Fps/Fes Kinase Regulates Cytoskeletal Reorganization and Migration of Mast Cells
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Mast cells are granulocytes that require signaling from the receptor protein-tyrosine kinase Kit and its ligand stem cell factor (SCF) for their maturation and function. In addition to providing growth and survival signals, the Kit receptor is involved in crosstalk to β1 integrins leading to mast cell adhesion, spreading, and migration on fibronectin (FN). Previous studies reported the involvement of the non-receptor protein-tyrosine kinases Fps/Fes and Fer in signaling downstream of the high affinity IgE receptor in mast cells and identified cell migration defects in Fer-deficient bone marrow-derived mast cells (BMMCs). Fps/Fes also becomes phosphorylated downstream of the Kit receptor in BMMCs, and this involves the action of the Src family kinase Fyn as an upstream activator of Fps/Fes. In this study, the Fps/Fes SH2 domain was observed to bind the phosphorylated Kit receptor in vitro, suggesting that the SH2 domain plays a role in the activation mechanism of Fps/Fes. To investigate the function of Fps/Fes in Kit signaling, BMMCs were generated from wild-type and Fps/Fes-null mice. Analysis of downstream effectors revealed that Fps/Fes is required for maximal p38 MAPK signaling. Further examination of Fps/Fes-deficient BMMCs revealed increases in adhesion, spreading, and a defect in cell polarization on full-length FN (a ligand for multiple β1 integrins), compared to wild-type BMMCs. Similar phenotypes are observed using an α5β1 integrin-specific FN fragment (9-11) as the matrix. Reduced phosphorylation of the putative Fps/Fes substrate HS1 (a cortactin homologue involved in actin regulation) is observed in Fps/Fes-deficient BMMCs, compared to control cells, and this may contribute to the observed cytoskeletal defects. Restoring Fps/Fes expression in Fps/Fes-deficient BMMCs by retroviral transduction results in a rescue of cell spreading, polarization, and chemotaxis defects to levels similar to those of wild-type cells. This thesis provides novel insights into the potential mode of Fps/Fes activation downstream of the Kit receptor, and a role for Fps/Fes in regulating crosstalk between Kit and α5β1 integrins to promote cytoskeletal reorganization and motility of mast cells.