The Cell and Molecular Biology of Glaucoma: Mechanisms of Retinal Ganglion Cell Death

Glaucomatous optic neuropathies are characterized by the progressive degeneration of the optic nerve and retinal ganglion cells. Conceptually, the process of ganglion cell loss can be considered a series of autonomous, self-destructing pathways.1 Retinal ganglion cells, for example, can be divided into basic regions, including the axon and synapse, the dendritic arbor, and the cell body. Each of these compartments, although contiguous with one another, can independently degenerate. A current model of glaucomatous pathogenesis suggests that the first compartment that is affected by intraocular pressure (IOP)–induced damage is the axon. The site of this damage is the laminar region of the eye, where the axons of the retinal ganglion cells exit the globe as they enter the optic nerve. The process of the damage to the axons is a matter of great speculation and ongoing research efforts and is not the principal subject of this essay. Basically, however, the strain placed on this region by elevated IOP causes molecular and functional changes to the resident cell population of the tissue. The principal cell population is made up of astrocytes, but lamina cribrosacytes, which have molecular profiles that are distinct from those of astrocytes, are also thought to reside there. One mechanism of axonal damage may be loss of support functions from the resident glial cells. Loss of energy support, for example, may lead to axons becoming unable to sustain energy-dependent processes such as axonal transport. Whatever the mechanism, rodent models of chronic glaucoma suggest that axon dysfunction and subsequent degeneration, are early events in the pathology of retinal ganglion cells.2,3 The level of damage to most axons is probably subacute, meaning that they are likely to retain connection and some level of communication with the ganglion cell soma. The evidence for subacute damage is that axons in glaucomatous mice degenerate by the dieback phenomenon at least in one model of glaucoma,2 although several studies have suggested that a proportion of axons also exhibit more severe Wallerian degeneration.3,4 Dieback is the process of degeneration that begins at the distal most point of the axon (the synapse) and progresses toward the cell body. This process could occur even if the initial site of damage is proximal to the cell body. A more acute injury, such as axotomy, would result in simultaneous degeneration of the axon throughout its length by Wallerian degeneration. Loss of axonal function leads to the activation of degenerative atrophy of the retinal ganglion cell bodies. The popular theory for this is that ganglion cell somas require neurotrophic input from target neurons in the brain, and failure of axonal transport to deliver these molecules results in a recapitulation of the apoptosis program, similar to what is believed to occur to supernumerary ganglion cells during development. This mechanism is still not proven, although there is clear evidence that trophic factor receptors, such as the TrkB receptor bound to brain-derived neurotrophic factor (BDNF), cannot be transported into the retina in a glaucomatous condition. A caveat regarding this notion is that molecules such as BDNF are produced directly by other cells in the retina, and so it is unclear why loss of transport of factors from remote neurons could be damaging. One possibility is that different molecular pathways are activated by identical trophic factors interacting with receptors in different compartments of a neuron.5 Thus, BDNF-binding in the synapse most likely activates a different signaling pathway from BDNF-binding receptors in the soma. Alternatively, in recent studies, Harder and Libby6 have clearly demonstrated that the initiator molecules necessary for developmental ganglion cell programmed cell death are different from those that regulate ganglion cell apoptosis after injury, suggesting that there may be distinct activation pathways. Once the cell death pathway is activated, ganglion cell somas undergo the intrinsic apoptosis program. This program is mediated by interactions of proteins of the Bcl2 gene family. Members of this family share similar amino acid domains (BCL homology or BH domains), which allows them to interact with each other.7 Some members, such as BCL-X and BCL2, have antiapoptotic roles. Others, such as BAX or BAK, have the apposing role of promoting apoptosis. The BH3-only domain–containing proteins are a third member of the family. These smaller peptides are involved in regulating the interaction between pro- and antiapoptotic Bcl2 family members. Speaking in a most general sense, the committed step of apoptosis requires activation of one or more BH3-only proteins, which then facilitate the activation of BAX and/or BAK, presumably by both direct interactions with the prodeath members and by sequestering antiapoptotic members and blocking their activity. Apoptosis is executed in cells when proapoptotic proteins can translocate and aggregate at the surface of the mitochondrial outer membrane. This aggregation event leads to membrane instability and/or pore formation, which in turn leads to the release of the electron transporter cytochrome c. Cytochrome c, once present in the cytoplasm, interacts with other protein complexes that lead to the activation of cysteine proteases, which selectively digest the cell from within. Activation of proapoptotic BAX and/or BAK is the committed step in the intrinsic apoptosis pathway, since this also leads to mitochondrial dysfunction, rendering these organelles unable to generate energy for the cell (loss of ATP production), and reactive oxygen species are typically generated in toxic amounts. Thus, even if caspases are inhibited, a cell in this condition is likely to die a slower, more necrotic death. Recent studies suggest that in healthy cells BCL-X and BAX are in a delicate balance that prevents BAX from aggregating at the surface of the mitochondria. The exact mechanism of how BCL-X is able to antagonize BAX function is not well understood, but recent studies8 suggest that in healthy cells BAX is present in an equilibrium, with an abundance of protein in the cytosol and a small fraction of protein bound to the mitochondrial outer membrane. The off-rate of BAX from the mitochondria is influenced by BCL-X, such that overexpression of this antiapoptotic protein can increase the off-rate by as much as 80%. Induction of BH3-only proteins appears to redistribute the equilibrium between mitochondrial and cytosolic BAX, by sequestering antiapoptotic proteins to decrease the mitochondrial off-rate, or by directly interacting with BAX to increase the on-rate, or perhaps by a combination of both. Several studies showing that multiple BH3-only proteins must be eliminated to replicate the Bax/Bak double-knockout phenotype (Bax−/− in neurons; see below) suggest that compete activation of BAX is dependent on a coordinated interaction of these activator proteins.