Glaucoma is a prevalent blinding disease characterized by the progressive loss of retinal ganglion cells. Previously, we used Bax knockout mice to show that this proapoptotic gene was essential for ganglion cell death stimulated by optic nerve crush and in a mouse model of spontaneous glaucoma. Further study also showed that early atrophy of ganglion cells occurred in Bax-deficient cells. This observation poses an important caveat to neuroprotective strategies; it is possible to block cell death while at the same time lose normal cell function. Several of the early atrophic events are linked to the activity of Histone Deacetylases (HDACs). In dying cells, HDAC3 translocates to the nucleus and appears to be critical for global histone deacetylation, nuclear atrophy, and cell death, but not ganglion cell-specific gene silencing. We are proposing a series of experiments to directly test the role of Hdac3 in these early events, using a combined genetic approach (conditional knock-out of Hdac3 in mouse ganglion cells) and selective HDAC inhibitors. These experiments will evaluate Hdac3 function in ganglion cell death in both acute and chronic (glaucoma) optic nerve damage paradigms. It is also important to explore the function of HDAC3 mechanistically, in precipitating cell death. This will be conducted in vitro using a novel approach of comparing the differential effects of exogenous HDAC3 on pre- and post-differentiated neurons. The atrophic event of gene silencing is also dependent on HDAC activity, but preliminary evidence suggests that this does not include HDAC3. The other prominent HDACs in the mouse retina are HDAC1 and HDAC2. We will use selective inhibitors of all three HDACs to help tease out the relative contributions of each in the silencing process. We hypothesize that the critical playe is HDAC2, and the inhibitor studies will be complemented using Hdac2 conditional knock-out mice to specifically interrogate the role that this specific gene plays. In addition, we will also extend these studies to monitor the contribution of a protein involved in chromatin remodeling (CBX5) and the recruitment of HDAC2 co- repressor complexes in the processes of ganglion cell atrophy and death. The ultimate objective of these studies is to determine if powerful HDAC inhibitors will someday be useful therapeutics to treat ganglion cell loss in glaucoma. Not only do they hold promise in preventing ganglion cell death, but they may act directly on the mechanism that cause dying ganglion cells to lose function long before committing to the cell death pathway.
This proposal is a continuation of studies of the process of retinal ganglion cell death. In the previous funding period, we continued to characterize the early events that lead to ganglion cell atrophy and the silencing of normal gene expression that precede the Bax-dependent stage of the intrinsic apoptotic program. These studies revealed the important role of histone deacetylases (HDACs) and the global deacetylation of nuclear histones in dying cells. Part of this new proposal is dedicated to directly characterizing the role that HDAC3 has in this process, and to evaluate further the molecular basis of HDAC3 toxicity to neurons.
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