or osteolysis. in animal models of rheumatoid arthritis, administration of csf-1 increases the severity of disease whereas csf1r loss-of-function reduces inflammation and joint erosion. in a rare bone disease called gorham‐stout disease, elevated production of csf-1 by lymphatic endothelial cells similarly produces excessive osteoclastogenesis and osteolysis. additionally, postmenopausal loss of estrogen has also been found to impact csf1r signaling and cause osteoporosis. estrogen deficiency causes osteoporosis by upregulating production of tnf-α by activated t cells. as in inflammatory arthritis, tnf-α stimulates stromal cells to produce csf-1 which increases csf1r signaling in osteoclasts. cancer schematic of tumor microenvironment of breast cancer. cancers secrete many signals including colony stimulating factor which activates tam csf1r and promotes tumor growth and survival. tumor-associated macrophages (tams) react to early stage cancers with anti-inflammatory immune responses that support tumor survival at the expense of healthy tissue. tumor infiltration by csf1r-expressing tams yields a negative prognosis and is correlated with poor survival rates for individuals with lymphoma and solid tumors. the tumor microenvironment often produces high levels of csf-1, creating a positive feedback loop in which the tumor stimulates survival of tams and tams promote tumor survival and growth. thus, csf1r signaling in tams is associated tumor survival, angiogenesis, therapy resistance, and metastasis. production of csf-1 by brain tumors called glioblastomas causes microglia (brain-resident macrophages) to exhibit immunosuppressive, tumor-permissive phenotypes. csf1r inhibition in mouse glioblastoma models is beneficial and improves survival by inhibiting tumor-promoting functions of microglia. mouse models of breast cancer also show that csf1r loss-of-function delays tam infiltration and metastasis. because anti-cancer macrophages and microglia rely on gm-csf and ifn-γ signaling instead csf-1, inhibition of csf1r signaling has been posited as a therapeutic target in cancer to preferentially deplete tumor-permissive tams. additionally, mutations in csf1r gene itself are associated with certain cancers such as chronic myelomonocytic leukemia and type m4 acute myeloblastic leukemia. adult-onset leukoencephalopathy because of the importance of the csf1r gene in myeloid cell survival, maturation, and function, loss-of-function in both inherited copies of the csf1r gene causes postnatal mortality. heterozygous mutations in the csf1r gene prevent downstream csf1r signaling and cause an autosomal dominant neurodegenerative disease called adult-onset leukoencephalopathy, which is characterized by dementia, executive dysfunction, and seizures. partial loss of csf1r in adult-onset leukoencephalopathy causes microglia to exhibit morphological and functional deficits (impaired cytokine production and phagocytosis) which is associated with axonal damage, demyelination, and neuronal loss. signaling by a dap12-trem2 complex in microglia is downstream of csf1r signaling and is needed for microglia phagocytosis of cellular debris and maintenance of brain homeostasis. trem2 deficiency in cultured myeloid cells prevents stimulation of proliferation by treatment with csf-1. similarities between nasu-hakola disease (caused by mutations in either dap12 or trem2) and adult-onset leukoencephalopathy suggest partial loss of microglia csf1r signaling promotes neurodegeneration. defects in neurogenesis and neuronal survival are also seen in adult-onset leukoencephalopathy due to impaired csf1r signaling in neural progenitor cells. other brain diseases and disorders schematic of brain microglia and infiltrating peripheral macrophages mounting immune responses against β-amyloid plaque. csf1r signaling is involved in several diseases and disorders of the central nervous system. research using animal models of epilepsy (kainic acid-induced seizures) suggests that csf1 signaling during seizures protects neurons by activating neuronal creb signaling. csf1r agonism during seizures increases neuronal survival whereas neuron-specific csf1r loss-of-function worsens kainic acid excitotoxicity, suggesting csf1r signaling in neurons directly protects against seizure-related neuronal damage. although csf1r signaling is beneficial in certain contexts, it is detrimental in diseases where microglia drive tissue damage. in charcot-marie-tooth disease type 1, csf-1 secretion from endoneurial cells stimulates proliferation and activation of macrophages and microglia that cause demyelination. likewise in multiple sclerosis, csf1r signaling supports the survival of inflammatory microglia which promote demyelination. csf1r inhibition prophylactically reduces demyelination in the experimental autoimmune encephalomyelitis animal model. the role of csf1r signaling in alzheimer's disease is more complicated because microglia both protect and damage the brain in response to alzheimer's disease pathology. csf-1 stimulates primary cultured human microglia to phagocytose toxic aβ1–42 peptides. microglia also initiate trem2-dependent immune responses to amyloid plaques which protects neurons. however, alzheimer's disease microglia also excessively secrete inflammatory cytokines and prune synapses promoting synapse loss, neuronal death, and cognitive impairment. both csf1r stimulation and inhibition improves cognitive function in alzheimer's disease models. thus, microglia seem to have both protective and neurotoxic functions during alzheimer's disease neurodegeneration. similar findings have been reported in lesion studies of the mouse brain, which showed that inhibition of csf1r after lesioning improves recovery but inhibition during lesioning worsens recovery. csf1r-targeting therapies for neurological disorders may impact both detrimental and beneficial microglia functions. therapeutics chemical structure of pexidartinib (plx3397), an fda-approved small molecule inhibitor of csf1r. structure of monoclonal antibody emactuzumab light and heavy chain complexed with csf1r. because tam csf1r signaling is tumor-permissive and can tumor treatment-resistance, csf1r signaling is a promising therapeutic target in the treatment of cancer. several studies have investigated the efficacy of csf1r inhibitor as a monotherapy and as a combination therapy in refractory and metastatic cancers. several small molecule inhibitors and monoclonal antibodies targeting csf1r are in clinical development for cancer therapy (table 2). pexidartinib (plx3397) is a small molecule inhibitor tyrosine of csfr (as well as ckit, flt3, and vegfr) with the most clinical development so far. several completed and concurrent clinical trials have tested the efficacy and safety of pexidartinib as a monotherapy for c-kit-mutated melanoma, prostate cancer, glioblastoma, classical hodgkin lymphoma, neurofibroma, sarcoma, and leukemias. in 2019, pexidartinib was fda-approved for treatment of diffuse-type tenosynovial giant cell tumors, a non-malignant tumor that develops from synovial tissue lining the joints. table 2. csf1r-targeted therapies in clinical development drug name form targets clinical trial diseases pexidartinib (plx3397) small molecule csf1r, c-kit, vegfr, and flt3 autoimmune diseases, alzheimer's disease, leukemia, acral melanoma, mucosal melanoma imatinib small molecule csf1r, abl, c-kit, and pdgfr-β osteoporosis, osteolysis, chronic myeloid leukemia (cml), breast cancer plx5622 small molecule csf1r rheumatoid arthritis, cancer, neuropathic pain, alzheimer's disease sotuletinib (blz945) small molecule csf1r, c-kit, pdgfrβ, and flt3 solid tumors, amyotrophic lateral sclerosis gw2580 small molecule csf1r arthritis, osteoporosis, cancer ki20227 small molecule csf1r, vegfr2, c-kit, and pdgfrβ osteolysis, breast cancer edicotinib (jnj-40346527) small molecule csf1r, c-kit, and flt3 alzheimer's disease, chl, rheumatoid arthritis, neurodegenerative diseases emactuzumab (rg7155) monoclonal antibody csf1r solid tumors imc-cs4 (ly3022855) monoclonal antibody csf1r solid tumors, breast cancer, prostate cancer amg820 monoclonal antibody csf1r solid tumors safety of csf1r inhibition the safety of csf1r inhibitors has been extensively characterized in clinical trials for the different small molecules and monoclonal antibodies in table 2. in some studies, csf1r inhibitors were not found to have dose-limiting toxicity while other studies did observe toxicity at high doses and have defined a maximum tolerated dose. across multiple studies, the most frequent adverse effects included fatigue, elevated liver enzymes (creatine kinase, lactate dehydrogenase, aspartate aminotransferase, alanine transaminase), edema, nausea, lacrimation, and reduced appetite, but no signs of liver toxicity were found. there are some differences in the side effects of monoclonal antibody compared to small molecule csf1r inhibitors. edema was more common with monoclonal antibody treatment compared to small molecules, suggesting that immune response to monoclonal antibodies may drive some side effects. additionally, some small molecule inhibitors are not specific for csf1r, and off-target effects could explain observed side effects. for example, pexidartinib treatment was found to change hair color, presumably by its impact on kit kinase. overall, csf1r inhibitors have favorable safety profiles with limited toxicity. controversy csf1r inhibitors such as plx5622 are widely used to study the role of microglia in mouse preclinical models of alzheimer's disease, stroke, traumatic brain injury, and aging. plx5622 is typically used for microglia research because plx5622 has higher brain bioavailability and csf1r-specificity compared to other csf1r inhibitors such as plx3397. in 2020, researchers david hume (university of queensland) and kim green (uci) published a letter in the academic journal pnas defending the use small molecule csf1r inhibitors to study microglia in brain disease. this letter was in response to a primary research paper published in pnas by lead correspondent eleftherios paschalis (hms) and others which provided evidence that microglia research using plx5622 is confounded by csf1r inhibition in peripheral macrophages. paschalis and colleagues published a subsequent letter in pnas defending the findings of their published research. interactions colony stimulating factor 1 receptor has been shown to interact with: cbl gene, fyn, grb2, suppressor of cytokine signaling 1, this receptor is also linked with the cells of mps.