The role of the fps/fes proto-oncogene in development and innate immunity

Fps/Fes and Fer comprise a distinct subfamily of non-receptor protein tyrosine kinases. fps/fes was originally isolated as a oncogene, while no oncogenic isolates of fer have been found. In tissue culture models these proteins have been shown to function in cytoskeletal organization, signalling from cell surface receptors, cellular proliferation and apoptosis. Since no human or mouse mutations have been described, these cell culture models represented a first line of investigation. However, the ultimate test of a proteins role in signalling must be performed in a physiological setting. In order to determine the role Fps/Fes and Fer play in normal development and cellular signalling we made three mouse models. As a first step, the mouse fps/fes gene was cloned and completely sequenced and for the first time the genomic organization of the mammalian fer gene was described. Using homologous recombination, mice that were incapable of expressing any Fps/Fes protein (Fps null) or expressed a catalytically inactive Fps/Fes protein (Fps KR) were made. The third mouse model expressed a catalytically inactive Fer protein (Fer DR). Surprisingly, all three mouse models were viable, questioning the functions that were described for these proteins using tissue culture models. The Fps null mice displayed minor defects in hematopoietic hemostasis. In addition, we described a role for Fps/Fes in innate immunity. These mice have a defect in erythropoietin receptor signalling that leads to an increase in erythrocyte production, hemoglobin content and hematocrit. These defects could all be rescued with a human fps/fes transgene, demonstrating that the defects are Fps/Fes specific. Fps/Fes has been described to signal downstream from the granulocyte macrophage-colony stimulating factor receptor (GM-CSFR), however, our mice showed no noticeable defects in the activation of signal transducers and activators of transcription (STAT) proteins downstream from the GM-CSFR. Since we showed that Fps/Fes played a role in innate immunity we also looked at lippopolysaccharide (LPS) signalling mediated by the Toll-like receptor 4 (TLR4). We could not demonstrate any defects in the pathways that are normally activated by LPS downstream from TLR4. Based upon analysis using compound homozygous mice we conclude that Fps/Fes and Fer appear to play redundant roles in myelopoiesis, lymphopoiesis and fertility. The subcellular localization of Fps/Fes indicated a possible role for the kinase in vesicular trafficking. Fps/Fes was localized to the endocytic and exocytic compartments, and may thus be involved with regulating secretion or internalization of signalling complexes. This might affect the release of cytokines and other events secondary to receptor signalling and help explain why we did not see defects directly downstream from some of the cell surface receptors. In the future, these mice will be invaluable tools in elucidating the functions Fps/Fes and Fer play in normal development and cellular signalling.