CSF3 (GCSF) is a cytokine that regulates production of neutrophils and granulocytes (reviewed in Panopoulos and Watowich 2008). CSF3 circulates extracellularly as a dimer and binds to the monomeric receptor CSF3R (GCSFR) on neutrophil precursors and mature neutrophils (reviewed in Futosi et al. 2013). CSF3R possesses no catalytic activity of its own and is constitutively associated with the kinases LYN (Corey et al. 1994) and JAK1 (Nicholson et al. 1994). Upon binding the CSF3 dimer, CSF3R dimerizes, is phosphorylated, and activates JAK-STAT signaling, RAS-RAF-MEK-ERK signaling, and PI3K signaling (reviewed in Basu et al. 2002, Roberts et al. 2005, Kendricks and Bogoyevitch 2007, Touw and van de Geijn 2007).
After dimerization of CSF3R, JAK1 associated with CSF3R is required for phosphorylation of tyrosine residues in the cytosolic domain of CSF3R which recruit further kinases such as JAK2, SYK, HCK, and TYK2 (reviewed in Sampson et al. 2007). Phosphorylated JAK1 and JAK2 then appear to act redundantly to phosphorylate STAT proteins (STAT1, STAT3, STAT5) which dimerize and transit to the nucleus to activate gene expression.
CSF3 signaling also activates the RAS pathway, resulting in activation of ERK1 and ERK2 and cellular proliferation. Phosphorylated CSF3R recruits both GRB2, which can act as a scaffold for RAS guanyl exchange factors SOS and VAV, and PTPN11 (SHP2), which activates RAS by dephosphorylating tyrosine-32 of RAS (Bunda et al. 2015). Association of SOS or VAV with the phosphorylated CSF3R has not yet been shown. The pathway to activation of PI3K is uncertain but appears to proceed via GAB2 bound to CSF3R.
Mutations in CSF3R can occur during the course of Kostmann disease, a severe congenital neutropenia (reviewed in Zeidler and Welte 2002, Zeidler 2005, Ward 2007, Vandenberghe and Beel 2011). Somatic mutations in CSF3R, principally truncations of the C-terminal region, are involved in the pathogenesis of severe congenital neutropenia and are associated with progression to acute myeloid leukemia (Dong et al. 1995, reviewed in Ward 2007, Beekman and Touw 2010, Xing and Zhao 2016). Loss or mutation of the C-terminal region of CSF3R interferes with inhibition and turnover of the receptor. Mutation of Thr-618 to Ile-618 in CSF3R causes spontaneous dimerization and consequent autoactivation leading to CSF3-independent signaling and chronic neutrophilic leukemia (Maxson et al. 2013).