ABSTRACT
PURPOSE: Downregulation of normal gene expression in dying retinal ganglion cells has been documented in both acute and chronic models of optic nerve disease. The authors examined the mechanism and timing of this phenomenon in DBA/2J mice, using genetically modified substrains of this inbred line. METHODS: DBA/2J mice, doubly congenic for the Bax mutant allele and the ganglion cell reporter gene Fem1c(Rosa3) (R3), were evaluated to elucidate the timing of loss of normal gene expression during the apoptotic process. The localization of histone deacetylase 3 (HDAC3) and nuclear histone H4 acetylation were examined by immunofluorescence in dying cells. The role of HDACs in gene silencing during glaucoma was interrogated using the global HDAC inhibitor trichostatin A (TSA). RESULTS: Silencing of the R3 allele occurred in Bax(-/-) ganglion cells, indicating that this process preceded the committed step of the intrinsic apoptotic pathway. Weekly TSA treatment, between the ages of 6 and 10 months, was able to attenuate the loss of R3 expression in the retina, but had no effect on optic nerve degeneration. Dying cells in aging DBA/2J mice exhibited nuclear localization of HDAC3 and a decrease in the level of H4 acetylation. CONCLUSIONS: Retinal ganglion cells exhibit a loss of normal gene expression as an early (pre-BAX involvement) part of their apoptotic program during glaucomatous degeneration. This process can be ameliorated, but not completely blocked, using HDAC inhibitors. Epigenetic changes to active chromatin, such as deacetylation, may be mediated by HDAC3 in dying neurons.
