Abstract
One type of membrane microdomain, enriched in glycosphingolipids and cholesterol and referred to as lipid rafts, has been implicated in the generation of activating signals triggered by a variety of stimuli. Several laboratories, including ours, have recently demonstrated that the B cell receptor (BCR) inducibly localizes to the rafts upon activation and that functional lipid rafts are important for BCR-mediated "positive" signaling. In the later phases of the immune response, coligation of the BCR and the inhibitory receptor Fc gamma RIIB1 leads to potent inhibition of BCR-induced positive signaling through the recruitment of the inositol phosphatase SHIP to Fc gamma RIIB1. One potential model is that the Fc gamma RIIB1 itself might be excluded from the rafts basally and that destabilization of raft-dependent BCR signaling might be part of the mechanism for the Fc gamma RIIB1-mediated negative regulation. We tested this hypothesis and observed that preventing BCR raft localization is not the mechanism for this inhibition. Surprisingly, a fraction of Fc gamma RIIB1 is constitutively localized in the rafts and increases further after BCR + FcR coligation. SHIP is actively recruited to lipid rafts under negative stimulation conditions, and the majority of Fc gamma RIIB1-SHIP complexes localize to lipid rafts compared with non-raft regions of the plasma membrane. This suggested that this negative feedback loop is also initiated in the lipid rafts. Despite its basal localization to the rafts, Fc gamma RIIB1 did not become phosphorylated after BCR alone cross-linking and did not colocalize with the BCR that moves to rafts upon BCR engagement alone (positive signaling conditions), perhaps suggesting the existence of different subsets of rafts. Taken together, these data suggest that lipid rafts play a role in both the positive signaling via the BCR as well as the inhibitory signaling through Fc gamma RIIB1/SHIP.
Highlights
§ Current address: United States Army Medical Research Institute of Infectious Diseases, Dept. of Biochemistry and Cell Biology, Rm. 902R, 1425 Porter St., Frederick, MD 21702
In the later phases of the immune response, coligation of the B cell receptor (BCR) and the inhibitory receptor Fc␥RIIB1 leads to potent inhibition of BCR-induced positive signaling through the recruitment of the inositol phosphatase SHIP to Fc␥RIIB1
Because BCR cross-linking has been shown to induce the tyrosine phosphorylation of specific set of proteins in the lipid rafts [33, 38], we first examined whether the pattern of tyrosine phosphorylation in the lipid rafts might be affected by BCR ϩ FcR coligation in murine A20 B cells
Summary
One type of specialized microdomains within the plasma membrane, known as “lipid rafts,” has been shown to play a key role in signal transduction by multiple cell surface receptors [27,28,29,30]. We have recently demonstrated that activation of B cell receptor results in rapid recruitment of BCR itself as well as PLC␥2 into lipid rafts, whereas the Src family kinase Lyn constitutively resides in this microdomain [33]. Weintraub and Goodnow [40] demonstrated that, in contrast to naive B cells, tolerant B lymphocytes are incapable of recruiting their surface Ig into GEMs, suggesting raft aggregation as a critical step in determining the fate of developing B cells Taken together, these reports strongly suggest a critical role for lipid rafts in BCR signaling at different stages of development and maturation
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