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G. Hoppe, Q. Ebrahem, B. W. Hanzlicek, S. L. Ball, J. E. Sears; Loss of Chromatin Remodeling Protein Hmgb1 Precedes Photoreceptor Degeneration. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2988.
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Abundant DNA-binding protein Hmgb1 was recently implicated as a neuroprotective nuclear factor responsible for suppression of environmental and hereditary genotoxicity. We analyzed expression and distribution of Hmgb1 in two models of photoreceptor degeneration one due to genetic mutation and another due to acute trauma as well as in cultured cells expressing a rhodopsin mutation.
Relative abundance and localization of Hmgb1 were detected by immunofluorescent microscopy in the rat model of retinitis pigmentosa (RCS rat) and experimental retinal detachment. RCS rats bred on a Long Evans background and control Long Evans rats were sacrificed as various time points of postnatal development from P7 (no degeneration) to P42 (80% degeneration). Retinal detachment in Long Evans rats was achieved by subretinal injections of albumin followed by 10 days of postoperative healing. Eye tissues were collected, immersion fixed in 4% paraformaldehyde and processed for frozen sections. In vitro model of cytotoxic rhodopsin mutation was established by expressing P23H mutant in CHO cells.
P23H rhodopsin mutation led to massive leakage of endogenous Hmgb1 from nuclei of CHO cells suggesting that hereditary cytotoxic neurodegeneration may lead to Hmgb1 loss. Levels and distribution of Hmgb1 in RCS rat were not different from the control in pre-degenerating retinas (P7-P14). Progressive disappearance of Hmgb1 from photoreceptor nuclei was evident starting at P21. At P42, when 20% of ONL is still present and functionally active, only few photoreceptors (less than 5% of the remaining nuclei) expressed Hmgb1. A significant reduction in Hmgb1 was also observed in areas of retinal detachment, but not in adjacent unaffected retina. The downregulation of Hmgb1 was more pronounced in the thinning ONL as well as in the INL above degenerating photoreceptors.
Photoreceptors appear to either effectively downregulate or passively lose major nuclear protein Hmgb1 implicated in DNA transcription and repair. Loss of Hmgb1 occurs prior to the functional demise of photoreceptor cells. Absence of Hmgb1 may lead to genomic instability and explain increased susceptibility of degenerating retina to oxidative stress and light damage.
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