Previous studies have shown that following whole-body irradiation bone marrow (BM)-derived cells can migrate into the central nervous system, including the retina, to give rise to microglia-like cells. The detailed mechanism, however, remains elusive. We show in this study that a single-dose whole-body γ-ray irradiation (8 Gy) induced subclinical damage (i.e., DNA damage) in the neuronal retina, which is accompanied by a low-grade chronic inflammation, para-inflammation, characterized by upregulated expression of chemokines (CCL2, CXCL12, and CX3CL1) and complement components (C4 and CFH), and microglial activation. The upregulation of chemokines CCL2 and CXCL12 and complement C4 lasted for more than 160 days, whereas the expression of CX3CL1 and CFH was upregulated for 2 weeks. Both resident microglia and BM-derived phagocytes displayed mild activation in the neuronal retina following irradiation. When BM cells from CX3CR1(gfp/+) mice or CX3CR1(gfp/gfp) mice were transplanted to wild-type C57BL/6 mice, more than 90% of resident CD11b(+) cells were replaced by donor-derived GFP(+) cells after 6 months. However, when transplanting CX3CR1(gfp/+) BM cells into CCL2-deficient mice, only 20% of retinal CD11b(+) cells were replaced by donor-derived cells at 6 month. Our results suggest that the neuronal retina suffers from a chronic stress following whole-body irradiation, and a para-inflammatory response is initiated, presumably to rectify the insults and maintain homeostasis. The recruitment of BM-derived myeloid cells is a part of the para-inflammatory response and is CCL2 but not CX3CL1 dependent.

Previous studies have shown that CCL2/CX3CR1 deficient mice on C57BL/6N background (with rd8 mutation) have an early onset (6 weeks) of spontaneous retinal degeneration. In this study, we generated CCL2−/−CX3CR1GFP/GFP mice on the C57BL/6J background. Retinal degeneration was not detected in CCL2−/−CX3CR1GFP/GFP mice younger than 6 months. Patches of whitish/yellowish fundus lesions were observed in 17∼60% of 12-month, and 30∼100% of 18-month CCL2−/−CX3CR1GFP/GFP mice. Fluorescein angiography revealed no choroidal neovascularisation in these mice. Patches of retinal pigment epithelium (RPE) and photoreceptor damage were detected in 30% and 50% of 12- and 18-month CCL2−/−CX3CR1GFP/GFP mice respectively, but not in wild-type mice. All CCL2−/−CX3CR1GFP/GFP mice exposed to extra-light (∼800lux, 6 h/day, 6 months) developed patches of retinal atrophy, and only 20–25% of WT mice which underwent the same light treatment developed atrophic lesions. In addition, synaptophysin expression was detected in the outer nucler layer (ONL) of area related to photoreceptor loss in CCL2−/−CX3CR1GFP/GFP mice. Markedly increased rhodopsin but reduced cone arrestin expression was observed in retinal outer layers in aged CCL2−/−CX3CR1GFP/GFP mice. GABA expression was reduced in the inner retina of aged CCL2−/−CX3CR1GFP/GFP mice. Significantly increased Müller glial and microglial activation was observed in CCL2−/−CX3CR1GFP/GFP mice compared to age-matched WT mice. Macrophages from CCL2−/−CX3CR1GFP/GFP mice were less phagocytic, but expressed higher levels of iNOS, IL-1β, IL-12 and TNF-α under hypoxia conditions. Our results suggest that the deletions of CCL2 and CX3CR1 predispose mice to age- and light-mediated retinal damage. The CCL2/CX3CR1 deficient mouse may thus serve as a model for age-related atrophic degeneration of the RPE, including the dry type of macular degeneration, geographic atrophy.

Deep sequencing has revealed the complexity of small RNA expression in the mouse retina and RPE/choroid. This knowledge will improve the design and interpretation of future functional studies of the role of miRNAs and other small RNAs in retinal disease.

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Overall, these results suggest that CF airway epithelial cells are more sensitive to apoptosis via increased levels of Fas and subsequent activation of the Fas death receptor pathway, which may be associated with dysfunctional CFTR.