Macrophage colony stimulating factor (M-CSF) is a microglial activator that is important in the cerebral inflammatory response. M-CSF expression is increased in AD and in the PDAPP transgenic mouse model for AD. The M-CSF receptor (M-CSFR) is increased on microglia in AD, in the PDAPP mouse, and in experimental models for brain injury. Overexpression of M-CSFR on microglia results in proliferation, increased expression of cytokines, and paracrine activation of other inflammatory cells. M-CSFR overexpression also results in aggressive microglial phagocytosis of Abeta by both FcgammaR-dependent and - independent mechanisms. Surprisingly, overexpression of M-CSFR on microglia protects neurons from excitotoxic injury in slice cultures transfected using biolistics, and in a microglial-hippocampal organotypic co-culture system. Also, M-CSF protects neurons in organotypic cultures from excitotoxic injury. In the proposed experiments, M-CSFR-induced microglial expression of potentially protective neurotrophins, cytokines, chemokines, and nitric oxide will be quantified after NMDA treatment in the biolistic transfection model and in the co-culture system. To localize differential expression to individual cell types, immunohistochemistry and laser capture microdissection will be used to obtain homogeneous populations of cells from slice cultures for real-time RT-PCR. To identify neuroprotective factors, hammerhead ribozymes will be used to block microglial expression and antibodies will be used to neutralize microglial factors in the co-culture. Candidate neuroprotective factors will also be tested for rescue of neurons from NMDA toxicity by adding them to organotypic culture medium. The signal transduction pathways activated by the M-CSFR that affect proliferation, cytokine expression, and phagocytosis are unknown. Hence, experiments will be performed to test activation of microglial Src kinases, PI3K, and Ras-mediated signaling, all of which are important in M-CSFR signal transduction in other cell types. These studies will define how activation of microglial M-CSFR protects neurons, and identify the molecular mechanisms by which the M-CSFR activates microglia and induces Abeta phagocytosis. Understanding the functions of M-CSFR and its ligand in microglia may lead to new strategies to promote Abeta clearance and to protect neurons in AD and other brain disorders. Much emphasis has been placed on the negative role of microglia. Although microglial neurotoxicity undoubtedly occurs in many disease states, it is important to understand how microglia might be induced to clear abnormal proteins from the brain or provide neuroprotection.
