Microglial phagocytosis and activation underlying photoreceptor degeneration is regulated by CX3CL1‐CX3CR1 signaling in a mouse model of retinitis pigmentosa

Matthew K Zabel,Xu Wang,Wai T Wong,Lian Zhao,Robert N Fariss,Wenxin Ma,Yikui Zhang,Shaimar R Gonzalez

doi:10.1002/glia.23016

Matthew K Zabel, Xu Wang + Show 6 more

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https://doi.org/10.1002/glia.23016

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Journal: Glia	Publication Date: Jun 17, 2016
Citations: 139	License type: CC BY 4.0

Affiliation: National Eye Institute, National Institutes of Health

Abstract

Retinitis pigmentosa (RP), a disease characterized by the progressive degeneration of mutation‐bearing photoreceptors, is a significant cause of incurable blindness in the young worldwide. Recent studies have found that activated retinal microglia contribute to photoreceptor demise via phagocytosis and proinflammatory factor production, however mechanisms regulating these contributions are not well‐defined. In this study, we investigate the role of CX3CR1, a microglia‐specific receptor, in regulating microglia‐mediated degeneration using the well‐established rd10 mouse model of RP. We found that in CX3CR1‐deficient (CX3CR1GFP/GFP) rd10 mice microglial infiltration into the photoreceptor layer was significantly augmented and associated with accelerated photoreceptor apoptosis and atrophy compared with CX3CR1‐sufficient (CX3CR1GFP/+) rd10 littermates. CX3CR1‐deficient microglia demonstrated increased phagocytosis as evidenced by (1) having increased numbers of phagosomes in vivo, (2) an increased rate of phagocytosis of fluorescent beads and photoreceptor cellular debris in vitro, and (3) increased photoreceptor phagocytosis dynamics on live cell imaging in retinal explants, indicating that CX3CR1 signaling in microglia regulates the phagocytic clearance of at‐risk photoreceptors. We also found that CX3CR1 deficiency in retinal microglia was associated with increased expression of inflammatory cytokines and microglial activation markers. Significantly, increasing CX3CL1‐CX3CR1 signaling in the rd10 retina via exogenous intravitreal delivery of recombinant CX3CL1 was effective in (1) decreasing microglial infiltration, phagocytosis and activation, and (2) improving structural and functional features of photoreceptor degeneration. These results indicate that CX3CL1‐CX3CR1 signaling is a molecular mechanism capable of modulating microglial‐mediated degeneration and represents a potential molecular target in therapeutic approaches to RP. GLIA 2016;64:1479–1491

Highlights

Retinitis pigmentosa (RP) comprises a category of inherited retinal diseases in which mutations in photoreceptor or retinal pigment epithelium genes result in progressive degeneration of photoreceptors (Hartong et al, 2006)
Isolated transferase dUTP Nick End Labeling (TUNEL)-labeled apoptotic photoreceptor nuclei were detected in the outer nuclear layer (ONL) of rd10; CX3CR1GFP/GFP animals but not in rd10;CX3CR1GFP/1 littermates
We found that protein levels of inflammatory cytokines, typical of those secreted by activated microglia, are generally elevated in rd10;CX3CR1GFP/GFP compared with rd10;CX3CR1GFP/1 retina (Fig. 4A)

Summary

Introduction

RP comprises a category of inherited retinal diseases in which mutations in photoreceptor or retinal pigment epithelium genes result in progressive degeneration of photoreceptors (Hartong et al, 2006). Affected patients, who bear mutations in one of a large (>200) set of causative genes (Daiger et al, 2013), progress typically to severe vision loss. RP lacks comprehensive treatment (Sacchetti et al, 2015) and is a leading cause of blindness in the young worldwide (Bunker et al, 1984; Gao et al, 2015; Haim, 2002). Elucidating cellular and molecular mechanisms common to various genetic etiologies of RP is of key importance in the. Published online June 17, 2016 in Wiley Online Library (wileyonlinelibrary.com).

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