Colony stimulating factor 1 receptor (CSF1R), also known as macrophage colony-stimulating factor receptor (M-CSFR), and CD115 (Cluster of Differentiation 115), is a cell-surface protein encoded by the human CSF1R gene (known also as c-FMS). CSF1R is a receptor that can be activated by two ligands: colony stimulating factor 1 (CSF-1) and interleukin-34 (IL-34). CSF1R is highly expressed in myeloid cells, and CSF1R signaling is necessary for the survival, proliferation, and differentiation of many myeloid cell types in vivo and in vitro. CSF1R signaling is involved in many diseases and is targeted in therapies for cancer, neurodegeneration, and inflammatory bone diseases.
Gene
In the human genome, the CSF1R gene is located on chromosome 5 (5q32), and in mice the Csf1r gene is located on chromosome 18 (18D). CSF1R is 60.002 kilobases (kbs) in length. Hematopoietic stem cells express CSF1R at low levels, but CSF1R is highly expressed in more differentiated myeloid cell types such as monocytes, macrophages, osteoclasts, myeloid dendritic cells, microglia, and Paneth cells. CSF1R expression is controlled by two alternative promoters that are active in specific tissue types. Exon 1 of CSF1R is specifically transcribed in trophoblastic cells whereas exon 2 is specifically transcribed in macrophages. Activation of CSF1R transcription is regulated by several transcription factors including Ets and PU.1. Macrophage expression of the CSF1R gene is regulated by the promoter upstream of exon 2 and another highly conserved region termed the fms intronic regulatory element (FIRE). The FIRE is a 250-bp region in intron 2 that regulates transcript elongation during transcription of CSF1R in macrophages. Specific deletion of FIRE prevents differentiation of only specific macrophage types such as brain microglia and macrophages in the skin, kidney, heart, and peritoneum whereas deletion of the entire mouse Csf1r gene widely prevents macrophage differentiation, causing profound developmental defects. Additionally, the first intron of the CSF1R gene contains a transcriptionally inactive ribosomal protein L7 processed pseudogene, oriented in the opposite direction to the CSF1R gene.
Protein
CSF1R, the protein encoded by the CSF1R gene is a tyrosine kinase transmembrane receptor and member of the CSF1/PDGF receptor family of tyrosine-protein kinases. CSF1R has 972 amino acids, is predicted to have a molecular weight of 107.984 kilodaltons, and is composed of an extracellular and a cytoplasmic domain. The extracellular domain has 3 N-terminal immunoglobulin (Ig) domains (D1-D3) which bind ligand, 2 Ig domains (D4-D5) which stabilize the ligand, a linker region, and a single-pass transmembrane helix. The cytoplasmic domain has a juxtamembrane domain and tyrosine kinase domain that is interrupted by a kinase insert domain. At rest, the juxtamembrane domain of CSF1R enters an autoinhibitory position to prevent signaling of the CSF1R cytosolic domain. Upon binding of ligand to extracellular Ig domains, CSF1R dimerizes noncovalently and autophosphorylates several tyrosine residues. This first wave of CSF1R tyrosine phosphorylation creates phosphotyrosine-binding domains to which effector proteins can bind and initiate various cellular responses. Many proteins become tyrosine phosphorylated in response to CSF1R signaling (Table 1) including p85, Cbl, and Gab3 which are important for survival, differentiation, chemotaxis, and actin cytoskeleton of myeloid cells. The first wave of tyrosine phosphorylation also leads to the covalent dimerization of CSF1R via disulfide bonds. Covalent CSF1R dimerization is important for a series of modifications to CSF1R itself including a second wave of tyrosine phosphorylation, serine phosphorylation, ubiquitination, and eventually endocytosis which terminates signaling by trafficking the ligand-CSF1R complex to the lysosome for degradation. Colony stimulating factor 1 (CSF-1) and interleukin-34 (IL-34) are both CSF1R ligands. Both ligands regulate myeloid cell survival, proliferation, and differentiation, but CSF-1 and IL-34 differ in their structure, distribution in the body, and the specific cellular signaling cascades triggered upon binding to CSF1R. +Table 1. CSF-1R signaling proteins in myeloid cellsProteinFull protein name; functionSFKSrc family tyrosine kinasesGrb2AdaptorMonaMonocyte adaptor; adaptorSocs1Suppressor of cytokine signaling-1; adaptorPLCγPhospholipase C-γp85 PI3KRegulatory subunit of PI3KCblCasitas B lineage; ubiquitin ligase, adaptorFMIPFMS-interacting protein; function unknownPP2AProtein phosphatase 2A; serine/threonine phosphatasePyk2Proline-rich and Ca2C-activated tyrosine kinasePaxilinFocal complex adaptorPTPφPTP for phosphopaxillinMAYP/PSTPIP2Macrophage actin-associated and tyrosine-phosphorylated protein; actin bundlingIba1Ionized Ca2C-binding adaptor protein 1; actin bundlingGab2Grb2-associated binder-2; AdaptorGab3Grb2-associated binder-3; adaptorSHIP1SH2-domain-containing polyinositol phosphatase-1SHP1SH2-domain-containing phosphatase-1; PTPSHP2SH2-domain-containing phosphatase-2; PTPPKC-δProtein kinase C-dPkarePKA-related gene (Pkare); protein kinaseMysPDZ110-kDa myosin XVIIIASTAT1, STAT3, STAT5Signal transducers and activators of transcription-1, -2, -3; transcription factorsDok1, Dok2, Dok3Downstream of kinase-1, -2, -3; adaptorsVavRho family guanine-nucleotide-exchange factorBLIMP-1B-lymphocyte-induced maturation protein-1; transcriptional repressor
Function
Osteoclasts
Osteoclast are multi-nucleated cells that absorb and remove bone which is critical for growth of new bones and maintenance of bone strength. Osteoclasts are critical for the bone remodeling cycle which is achieved by the building of bone by osteoblasts, reabsorption by osteoclasts, and remodeling by osteoblasts. Osteoclasts precursor cells and mature osteoclast require stimulation of CSF1R for survival. Blockage of CSF1R signaling prevents osteoclast precursor cells from proliferating, maturing, and fusing into multi-nucleated cells. Stimulation of CSF1R promotes osteoclastogenesis (differentiation of monocytes into osteoclasts). CSF1R signaling in osteoclasts precursors promotes survival by upregulation of the Bcl-X(L) protein, an inhibitor of pro-apoptotic caspase-9. CSF1R signaling in mature osteoclasts promotes survival by stimulating mTOR/S6 kinase and the Na/HCO3 co-transporter, NBCn1. CSF1R signaling also directly regulates osteoclast function. Osteoclasts migrate along the bone surface, then adhere to the bone to degrade and reabsorb the bone matrix. CSF1R signaling positively regulates this behavior, increasing osteoclast chemotaxis and bone reabsorption.
Monocytes and macrophages
Monocytes and macrophages are mononuclear phagocytes. Monocytes circulate in the blood and are capable of differentiating into macrophages or dendritic cells, and macrophages are terminally differentiated tissue-resident cells. CSF1R signaling is necessary for differentiation of microglia and Langerhans cells which are derived from yolk sac progenitor cells with high expression of CSF1R. CSF1R signaling is only partially required for other tissue macrophages, and it is not necessary for monocytopoiesis (production of monocytes and macrophages) from hematopoietic stem cells. Macrophages of thymus and lymph nodes are almost completely independent of CSF1R signaling. In macrophages whose survival is fully or partially dependent on CSF1R signaling, CSF1R promotes survival by activating PI3K. CSF1R signaling also regulates macrophage function. One function of CSF1R signaling is to promote tissue protection and healing following damage. Damage to the kidney causes upregulation of CSF-1 and CSF1R in tubular epithelial cells. This promotes proliferation and survival of injured tubular epithelial cells and promotes anti-inflammatory phenotypes in resident macrophage to promote kidney healing. Lastly, activation of CSF1R is a strong chemokinetic signal, inducing macrophage polarization and chemotaxis towards the source of CSF1R ligand. This macrophage response requires rapid morphological changes which is achieved by remodeling of the actin cytoskeleton via the Src/ Pyk2 and PI3K signaling pathways.
Microglia
Microglia are the tissue-resident phagocytes of the central nervous system. CSF1R signaling promotes migration of primitive microglia precursor cells from the embryonic yolk sac to the developing brain prior to formation of the blood-brain-barrier. In perinatal development, microglia are instrumental in synaptic pruning, a process in which microglia phagocytose weak and inactive synapses via binding of microglial complement receptor 3 (CR3) (complex of CD11b and CD18) to synapse-bound iC3b. Csf1r loss-of-function inhibits synaptic pruning and leads to excessive non-functional synapses in the brain. In adulthood, CSF1R is required for the proliferation and survival of microglia. Inhibition of CSF1R signaling in adulthood causes near-complete (>99%) depletion (death) of brain microglia, however reversal of CSF1R inhibition stimulates remaining microglia to proliferate and repopulate microglia-free niches in the brain. Production of CSF1R ligands CSF-1 and IL-34 is increased in the brain following injury or viral infection, which directs microglia to proliferate and execute immune responses.
Neural progenitor cells
CSF1R signaling has been found to play important roles in non-myeloid cells such as neural progenitor cells, multipotent cells that are able to self-renew or terminally differentiate into neurons, astrocytes and oligodendrocytes. Mice with Csf1r loss-of-function have a significantly more neural progenitor cells in generative zones and fewer matured neurons in Forebrainforebrain laminae due to failure of progenitor cell maturation and radial migration. These phenotypes were also seen in animals with Csf1r conditional knock-out specifically in neural progenitor cells, suggesting that CSF1R signaling by neural progenitor cells is important for maturation of certain neurons. Studies using cultured neural progenitor cells also show that CSF1R signaling stimulates neural progenitor cells maturation.
Germline cells CSF1R is expressed in oocytes, the trophoblast, and fertilized embryos prior to implantation in the uterus. Studies using early mouse embryos in vitro have shown that activation of CSF1R stimulates formation of the blastocyst cavity and enhances the number of trophoblast cells. Csf1r loss-of-function mice exhibit several reproductive system abnormalities in the estrous cycle and ovulation rates as well as reduced antral follicles and ovarian macrophages. It is not clear whether ovulation dysfunction in Csf1r loss-of-function mice is due to loss of the protective effects of ovarian macrophages or loss of CSF1R signaling in oocytes themselves.
Clinical significance
Bone disease