Sean Bray

Introduction. Radiation therapy is the main treatment for primary brain tumors and metastases. Although effective, it impairs neuronal, glial, and vascular compartments of the brain leading to molecular, cellular and functional changes, most importantly to the hippocampus. Generally, this is referred to as radiation-induced brain injury (RBI).¹ Microglia which depend on colony-stimulating factor 1 receptor (CSF1R) signaling play a role in RBI by releasing proinflammatory mediators such as cyclooxygenase-2 (COX-2), tumor necrosis factor α (TNFα), interleukin-6 (IL-6), and toll-like receptor-8 (TLR-8) in response to release of adenosine triphosphate (ATP) from damaged neurons.² High ATP concentrations activate purinergic 2X7 Receptor (P2X7R ) which in turn activates more microglia through induction of PI3K/AKT and NF-kB pathways.^3,4,5 Antagonists for CSF1R and P2X7R have shown promise but continued research is needed to prove their efficacy.^6,7,8 Methods. Immunofluorescence of microglial markers ionized calcium-binding adapter molecule 1 (IBA-1+) and CD-68+ in the medial pre-frontal cortex (mPFC) of mice was evaluated 6 weeks post-irradiation and then a six week treatment with PXL5622, a CSF1R inhibitor, was evaluated.⁵ NF-κB and PI3K/AKT pathways were evaluated for their involvement in microglial activation and paracrine signaling in RBI by their responses to irradiation combined with application of brilliant blue G (BBG) and specific inhibitors to each (pyrrolidine dithiocarbamate (PDTC) and LY294002 a specific inhibitor of PI3K respectively.³ Results. Irradiation showed elevated IBA-1+ and CD68+ and associated increase in microglia (40% and 25% respectively) in the pre-limbic (PrL) and infra-limbic (IL) cortices of the medial pre-frontal cortex. A six week administration of PLX5622 eliminated 80-90% of IBA-1+ and CD68+ microglia from the control and irradiated mPFC.⁵ PI3K phosphorylation of p65 was increased after irradiation of primary microglia, inhibited by BBG, and PDTC had no effect on expression of P2X7R but did reverse irradiation-induced COX-2 upregulation and phosphorylated p65. Expression of phosphorylated AKT (p-AKT) was increased after irradiation, attenuated by BBG, and LY294002 reversed upregulation of p-AKT and COX-2 while having no effect on P2X7R. The findings suggest that NF-kB and PI3K/AKT pathways play a role in activation and paracrine signaling of irradiated microglia after P2X7R activation.³ Conclusion. Studies of CSF1R and microglia show microglial dependence on CSF1R for their proliferation and therefore CSF1R blockade presents an attractive target for pharmacotherapy. NF-kB and PI3K/AKT are downstream of P2X7R implicating them in the activation and paracrine signaling of irradiated microglia which also presents an attractive target for pharmacotherapy.

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