The crosstalk between B-cell receptor mediated signaling and the actin cytoskeleton
MetadataShow full item record
Oligomerization of the B-cell receptor (BCR) by antigen leads to both signal transduction and antigen internalization for processing and presentation. Previous studies indicate that these processes intersect at the actin cytoskeleton to coordinate the two cellular processes for the optimal activation of B-cells. The exact mechanism by which signals are transduced via the actin cytoskeleton into the efficient internalization and transport of BCR-antigen complexes is not well delineated. In this thesis, I demonstrate that Bruton's tyrosine kinase (Btk), a Tec kinase in the early signaling pathway of the BCR, is able to transduce signals from the BCR to actin regulatory proteins such as WASP and N-WASP. Upon BCR activation, Btk modulates actin dynamics by increasing the levels of phosphorylated, active WASP and N-WASP in B-cells. Btk regulates the activity of WASP and N-WASP by increasing the levels of PtdIns-4,5-P2 and phosphorylated Vav, both of which are required for WASP and N-WASP activation. Inhibition of Btk activity by a point mutation or a specific inhibitor prevents BCR-induced increases in PtdIns-4,5-P2 as well as in phosphorylated WASP, N-WASP and Vav. Furthermore, Btk deficiency or inhibition leads to a severe reduction in BCR-mediated antigen internalization, processing, and presentation to cognate T-cells. Further studies on the role of WASP show no significant effect of WASP deficiency on BCR internalization, while WASP deficiency affects B-cell development, decreasing the numbers of T1/T2 immature B-cells and marginal zone B-cells. Intriguingly, the protein expression levels of N-WASP and WAVE-2, homologues of WASP, increase in WASP-/- B-cells, implicating a compensatory role for WASP homologues in the absence of WASP. Over-expression of N-WASP's proline-rich domain inhibits BCR-mediated antigen uptake and intracellular transport. All of these data indicate that Btk, which is activated upon BCR binding to antigen, regulates actin dynamics and consequently antigen uptake and transport, by activating WASP and N-WASP via Vav and phosphatidylinositides. This presents a novel mechanism by which BCR-mediated signaling regulates BCR-mediated antigen processing and presentation.