Macrophage colony stimulating factor (CSF1) is a cytokine that is upregulated in several diseases of the central nervous system (CNS). To examine the effects of CSF1 over-expression on microglia, transgenic mice that over-express CSF1 in the glial fibrillary acidic protein (GFAP) compartment were generated. CSF1 over-expressing mice have increased microglial proliferation and increased microglial numbers compared to controls. Treatment with PLX3397, a small molecule inhibitor of the CSF1 receptor CSF1R and related kinases, decreases microglial numbers by promoting microglial apoptosis in both CSF1 over-expressing and control mice. Microglia in CSF1 over-expressing mice exhibit gene expression profiles indicating that they are not basally M1 or M2 polarized, but they do have defects in inducing expression of certain genes in response to the inflammatory stimulus lipopolysaccharide (LPS). These results indicate that the CSF1 over-expression observed in CNS pathologies likely has pleiotropic influences on microglia. Furthermore, small molecule inhibition of CSF1R has the potential to reverse CSF1-driven microglial accumulation that is frequently observed in CNS pathologies, but can also promote apoptosis of normal microglia.

Homologous recombination (HR) is critical for the repair of double strand breaks and broken replication forks. Although HR is mostly error free, inherent or environmental conditions that either suppress or induce HR cause genomic instability. Despite its importance in carcinogenesis, due to limitations in our ability to detect HR in vivo, little is known about HR in mammalian tissues. Here, we describe a mouse model in which a direct repeat HR substrate is targeted to the ubiquitously expressed Rosa26 locus. In the Rosa26 Direct Repeat-GFP (RaDR-GFP) mice, HR between two truncated EGFP expression cassettes can yield a fluorescent signal. In-house image analysis software provides a rapid method for quantifying recombination events within intact tissues, and the frequency of recombinant cells can be evaluated by flow cytometry. A comparison among 11 tissues shows that the frequency of recombinant cells varies by more than two orders of magnitude among tissues, wherein HR in the brain is the lowest. Additionally, de novo recombination events accumulate with age in the colon, showing that this mouse model can be used to study the impact of chronic exposures on genomic stability. Exposure to N-methyl-N-nitrosourea, an alkylating agent similar to the cancer chemotherapeutic temozolomide, shows that the colon, liver and pancreas are susceptible to DNA damage-induced HR. Finally, histological analysis of the underlying cell types reveals that pancreatic acinar cells and liver hepatocytes undergo HR and also that HR can be specifically detected in colonic somatic stem cells. Taken together, the RaDR-GFP mouse model provides new understanding of how tissue and age impact susceptibility to HR, and enables future studies of genetic, environmental and physiological factors that modulate HR in mammals.

Current therapies for high-grade gliomas extend survival only modestly. The glioma microenvironment, including glioma-associated microglia/macrophages (GAMs), is a potential therapeutic target. The microglia/macrophage cytokine CSF1 and its receptor CSF1R are overexpressed in human high-grade gliomas. To determine if the other known CSF1R ligand IL-34 is expressed in gliomas, we examined expression array data of human high-grade gliomas and performed RT-PCR on glioblastoma sphere-forming cell lines (GSCs). Expression microarray analyses indicated that CSF1, but not IL-34, is frequently overexpressed in human tumors. We found that while GSCs did express CSF1, most GSC lines did not express detectable levels of IL-34 mRNA. We therefore studied the impact of modulating CSF1 levels on gliomagenesis in the context of the GFAP-V12Ha-ras-IRESLacZ (Ras*) model. Csf1 deficiency deterred glioma formation in the Ras* model while CSF1 transgenic overexpression decreased the survival of Ras* mice and promoted the formation of high-grade gliomas. Conversely, CSF1 overexpression increased GAM density, but did not impact GAM polarization state. Regardless of CSF1 expression status, most GAMs were negative for the M2 polarization markers ARG1 and CD206; when present, ARG1+ and CD206+ cells were found in regions of peripheral immune cell invasion. Therefore, our findings indicate that CSF1 signaling is oncogenic during gliomagenesis through a mechanism distinct from modulating GAM polarization status.