Glaucoma, the second leading cause of blindness worldwide, is a neurodegenerative disease often associated with elevated intraocular pressure (IOP) and characterized by the progressive loss of retinal ganglion cells (RGCs) leading to visual loss. Among the various types of cell death associated with neuropathologies, the major cellular pathways underlying apoptosis and necrosis have been well characterized. However, in addition to these intrinsic mechanisms underlying primary cell death, intercellular communication via gap junctions (GJs) appear to play a major, yet poorly understand, role in secondary or `bystander' cell death. GJs, proteins that form cytoplasmic bridges between neighboring cells, act as conduits by which toxic materials are passed from dying cells to their neighbors leading to their death. Studies in CNS suggest that progressive secondary cell death mediated by GJs may, in fact, account for the majority of cell death associated with a number of insults, including ischemia, excitotoxicity, and trauma. Consistent with these findings, our preliminary data in retina show that blockade or ablation of GJs can significantly reduce the loss of RGCs and amacrine cells (ACs) in experimental glaucoma. We therefore posit that GJ-mediated secondary cell death is a crucial mechanistic element of glaucomatous injury, resulting in the majority of RGC and amacrine cell (AC) loss, and thereby offers a novel target for neuroprotective treatment. To test this hypothesis, we propose to use mouse models of experimental glaucoma to show directly that pharmacological blockade of GJs or genetic ablation of their constituent connexin subunits results in a significant reduction in in the loss o RGCs and the ACs to which they are coupled. Different GJs, based on the composition of their connexins, can have selective permeabilities, suggesting that the makeup of a GJ may determine whether it promotes cell death following injury. We will therefore use knockout mice in which selective connexins are ablated to determine which GJ cohorts support cell loss in glaucoma and should thus be targeted to promote neuroprotection. Finally, we will carry out electrophysiological and behavioral experiments to assess whether increasing RGC and AC survivability in glaucoma by blocking GJs results in preservation of visual function. In these experiments, we will record the electroretinogram (ERG), and use patch-clamp and microelectrode array recording techniques to assess retinal function. We will also record visual evoked potentials (VEP) and measure the optokinetic response (OKR) to assess central visual function. The results of this study should elucidate a new and important mechanism of RGC degeneration associated with glaucoma and, in so doing, reveal a novel target for neuroprotective treatment. While the proposed work will be directed at glaucoma, the therapeutic strategies that emerge should be applicable to the treatment of neurodegenerative diseases seen in other parts of the CNS.
The death of retinal ganglion cells is a hallmark of glaucoma, which leads to diminished visual function. In addition to the well-characterized intracellular cascades responsible for primary cell death in glaucoma, intercellular movement of toxic molecules via gap junctions between dying cells and their neighbors appears to play a crucial role in the progression of cell death across the retina. The proposed research program will determine the role of gap junction-mediated secondary cell death in the glaucomatous retina so as to reveal novel therapeutic strategies to protect neurons and thereby preserve visual function.
|Akopian, Abram; Kumar, Sandeep; Ramakrishnan, Hariharasubramanian et al. (2017) Targeting neuronal gap junctions in mouse retina offers neuroprotection in glaucoma. J Clin Invest 127:2647-2661|
|Akopian, Abram; Kumar, Sandeep; Ramakrishnan, Hariharasubramanian et al. (2016) Amacrine cells coupled to ganglion cells via gap junctions are highly vulnerable in glaucomatous mouse retinas. J Comp Neurol :|