinositol (GPI) anchored proteins. The aggregation of lipid raft components induces the same response in Jurkat T cells as the ligation of an antigen to the TCR. Changes in membrane order linked with reorganization of the plasma membrane upon Jurkat T cell activation were followed at 37°C. Fluorescent images were analyzed for generalised polarisation values - a measure of the relative abundance of liquid ordered and liquid disordered domains. TCR patching does not increase the overall membrane order suggesting that membrane domains of high order are brought together in the patches. This supports the existence of small ordered membrane domains in resting T cells that aggregate upon activation. Patching of GM1, the GPI-anchored protein CD59 and the non lipid raft marker CD45 significantly increases the overall membrane order. So does general crosslinking of membrane components with Concanavalin A. Remodelling of the actin cytoskeleton is an integral part of TCR signaling and T cell activation. Disrupting actin polymerization using latrunculin B decreases membrane order and stabilizing actin filaments with jasplakinolide increases membrane order. An increase in membrane order appears to be a general effect of plasma membrane component patching and is likely due to a global induction of actin polymerization at the plasma membrane.