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Neutrophilic polymorphonuclear leukocytes contain glycosphingolipid- and cholesterol-enriched lipid raft microdomains within the plasma membrane. Although there is evidence that lipid rafts function as signaling platforms for CXCR chemokine receptors, their role in recognition systems for other chemotaxins such as leukotriene B4 (LTB4) and fMLP is unknown. To address this question, human neutrophils were extracted with 1% Brij-58 and fractionated on sucrose gradients. B leukotriene receptor-1 (BLT-1), the primary LTB4 receptor, partitioned to low density fractions, coisolating with the lipid raft marker, flotillin-1. By contrast, formyl peptide receptor (FPR), the primary fMLP receptor, partitioned to high density fractions, coisolating with a non-raft marker, Cdc42. This pattern was preserved after the cells were stimulated with LTB4 or fMLP. Fluorescence resonance energy transfer (FRET) was performed to confirm the proximity of BLT-1 and FPR with these markers. FRET was detected between BLT1 and flotillin-1 but not Cdc42, whereas FRET was detected between FPR and Cdc42, but not flotillin-1. Pretreating neutrophils with methyl-β-cyclodextrin, a lipid raft-disrupting agent, suppressed intracellular Ca2+ mobilization and ERK1/2 phosphorylation in response to LTB4 but had no effect on either of these responses to fMLP. We conclude that BLT-1 is physically located within lipid raft microdomains of human neutrophils and that disrupting lipid raft integrity suppresses LTB4-induced activation. By contrast, FPR is not associated with lipid rafts, and fMLP-induced signaling does not require lipid raft integrity. These findings highlight the complexity of chemotaxin signaling pathways and offer one mechanism by which neutrophils may spatially organize chemotaxin signaling within the plasma membrane.

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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 6 1.46 and 149.9 6 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-b-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.Keywords: neutrophils; chemotaxis; lipid rafts; inflammation Inflammation mediated by polymorphonuclear neutrophils (PMNs) requires the cells to recognize and migrate toward chemotactic signals. Coupled with morphologic polarization, there must be complex spatial redistribution of signaling molecules, cytoskeletal structures, and membrane constituents for directionally biased movement (1-4). Although the distribution of proteins has been examined extensively, there is growing appreciation for the importance of compartmentalized membrane lipids and lipid-mediated signaling (2, 4-6). Earlier models have depicted a uniform lipid bilayer in the plasma membrane, but considerable evidence indicates that it is highly heterogeneous, containing small phase-separated regions often designated as lipid raft microdomains (LRMs) (reviewed in Refs. 2, 7-9). These microdomains are rigid, gel-like regions enriched in tightly packed sphingolipids, resulting in a liquidordered (Lo) state that is stabilized by cholesterol. It is believed that LRMs act as platforms where multiple constituents colocalize to form competent signaling complexes. Cell fractionation experiments have been used to isolate detergent-resistant/lowdensity membrane fractions containing an array of receptor ...

Purified 2,4-dichlorophenoxyacetic acid (2,4-D)/␣-ketoglutarate dioxygenase (TfdA) was shown to use 4-nitrophenoxyacetic acid (K m ‫؍‬ 0.89 ؎ 0.04 mM, k cat [catalytic constant] ‫؍‬ 540 ؎ 10 min ؊1), producing intensely yellow 4-nitrophenol. This reagent was used to develop a rapid, continuous, colorimetric assay for the detection of TfdA and analogous activities in 2,4-D-degrading bacterial cells and extracts.