Tyrosine residues of the granulocyte colony-stimulating factor receptor transmit proliferation and differentiation signals in murine bone marrow cells

Granulocyte colony-stimulating factor (G-CSF) is the major regulator of granulopoiesis and acts through binding to its specific receptor (G-CSF-R) on neutrophilic granulocytes. Previous studies of signaling from the 4 G-CSF-R cytoplasmic tyrosine residues used model cell lines that may have idiosyncratic, nonphysiological responses. This study aimed to identify specific signals transmitted by the receptor tyrosine residues in primary myeloid cells. To bypass the presence of endogenous G-CSF-R, a chimeric receptor containing the extracellular domain of the epidermal growth factor receptor in place of the entire extracellular domain of the G-CSF-R was used. A series of chimeric receptors containing tyrosine mutations to phenylalanine, either individually or collectively, was constructed and expressed in primary bone marrow cells from G-CSF-deficient mice. Proliferation and differentiation responses of receptor-expressing bone marrow cells stimulated by epidermal growth factor were measured. An increased 50% effective concentration to stimulus of the receptor Y_null mutant indicated that specific signals from tyrosine residues were required for cell proliferation, particularly at low concentrations of stimulus. Impaired responses by mutant receptors implicated G-CSF-R Y₇₆₄ in cell proliferation and Y₇₂₉ in granulocyte differentiation signaling. In addition, different sensitivities to ligand stimulation between mutant receptors indicated that G-CSF-R Y₇₄₄ and possibly Y₇₂₉ have an inhibitory role in cell proliferation. STAT activation was not affected by tyrosine mutations, whereas ERK activation appeared to depend, at least in part, on Y₇₆₄. These observations have suggested novel roles for the G-CSF-R tyrosine residues in primary cells that were not observed previously in studies in cell lines.