Focal Adhesion Kinase (FAK) Is Required for CXCL12-Induced Dhemotaxis and Adhesion to VCAM-1 in B Lymphocytes and Hematopoietic Stem/Progenitor Cells.

The chemokine CXCL12 (SDF-1) and its receptor CXCR4 play a critical role in hematopoietic progenitor cell migration and positioning within the bone marrow (BM). However, CXCL12/CXCR4-induced signaling pathways in blood cell migration are poorly characterized. Based on previous studies of BM progenitor B cells indicating a strong correlation between FAK signaling and CXCL12 induced adhesion to VCAM-1, we speculated that FAK might be an important signaling component in CXCL12-induced chemotaxis and integrin-mediated adhesion. Here, we used two approaches to reduce FAK expression in (human and mouse) progenitor B cells and mouse Sca-1⁺, Kit⁺, Lin⁻ stem/progenitor cells. FAK specific siRNAs reduced FAK expression by 80% and abolished both CXCL12-induced chemotaxis and adhesion to VCAM-1 in the pro-B cell line, REH. FAK knock-down did not change expression levels of CXCR4 and VLA-4 integrin. FAK expression was rescued by transfection with wild type, chicken FAK, which also restored both CXCL12-induced chemotaxis and adhesion. Furthermore, we found that in FAK deficient cells CXCL12-induced activation of the GTP-ase Rap1 was reduced, suggesting the importance of FAK in CXCL12-mediated inside-out integrin activation. CXCL12-mediated chemotaxis was also impaired in primary progenitor B cells and hematopoietic stem/progenitor cells (HSC/P) isolated from FAK floxed mice, in which FAK was deleted by Cre-mediated excision of FAK floxed alleles. Cre-mediated FAK deletion did not affect cell viability or induce apoptosis. These studies suggest that FAK may function as a key intermediary in signaling pathways controlling hematopoietic cell migration and lineage development.