activation in the non-irradiated tissue. Scale bar, 100 mm (A, B), 10 mm (C, D). (TIF) Acknowledgments We thank N. Marsh-Armstrong for comments on the manuscript. References 1. Quigley HA, Broman AT (2006) The number of people with glaucoma worldwide in 2010 and 2020. Br J Ophthalmol 90: 262–267. 2. Sommer A (1996) Glaucoma risk factors observed in the Baltimore Eye Survey. Current opinion in ophthalmology 7: 93–98. 3. Gupta N, Yucel YH (2007) Glaucoma as a neurodegenerative disease. Curr Opin Ophthalmol 18: 110–114. 4. Burgoyne CF (2011) A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma. Experimental eye research 93: 120– 132. 5. Crish SD, Calkins DJ (2011) Neurodegeneration in glaucoma: progression and calcium-dependent intracellular mechanisms. Neuroscience 176: 1–11. 6. Stevens B, Allen NJ, Vazquez LE, Howell GR, Christopherson KS, et al. (2007) The classical complement cascade mediates CNS synapse elimination. Cell 131: 1164–1178. 7. Whitmore AV, Libby RT, John SW (2005) Glaucoma: thinking in new ways-a role for autonomous axonal self-destruction and other compartmentalised processes? Prog Retin Eye Res 24: 639–662. 8. Johnson EC, Morrison JC (2009) Friend or foe? Resolving the impact of glial responses in glaucoma. J Glaucoma 18: 341–353. 9. Hanisch UK, Kettenmann H (2007) Microglia: active sensor and versatile effector cells in the normal and pathologic brain. Nat Neurosci 10: 1387–1394. 10. Block ML, Zecca L, Hong JS (2007) Microglia-mediated neurotoxicity: uncovering the molecular mechanisms. Nat Rev Neurosci 8: 57–69. 11. Bosco A, Steele MR, Vetter ML (2011) Early microglia activation in a mouse model of chronic glaucoma. J Comp Neurol 519: 599–620. 12. Neufeld AH (1999) Microglia in the optic nerve head and the region of parapapillary chorioretinal atrophy in glaucoma. Arch Ophthalmol 117: 1050– 1056. 13. Libby RT, Anderson MG, Pang IH, Robinson ZH, Savinova OV, et al. (2005) Inherited glaucoma in DBA/2J mice: pertinent disease features for studying the neurodegeneration. Vis Neurosci 22: 637–648. 14. Howell GR, Libby RT, Jakobs TC, Smith RS, Phalan FC, et al. (2007) Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma. J Cell Biol 179: 1523–1537. 15. Soto I, Oglesby E, Buckingham BP, Son JL, Roberson ED, et al. (2008) Retinal ganglion cells downregulate gene expression and lose their axons within the optic nerve head in a mouse glaucoma model. J Neurosci 28: 548–561. 16. Inman DM, Horner PJ (2007) Reactive nonproliferative gliosis predominates in a chronic mouse model of glaucoma. Glia 55: 942–953. 17. Steele MR, Inman DM, Calkins DJ, Horner PJ, Vetter ML (2006) Microarray analysis of retinal gene expression in the DBA/2J model of glaucoma. Invest Ophthalmol Vis Sci 47: 977–985. 18. Fan W, Li X, Wang W, Mo JS, Kaplan H, et al. (2010) Early Involvement of Immune/Inflammatory Response Genes in Retinal Degeneration in DBA/2J Mice. Ophthalmology and eye diseases 1: 23–41. 19. Howell GR, Macalinao DG, Sousa GL, Walden M, Soto I, et al. (2011) Molecular clustering identifies complement and endothelin induction as early events in a mouse model of glaucoma. The Journal of clinical investigation 121: 1429–1444. 20. Ahmed F, Brown KM, Stephan DA, Morrison JC, Johnson EC, et al. (2004) Microarray analysis of changes in mRNA levels in the rat retina after experimental elevation of intraocular pressure. Invest Ophthalmol Vis Sci 45: 1247–1258. 21. Johnson EC, Jia L, Cepurna WO, Doser TA, Morrison JC (2007) Global changes in optic nerve head gene expression after exposure to elevated intraocular pressure in a rat glaucoma model. Invest Ophthalmol Vis Sci 48: 3161–3177. 22. Lam TT, Kwong JM, Tso MO (2003) Early glial responses after acute elevated intraocular pressure in rats. Invest Ophthalmol Vis Sci 44: 638–645. 23. Miyahara T, Kikuchi T, Akimoto M, Kurokawa T, Shibuki H, et al. (2003) Gene microarray analysis of experimental glaucomatous retina from cynomo- logous monkey. Invest Ophthalmol Vis Sci 44: 4347–4356. 24. Bosco A, Inman DM, Steele MR, Wu G, Soto I, et al. (2008) Reduced retina microglial activation and improved optic nerve integrity with minocycline treatment in the DBA/2J mouse model of glaucoma. Invest Ophthalmol Vis Sci 49: 1437–1446. 25. Anderson MG, Libby RT, Gould DB, Smith RS, John SW (2005) High-dose radiation with bone marrow transfer prevents neurodegeneration in an inherited glaucoma. Proc Natl Acad Sci U S A 102: 4566–4571. 26. Howell GR, Soto I, Zhu X, Ryan M, Macalinao DG, et al. (2012) Radiation treatment inhibits monocyte entry into the optic nerve head and prevents Irradiation Abates Early Microglia Activation Author Contributions Conceived and designed the experiments: AB MLV. Performed the experiments: AB SDC MRS COR DMI. Analyzed the data: AB MRS DMI DJC. Contributed reagents/materials/analysis tools: MLV PJH DJC. Wrote the paper: MLV. Edited the manuscript: AB SDC DMI PJH DJC MLV. neuronal damage in a mouse model of glaucoma. The Journal of clinical investigation. 27. Howell GR, Libby RT, Marchant JK, Wilson LA, Cosma IM, et al. (2007) Absence of glaucoma in DBA/2J mice homozygous for wild-type versions of Gpnmb and Tyrp1. BMC Genet 8: 45. 28. John SW, Smith RS, Savinova OV, Hawes NL, Chang B, et al. (1998) Essential iris atrophy, pigment dispersion, and glaucoma in DBA/2J mice. Invest Ophthalmol Vis Sci 39: 951–962. 29. Jung S, Aliberti J, Graemmel P, Sunshine MJ, Kreutzberg GW, et al. (2000) Analysis of fractalkine receptor CX(3)CR1 function by targeted deletion and green fluorescent protein reporter gene insertion. Mol Cell Biol 20: 4106–4114. 30. Buckingham BP, Inman DM, Lambert W, Oglesby E, Calkins DJ, et al. (2008) Progressive ganglion cell degeneration precedes neuronal loss in a mouse model of glaucoma. J Neurosci 28: 2735–2744. 31. Crish SD, Sappington RM, Inman DM, Horner PJ, Calkins DJ (2010) Distal axonopathy with structural persistence in glaucomatous neurodegeneration. Proceedings of the National Academy of Sciences of the United States of America 107: 5196–5201. 32. Inman DM, Sappington RM, Horner PJ, Calkins DJ (2006) Quantitative correlation of optic nerve pathology with ocular pressure and corneal thickness in the DBA/2 mouse model of glaucoma. Invest Ophthalmol Vis Sci 47: 986– 996. 33. Sappington RM, Carlson BJ, Crish SD, Calkins DJ (2010) The microbead occlusion model: a paradigm for induced ocular hypertension in rats and mice. Investigative ophthalmology & visual science 51: 207–216. 34. Libby RT, Li Y, Savinova OV, Barter J, Smith RS, et al. (2005) Susceptibility to neurodegeneration in a glaucoma is modified by Bax gene dosage. PLoS Genet 1: 17–26. 35. Reichstein D, Ren L, Filippopoulos T, Mittag T, Danias J (2007) Apoptotic retinal ganglion cell death in the DBA/2 mouse model of glaucoma. Experimental eye research 84: 13–21. 36. Schlamp CL, Li Y, Dietz JA, Janssen KT, Nickells RW (2006) Progressive ganglion cell loss and optic nerve degeneration in DBA/2J mice is variable and asymmetric. BMC Neurosci 7: 66. 37. Kalm M, Lannering B, Bjork-Eriksson T, Blomgren K (2009) Irradiation- induced loss of microglia in the young brain. Journal of neuroimmunology 206: 70–75. 38. Hwang SY, Jung JS, Kim TH, Lim SJ, Oh ES, et al. (2006) Ionizing radiation induces astrocyte gliosis through microglia activation. Neurobiology of disease 21: 457–467. 39. Wada R, Tifft CJ, Proia RL (2000) Microglial activation precedes acute neurodegeneration in Sandhoff disease and is suppressed by bone marrow transplantation. Proceedings of the National Academy of Sciences of the United States of America 97: 10954–10959. 40. Yoshiyama Y, Higuchi M, Zhang B, Huang SM, Iwata N, et al. (2007) Synapse loss and microglial activation precede tangles in a P301S tauopathy mouse model. Neuron 53: 337–351. 41. Giuliari GP, Sadaka A, Hinkle DM, Simpson ER (2011) Current treatments for radiation retinopathy. Acta oncologica 50: 6–13. 42. Gupta A, Dhawahir-Scala F, Smith A, Young L, Charles S (2007) Radiation retinopathy: case report and review. BMC ophthalmology 7: 6. 43. Horgan N, Shields CL, Mashayekhi A, Shields JA (2010) Classification and treatment of radiation maculopathy. Current opinion in ophthalmology 21: 233–238. 44. Fike JR (2011) Physiopathology of radiation-induced neurotoxicity. Revue neurologique 167: 746–750. 45. Sarkissian V (2005) The sequelae of cranial irradiation on human cognition. Neuroscience letters 382: 118–123. 46. Ben Abdallah NM, Slomianka L, Lipp HP (2007) Reversible effect of X- irradiation on proliferation, neurogenesis, and cell death in the dentate gyrus of adult mice. Hippocampus 17: 1230–1240. 47. Wu KL, Li YQ, Tabassum A, Lu WY, Aubert I, et al. (2010) Loss of neuronal protein expression in mouse hippocampus after irradiation. Journal of neuropathology and experimental neurology 69: 272–280. 48. Rosi S, Andres-Mach M, Fishman KM, Levy W, Ferguson RA, et al. (2008) Cranial irradiation alters the behaviorally induced immediate-early gene arc (activity-regulated cytoskeleton-associated protein). Cancer research 68: 9763– 9770. 49. Ajami B, Bennett JL, Krieger C, Tetzlaff W, Rossi FM (2007) Local self-renewal can sustain CNS microglia maintenance and function throughout adult life. Nat Neurosci 10: 1538–1543. PLOS ONE | www.plosone.org 10 August 2012 | Volume 7 | Issue 8 | e43602