Focal Adhesion Kinase Regulates Critical Functions in Hematopoiesis.

Focal adhesion kinase (FAK) initially identified as a unique cytoplasmic tyrosine kinase involved in focal adhesions, has been studied extensively in fibroblasts. In these cells, FAK has been shown to play an essential role in bridging signals between integrin and growth factor receptors such as the PDGF and the EGF receptor. In fibroblasts, FAK localizes to regions of the cell that attach to the extracellular matrix and coordinates signals from integrins, cytokines, growth factor receptors, and oncogenes. In some tumors, FAK is over-expressed or constitutively activated, which correlates with increased motility, invasiveness, and proliferation. More recently, expression of FAK in acute myeloid leukemia was associated with enhanced blast migration, increased cellularity, and poor prognosis. However, virtually nothing is known about FAKs role in normal hematopoiesis. FAK is expressed in blood cells, including in bone marrow derived KIT+, Gr-1+, Mac-1+, CD4+, CD8+ and B220+ cells. To determine how loss of FAK affects hematopoiesis, we have generated a mouse model with hematopoietic restricted deletion of FAK. We deleted FAK in bone marrow cells by crossing the FAK-flox mice to Mx.Cre+ expressing mice and by treating Mx.cre+FAK+/+ and Mx.cre+FAKflox/flox mice with poly (I)-poly(C) and then analyzing mice 1 month after the last injection. After one month of poly(I)-poly(C) induction, the progeny failed to express detectable levels of FAK in bone marrow, spleen as well as in bone marrow derived macrophages as determined by PCR and western blotting. Evaluation of peripheral blood counts in control and FAK deleted mice revealed modest but significant differences in different lineages (WBC k/μl: FAK; 14 vs. FAK^−/−; 10.3, n=7, *p<0.05, LY k/μl: FAK; 10.48 vs. FAK^−/−; 7.26, n=7, *p<0.005, RBC k/μl: FAK; 9.76 X10⁶ vs. FAK^−/−;8.58 X10⁶ n=7 *p<0.003, PLT k/μl: FAK; 644 vs. FAK^−/−; 434 n=7 *p<0.007). Since macrophages express abundant levels of FAK and are rapidly recruited in large numbers to sites of infection, we initially examined the role of FAK in macrophages by creating a well studied model of aseptic thioglycolate-induced peritonitis. Our results demonstrate a ∼1.5 fold reduction in the migration of macrophages to the peritoneal cavity of FAK−/− mice compared to controls (n=5, FAK; 1.8 X 10⁶ vs. FAK^−/−; 1.213 X10⁶, *p<0.03). The reduction in recruitment of FAK−/− macrophages was observed in spite of comparable levels of F4/80 expression (WT; 92.98% vs. FAK^−/−; 94.55%) as well as integrin (α4β1 & α5β1) expression (WT; 68% & 83.79% vs. FAK^−/−; 60.39% & 83.17%, respectively) between WT and FAK−/− macrophages. Further analysis of FAK−/− macrophages revealed a significant decrease in extracellular matrix/integrin directed migration of these cells in response to M-CSF on fibronectin (40% reduction), laminin (55% reduction) and collagen (60% reduction) (n=3, *p<0.004) coated plates as well as a decrease in migration in a wound healing assay (n=3, *p<0.003). The reduction in migration of FAK−/− macrophages was associated with a significant decrease in adhesion on fibronectin (63%), laminin (52%) and collagen (56%) as well as spreading (n=3, *p<0.03). Taken together, our results provide a critical physiologic role for FAK in regulating several adhesive and migratory functions in cells of myeloid lineage. Additional functions of FAK in other hematopoietic lineages will be discussed.