Swiprosin-1/EFhd2 controls B cell receptor signaling through the assembly of the B cell receptor, Syk, and phospholipase C gamma2 in membrane rafts

Compartmentalization of the BCR in membrane rafts is important for its signaling capacity. Swiprosin-1/EFhd2 (Swip-1) is an EF-hand and coiled-coil-containing adaptor protein with predicted Src homology 3 (SH3) binding sites that we identified in membrane rafts. We showed previously that Swip-1 amplifies BCR-induced apoptosis; however, the mechanism of this amplification was unknown. To address this question, we overexpressed Swip-1 and found that Swip-1 amplified the BCR-induced calcium flux in WEHI231, B62.1, and Bal17 cells. Conversely, the BCR-elicited calcium flux was strongly attenuated in Swip-1-silenced WEHI231 cells, and this was due to a decreased calcium mobilization from intracellular stores. Complementation of Swip-1 expression in Swip-1-silenced WEHI231 cells restored the BCR-induced calcium flux and enhanced spleen tyrosine kinase (Syk) tyrosine phosphorylation and activity as well as SLP65/BLNK/BASH and phospholipase C gamma2 (PLCgamma2) tyrosine phosphorylation. Furthermore, Swip-1 induced the constitutive association of the BCR itself, Syk, and PLCgamma2 with membrane rafts. Concomitantly, Swip-1 stabilized the association of BCR with tyrosine-phosphorylated proteins, specifically Syk and PLCgamma2, and enhanced the constitutive interaction of Syk and PLCgamma2 with Lyn. Interestingly, Swip-1 bound to the rSH3 domains of the Src kinases Lyn and Fgr, as well as to that of PLCgamma. Deletion of the predicted SH3-binding region in Swip-1 diminished its association and that of Syk and PLCgamma2 with membrane rafts, reduced its interaction with the SH3 domain of PLCgamma, and diminished the BCR-induced calcium flux. Hence, Swip-1 provides a membrane scaffold that is required for the Syk-, SLP-65-, and PLCgamma2-dependent BCR-induced calcium flux.

The novel adaptor protein Swiprosin-1 enhances BCR signals and contributes to BCR-induced apoptosis

B-cell receptor (BCR) signals are essential for B-cell differentiation, homeostasis and negative selection, which are regulated by the strength and quality of BCR signals. Recently, we identified a new adaptor protein, Swiprosin-1, in lipid rafts of B-cell lines that undergo apoptosis after BCR stimulation. During murine B-cell development, Swiprosin-1 exhibited highest expression in immature B cells of the bone marrow, but was also expressed in resting and activated splenic B cells and in non-lymphoid tissue, especially in the brain. Ectopic expression of Swiprosin-1 in the immature murine B-cell line WEHI231 enhanced spontaneous and BCR-induced apoptosis. In contrast, short hairpin RNA (shRNA)-mediated downregulation of Swiprosin-1 impaired specifically spontaneous and BCR-elicited apoptosis, but not BCR-induced G1 cell cycle arrest and upregulation of the cell cycle inhibitor p27(Kip1). In accordance, Swiprosin-1 abundance regulated net cell growth of WEHI231 cell populations through reciprocal regulation of Bcl-xL, but not Bim, thereby controlling spontaneous apoptosis. Swiprosin-1-enhanced apoptosis was blocked through nuclear factor kappaB-activating stimuli, namely B-cell-activating factor of the TNF family, anti-CD40 and lipopolysaccharide (LPS). This correlated with enhanced BCR-induced IkappaB-alpha phosphorylation and degradation in cells expressing a Swiprosin-1-specific shRNA. Finally, ectopic Swiprosin-1 expression enhanced BCR-induced cell death in primary, LPS-stimulated splenic B cells. Hence, Swiprosin-1 may regulate lifespan and BCR signaling thresholds in immature B cells.