| Antigen (Ag) binding to the BCR rapidly initiates two important events: a phosphorylation cascade that results in the production of secondary signaling intermediaries and the internalization of Ag-BCR complexes. Previous studies using anti-BCR antibodies (Ab) have suggested that BCR signaling is an essential requirement for BCR endocytosis and have further implicated lipid rafts as essential platforms for both BCR functions. More importantly, a model where BCR signaling and rafts are required for BCR internalization supports the hypothesis that both BCR functions are mediated sequentially by the same population of BCRs. However, published data from our laboratory indicates that lipid-rafts and consequently raft-mediated signaling are dispensable for BCR-mediated internalization of a BCR-specific Ag, suggesting that signaling and internalization might be mediated by two different BCR populations. Therefore, to address this controversy, we investigated the relationship between BCR signaling and endocytosis by defining the role of early kinase signaling in the BCR mediated internalization of a model Ag (haptenated protein).{09}The results demonstrate that internalization of BCR-Ag complexes proceeds normally in the absence of Src-family kinases activity and Syk-mediated BCR signaling thus supporting the hypothesis that BCR signaling and internalization are mediated by two exclusive populations of BCR. Moreover, by comparing Ag and Ab in the same BCR system, it was determined that, while both ligands localize to clathrin-coated pits, the internalization of Ab-BCR complexes is more susceptible to inhibition of signaling and highly sensitive to disruption of lipid rafts and the actin cytosleleton compared to Ag-BCR complexes. Thus, these results demonstrate that the nature of the ligand ultimately determines the functional requirements and relative contribution of lipid rafts and other membrane structures to the internalization of BCR-ligand complexes.; Although the role of BCR signaling in internalization is controversial, the role of BCR signaling in the trafficking of BCR-ligand complexes is well established. A model where BCR internalization and signaling are mediated by two populations of BCRs poses the additional challenge to explain how BCR signaling affects BCR trafficking. In this regard, our results suggest that BCR signaling is also necessary for proper targeting of BCR-Ag complexes in our Ag-specific model. Our data suggest that BCR signaling alters trafficking by facilitating the association of antigen-containing vesicles with cytoskeletal components. |
