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Elena Pipi, Paul Ibbett, Andrew J Lotery, Imre Lengyel, V. Hugh Perry, Jessica Teeling; Hydroxyapatite induced experimental retinal degeneration in a murine model. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2272.
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Current models of experimental age-related macular degeneration fail to recapitulate the hallmarks of human AMD pathology, including drusen formation and deposition. Recent human studies using post-mortem aged tissue indicate that hydroxyapatite (HAP) spherules are present in all types of sub-RPE deposits. These spherules act as a scaffold for proteins to initiate drusen formation under the RPE. We believe that these spherules could come into direct contact with the neurosensory retina in geographic atrophy (GA), leading to retinal damage. To test the consequences of this contact, we developed a murine model using intraocular injections of HAP to form drusen-like deposits and then examined retinal function and cellular pathology.
Female C57BL/6J (5 weeks, n=8) received an intravitreal injection of ~500 HAP chromatography beads (20 µm diameter, BioRad) in the right eye, while the untreated left eye served as a control. Retinal structure and function was examined at 4- and 8-weeks post-injection with electroretinography (ERG) and optical coherence tomography (OCT). At 8-weeks post injection eyes were enucleated, embedded in OCT and snap frozen for immunohistochemical analysis for indicators of pathology (CD11b, FcγRI and GFAP).
ERG (b-wave) was significantly reduced in HAP injected eyes compared to control eyes. These ERG deficits were progressive from 4- to 8-weeks in 5 out of 8 mice. Both the number of retinal deposits seen in OCT and the ERG (b-wave) deficits (right to left eye) correlated strongly with reduced thickness of the ONL, as determined by OCT segmentation. Histology revealed that HAP spherules deposit at the outer retina and acquire autofluorescent properties. Finally, CD11b, FcγRI and GFAP expression was significantly increased following HAP injection compared to control, suggesting that HAP induced retinal inflammation and gliosis.
Our results support the hypothesis that HAP deposition is directly involved in retinal degeneration in the mouse retina. These findings might be relevant for human AMD pathology, particularly in atrophic AMD where the neurosensory retina is likely to be directly exposed to the calcifications in and around Bruch’s membrane. The full characterisation of this HAP-induced retinopathy may prove helpful in unravelling the molecular steps leading to retinal degeneration.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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