Meta-Biol

• Pathways

Colony stimulating factor-1 (CSF1, CSF-1, also called macrophage colony stimulating factor, M-CSF) is a disulfide-linked dimer that stimulates the proliferation and differentiation of mononuclear phagocytes and the survival, proliferation, motility, and anti-inflammatory activity of macrophages (reviewed in Mouchemore et al. 2012, Stanley and Chitu 2014, Ushach and Zlotnik 2016, Dwyer et al. 2017 and inferred from mouse homologs in Caescu et al. 2015). The unliganded CSF1 receptor, CSF1R (CSF-1R) is either clustered or undergoing rapid dimer-monomer transitions at the cell surface (Li and Stanley 1991). The CSF1 dimer initially binds the D2 and D3 extracellular domains of a monomer of CSF1R (Wang et al. 1993, Chihara et al. 2010, Ma et al. 2012, Felix et al. 2015, and inferred from mouse homologs). A second monomer of CSF1R then binds the CSF1:CSF1R complex and the resulting dimerization of CSF1R activates its kinase activity (Elegheert et al. 2011, Felix et al. 2015, and inferred from mouse homologs). CSF1R initially trans-autophosphorylates tyrosine-561 in the juxtamembrane domain, relieving negative autoinhibition of kinase activity, resulting in the trans-autophosphorylation of 7 more tyrosine residues in its cytoplasmic domain (Rohrschneider et al. 1997, Chihara et al. 2010, and inferred from mouse homologs in Xiong et al. 2011).
The PIK3R1 (p85alpha) regulatory subunit of phosphatidylinositol 3-kinase (PI3K) binds phosphotyrosine-723 of CSF1R, phosphorylated SRC binds phosphotyrosine-561 of CSF1R, phosphorylated CBL binds CSF1R associated with SHC, and GRB2:SOS binds CSF1R (Saleem et al. 1995, and inferred from mouse homologs). The resulting activation of the catalytic subunit of PI3K (PIK2CA,B,G) produces phosphatidylinositol 3,4,5-trisphosphate which recruits effectors containing pleckstrin homology domains (PH domains) such as PKB (also called Akt) to the plasma membrane. Pathways activated by PI3K appear to both enhance proliferation, survival, and migration of macrophages (reviewed in Dwyer et al. 2017) and, via induction of miR21, suppress the inflammatory response by targeting mRNAs encoding multiple proinflammatory molecules.
Phospholipase C gamma2 (PLCG2) binds phosphotyrosine-723 of CSF1R, hydrolyzes phosphatidylcholine to yield choline phosphate (phosphocholine) and diacylglycerol, and promotes survival and differentiation of macrophages via PKCdelta (PRKCD) (inferred from mouse homologs).
GRB2 bound to SOS1 (GRB2:SOS1) transiently interacts with phosphotyrosine-699 of CSF1R. SOS1 promotes the exchange of GDP for GTP by KRAS, activating the RAS-RAF-ERK1,2 pathway that causes proliferation of macrophage precursors (inferred from mouse homologs). CBL transiently associates with and ubiquitinates the CSF1R, then is deubiquitinated and returned to the cytoplasm (inferred from mouse homologs).
Phosphorylated CSF1R also recruits STAT1 and STAT3, which are then phosphorylated (inferred from mouse homologs). The role of phosphorylated STAT1,3 in signaling by CSF1R is incompletely characterized.
CSF1R is a target for therapeutics, such as imatinib (reviewed in Kumari et al. 2018).

Cytokines are small proteins that regulate and mediate immunity, inflammation, and hematopoiesis. They are secreted in response to immune stimuli, and usually act briefly, locally, at very low concentrations. Cytokines bind to specific membrane receptors, which then signal the cell via second messengers, to regulate cellular activity.

Humans are exposed to millions of potential pathogens daily, through contact, ingestion, and inhalation. Our ability to avoid infection depends on the adaptive immune system and during the first critical hours and days of exposure to a new pathogen, our innate immune system.