Abstract
Highly ordered sphingolipid-enriched lipid raft microdomains (LRMs) within plasma membranes purportedly function as specialized signaling platforms. Leukocyte migration is believed to entail LRM redistribution, but progress in studying LRMs in situ during cell movement has been limited. By using an improved method for imaging the spectral shift of the environmentally sensitive probe, laurdan (expressed as a generalized polarization function), the plasma membrane order (i.e., tight packing of membrane bilayer lipids) of human polymorphonuclear neutrophils (PMNs) was mapped in real time during migration. Morphologically polarized PMNs exhibited prominent LRM clusters at the uropod, where in every instance membrane order was found to oscillate with mean periodicities of 37.0 ± 1.46 and 149.9 ± 9.0 seconds (P < 0.01). LRM aggregates were also demonstrated in punctate and clustered distributions of nonpolarized cells and transiently at the lamellipodia of polarized PMNs. Cellular polarization was not accompanied by an overall increase in membrane order. LRM disorganization with methyl-β-cyclodextrin had small negative effects on cell velocity, but it abrogated directionally biased migration toward chemotactic gradients of FMLP or leukotriene B(4). LRMs disruption also caused redistribution of Rac 1/2 GTPase and GM3 ganglioside away from the lamellipodium, as well as extension of multiple pseudopods simultaneously or in rapid succession, rather than formation of a defined leading edge. Thus, we demonstrate that the plasma membrane order of migrating PMNs changes dynamically, with prominent oscillations consistently seen at the uropod. These findings solidify the existence of rapidly reorganizing LRMs in situ and support a role for LRMs in chemotaxin responsiveness.