Protein-tyrosine kinase, Syk, is required for CXCL12-induced polarization of B cells.

Cell polarization and migration in response to CXCL12 is essential for hematopoiesis. To investigate the role of Syk in CXCL12/CXCR4-induced signaling, wild-type Syk or its dominant-negative form (DN-Syk) was introduced in mouse pro-B cells, BAF3. With CXCL12 stimulation, BAF3 cells became polarized with the formation of a leading edge and contractile uropod at the rear end with increased motility. Overexpression of wild-type Syk caused enhanced polarization, whereas DN-Syk inhibited cell polarity due to the loss of contractile structure at the rear end, and the altered phenotype was enhanced after CXCL12 stimulation. Motility of mutant BAF3 containing DN-Syk increased independent of CXCL12 stimulation. As beta1 integrin-mediated cell adhesion was inhibited, decreased adhesion might promote motility. CXCL12 stimulation led to prompt activation of RhoA, but expression of DN-Syk suppressed RhoA activation. These results demonstrate that Syk participates in CXCL12-induced cell polarization, which occurs in concert with cell adhesion mediated by beta1 integrin.