Cultures of mature differentiated RGCs are difficult to maintain, even for short periods, so that most culture studies are done with immature cells from neonatal, or a few days postnatal, rodent retinas. Thus, it is uncertain to what extent RGC cultures reflect adult differentiated RGCs in vivo. The most common acute in vivo animal models that have been used for neuroprotection studies relevant to glaucoma include cutting or crushing of the optic nerve, acute retinal ischemia-reperfusion, and, to a lesser extent, intravitreal injection of an excitotoxic neurotransmitter or analog (e.g., glutamate, N-methyl-D-aspartate [NMDA], and kainic acid). Laser photodamage of the trabecular meshwork in the monkey eye is currently the best available chronic elevated intraocular pressure (IOP) glaucoma model in a primate, but it is seldom available for neuroprotection studies. Considerable progress has been made toward developing induced chronic elevated IOP glaucoma models in rodent eyes, and spontaneous development of elevated IOPs has recently been described in one strain of mouse^1,2 (DBA/2) and in a rat.³ However, these elevated IOP models are not yet as well established as the acute rodent models of optic nerve crush or ocular ischemia in which most in vivo neuroprotection studies have been done.

Although RGCs appear to be preferentially vulnerable to damage and death in all the rodent models cited, caution must be added about their extrapolation to clinical glaucoma because pathology affects all layers of the retina and is much more rapid and extensive than in primate eyes exposed to chronic elevated IOP. Justification for the optic nerve crush, ischemia, and excitotoxic models is based on theories for the initiation and development of retinal pathology in glaucoma. Each model is thought to mimic at least one component of glaucomatous optic neuropathy, and the evaluation of potential neuroprotective agents for glaucoma in models is based on this premise. A wide range of drug classes shows neuroprotective activity in RGC culture systems, but fewer have been tested and found active in the acute in vivo models of RGC death, and fewer still in chronic high-IOP models, which arguably have the closest relevance to glaucoma. None of these agents have yet reached clinical use for treating glaucoma at the level of the retina or optic nerve head, although one, memantine, is currently in a clinical trial. For the purposes of this volume, this chapter emphasizes the mechanisms leading to RGC death and neuroprotective studies done on in vivo model systems and excludes studies on regeneration.

There may be an important interaction between vascular dysfunction and elevated IOP.⁷ One may hypothesize a defective regulation of optic nerve head or retinal blood flow in response to IOP compression of the microcirculation to be the predisposing factor for glaucomatous RGC loss. Subjects with normal microvascular regulation may be able to counteract moderate increases in IOP and thus avoid ischemic damage and glaucomatous retinal pathology (as in patients with ocular hypertension?). Some degree of vascular dysregulation may not be pathologic by itself but may become so with the added stress when IOPs are elevated above normal (as in primary open-angle glaucoma?), whereas a greater degree of vascular dysregulation could be pathologic even with normal IOP levels (as in normal-tension glaucoma?).

Any hypothesis for the initiation and progression of glaucomatous neuropathy needs to account for the focal and chronic nature of RGC and nerve fiber layer (nfl) loss. In vascular dysfunction, inability of retinal microvessels in a localized area to autoregulate against IOP compression could lead to hypoperfusion and a focal ischemia that in turn triggers overriding autoregulatory mechanisms that temporarily restore perfusion (e.g., mediated by the localized rise in pCO₂ level). This response cycle could result in repetitive transient episodes of ischemia-reperfusion over an extended period of time, causing a gradual focal loss of RGCs and thinning of the nfl leading to that location. A similar postulate can be offered for ischemic-reperfusion episodes occurring in the optic nerve head/lamina cribrosa region when subjected to compression by elevated IOP. Localization of the microvascular pathology to subregions of the optic nerve head could account for the observed focal pattern of RGC death and nfl thinning. These conjectures for a vascular pathology initiating glaucomatous optic neuropathy provide some justification for using ischemia-reperfusion models to develop neuroprotective agents for glaucoma. This vascular hypothesis includes both current general theories about where retinal glaucoma pathology actually begins, namely, axon pathology leading to RGC loss (termed primary, or retrograde cell death) or damage to or death of the ganglion cell body first, followed by axon degeneration (termed secondary, or anterograde death). As discussed subsequently, it is likely that both types of death processes occur in glaucoma.

Axon degeneration can be triggered by a variety of insults, especially physical damage and ischemia, and it appears to be an autodestructive process with similarity to apoptosis. Axonal injury causes the release of cytochrome C from mitochondria but does not appear to follow subsequent apoptotic pathway steps involving caspases,¹⁰ as diagrammed for the cell body in Figure 1. In normal mice transected axons distal to their soma fail to conduct an action potential within 1 day of the injury, but in the Wallerian degeneration-Slow (WldS) mouse mutant¹¹ such axons remain structurally intact and can conduct action potentials for up to 3 weeks. It is likely that axon preservation in the WldS mouse applies also to the proximal part of a damaged optic nerve axon and may increase survival of the RGC.

The WldS mutation causes overexpression of a chimeric protein composed of a component of the ubiquitin proteosome proteolytic pathway (UPP) and the NAD synthesizing enzyme nicotinamide mononucleotide adenyltransferase-1. Increased NAD synthesis seems to be an important axon-sparing component of the WldS phenotype.¹² This recent finding suggests neuroprotective strategies aimed at increasing the cellular NAD supply, and it may explain earlier reports on the beneficial effect of nicotinamide treatment in several neurodegeneration models. However, the UPP may also play a role in the WldS phenotype because this polyfunctional proteolytic system degrades transcription factors and regulatory proteins involved in death signaling, including some that control the release of pro-apoptotic factors from mitochondria (see later discussion and Fig. 1). Further evidence for a clear difference in degenerative pathways for RGC axons and their somata is in a recent report¹³ on spontaneously glaucomatous DBA/2 mice that were also deficient for the pro-apoptotic protein BCL-2-associated X protein (BAX) (Fig. 1). The optic nerve axons degenerated in these mice without loss of the RGC, suggesting that BAX is essential for completion of the apoptotic program in the soma but is not required for axon degeneration.

The damaging initial effect of ischemia on axons that has been most studied is a persistent depolarization that occurs through opening of Na⁺ and/or Ca²⁺ entry channels. Sodium channel blockade by some anesthetics, anticonvulsant drugs, and β-adrenergic receptor blockers can significantly reduce axon injury in such models. These agents include, for example, lidocaine, riluzole, flunarizine, betaxolol,¹⁴ diazepam, carbamazepine, and phenytoin. The last two agents listed were found to protect against ischemic injury in an in vitro acute rat optic nerve anoxia model at effective drug concentrations below those used to treat epilepsy.¹⁵ The wide chronic therapeutic use of these two agents thus provides an opportunity for a retrospective clinical study to assess whether Na⁺ channel blockade is of value in glaucoma therapy. More specific Ca²⁺ channel blockers, such as nifedipine and nimodipine, which are primarily used as vasodilators, gave marginally positive results when evaluated in a clinical study.¹⁶ The present status in glaucoma therapy of established channel blocking drugs that affect axons, alone or combined with a vascular effect, is that they may have limited value in specific instances,¹⁷ but their general use is probably unwarranted. Because of the multiplicity of neural Na⁺ and Ca²⁺ channel subtypes, and the state-dependent binding at multiple sites of potential blockers, we may not yet have identified the appropriate channel type or the kind of blocking agent needed for a more effective glaucoma therapeutic agent of this drug class.

In contrast with causing damage, reactive glial cells in the retina could potentially have some beneficial effects on the basis of up-regulation of various survival/neurotrophic/growth factors, such as insulin-like growth factor (IGF)-1, IL-10, ciliary neurotrophic factor (CNTF), and members of the chaperone/heat stress proteins (HSPs), such as HSP-27.²³ In a similar apparent paradox, nitric oxide and TNF-α can also have protective effects (these two factors are discussed further below).

The foregoing discussion indicates several potential mechanisms that could be targeted for developing neuroprotective agents that interfere with destructive actions by activated astrocytes or other glial cells, namely, inhibitors of NOS-2 or enzymes that modify the ecm, and agents or gene therapy that promote local neurotrophin/survival factor synthesis and release to make up for the interruption of axonal supply of such factors from the optic tectum.

There are some experimental indications that activation of receptors mediating neurotransmission can affect signaling by BDNF and other neurotrophins. Elevated levels of cyclic adenosine monophosphate (cAMP) increase the survival effect of neurotrophins in RGC cultures. As indicated in Figure 1 (green), BDNF receptor activation leads to an intracellular anti-apoptosis signal pathway involving the activation of PI3-k and the serine-threonine kinase Akt, resulting in inactivation of pro-apoptotic proteins, such as BAX or BCL-2-associated death protein (BAD), by phosphorylation (see subsequent discussion). As diagrammed in Figure 1, cAMP, acting through the cAMP response element binding protein-activated transcription factor, can potentiate the BDNF signal cascade either by blocking BAX/BAD pro-apoptotic signals or up-regulating anti-apoptotic BCL-2 expression.²⁷ On this basis one might expect cAMP activators to be beneficial in optic nerve damage models, but there is as yet no evidence for this in vivo. Instead, contrary to this expectation, it has been found that a β-adrenergic antagonist, betaxolol, is beneficial in the ischemia model²⁸ and also up-regulates mRNA for BDNF²⁹ and other neurotrophic/growth factors.³⁰ This response, and other mechanisms of some β-adrenergic antagonists, such as ion-channel blockade, suggests that their neuroprotective effect involves signal systems other than the β-adrenergic receptor/adenylyl cyclase system.

Adult RGC survival and regeneration in vivo after optic nerve damage or ischemia can also be promoted by CNTF,³¹ β-FGF,³² and IGF-1.³³ FGF can also be up-regulated in the retina by activation of other receptors, for example, by α₂-adrenergic agonists,³⁴ and this may account in part for the neuroprotective effect of this class of drug in the rat optic nerve crush, ischemia, and elevated IOP models.^35–37 These findings on β-antagonists and α-agonists affecting growth factors hold out the possibility that conventional pharmaceutical agents acting through well-established receptor systems (such as those linked to cAMP), or have novel drug mechanisms that modulate neurotrophin/growth factor expression, may be a useful therapeutic approach to minimize loss of trophic support caused by damaged axons. For the near term, this approach deserves further investigation because it seems more clinically feasible than the promising but long-term approach of direct replacement of growth factors and neurotrophins by gene therapy.

This protective injury phenomenon may extend to the primate eye and could account for the finding that laser-induced damage to a focal area of the outer retina in monkeys specifically preserved the RGCs located over the injury area in comparison with RGC loss in areas of undamaged outer retina when glaucoma was subsequently induced by elevated IOP (by laser-scarring of the trabecular meshwork).⁴¹

Other examples of this phenomenon include ischemic preconditioning of the retina⁴² and heat stress, which attenuate RGC death in response to a more severe ischemic episode, excitotoxicity, or optic nerve crush. A mild inflammation of the retina caused by immunization with a peptide derived from a retina-specific protein (interphotoreceptor retinal binding protein) protects RGC against optic nerve crush injury.⁴³ Such a protective immune response can also be elicited in these acute models and in a rat glaucoma model of elevated IOP by immunization with a synthetic peptide capable of targeted auto-antigen mimicry, such as COP-1⁴⁴ (a random copolymer of tyrosine, alanine, lysine, and glutamic acid in specific proportions). The protective action of agents such as COP-1 may be quite general, as is heat stress, and appears to be effective against autoimmune conditions including experimental autoimmune uveitis.⁴⁵ The protective immune response by agents such as COP-1 appears to be T-cell dependent and may involve up-regulation of BDNF.⁴⁶

Ischemic and heat shock preconditioning are the best understood cellular mechanisms for endogenous neuroprotection. Responses to hypoxia are mediated by hypoxia-inducible factors (HIF-1,2) that promote the transcription of a specific set of genes with hypoxia-response elements in their regulatory domain. The expression of HIFs can be up-regulated by cytokines, such as IGF, but their activation is controlled posttranslationally. Under normal conditions (normoxia) the oxygen-dependent enzymatic hydroxylation of the low constitutive level of HIF-1 targets the protein for rapid degradation through the UPP, resulting in a half-life of only a few minutes in the cytoplasm. However, during hypoxic stress the UPP-mediated destruction is attenuated and the HIFs translocate to the nucleus.

Genes that are up-regulated by HIFs include those required by cells to adapt to decreased oxygen by shifting their metabolism to glycolysis; genes for erythropoietin, which is neuroprotective in many model systems including RGC after optic nerve axotomy⁴⁷; and genes for vascular endothelial growth factor, which is involved in neovascularization that is often a consequence of ischemia.⁴⁸

Heat stress in particular (but other stresses as well) induces a set of protective heat shock proteins in most cell types termed HSPs, which are members of the larger chaperonin family. Constitutive expression of HSPs in retinal neurons and glia is enhanced in glaucomatous primate retinas, especially in the RGC and nfl layers,⁴⁹ but also in the optic nerve head.⁵⁰ Induction of some HSP proteins by systemic treatment with Zn²⁺ or geranylgeranylacetone is reported to be neuroprotective of RGC in a high IOP glaucoma model in the rat,^51,52 as is HSP-27 applied to the eye by electroporation in the rat ischemia-reperfusion model.⁵³ The expression of HSPs is controlled in part by a transcription factor, HSF-1. Similar to the HIF described previously, HSF-1 does not translocate to the nucleus in the absence of a stress. A new agent, bimoclomol, has recently been described that binds to HSF-1, prolonging its activation, DNA-binding, and transcriptional activity.⁵⁴ Bimoclomol is a hydroxylamine derivative and the co-inducing or potentiating effect of stress-induced HSP expression by this class of drug has already shown promise in one model of severe neurodegeneration.⁵⁵ Agents, such as bimoclomol, that act on stress-activated transcription of protective genes represent an important lead-compound toward the goal of harnessing endogenous neuroprotection for future therapies. This endogenous response is likely to be more effective than the mostly single-action agents currently being studied because multiple protective pathways are activated.

Apoptotic pathways involve complex multiply linked signaling steps that can be subdivided into three phases: initiation, decision, and degradation. Figure 1 is a schematic representation of these phases that depicts some of the known signal pathways and proteins, identified mostly from studies on cultured neurons, and it includes both pro-apoptotic and anti-apoptotic pathways. Studies on cell culture systems have also established the proof-of-principle that blocking one or more steps in pro-apoptotic signaling can delay the demise of a cell if not prevent it. It is evident that the earlier the pro-apoptotic signal cascade is interrupted the greater the likelihood will be of reversing it and thus preventing cell death. At present, most agents studied in model systems to establish proof-of-principle have been targeted to the post-mitochondrial degradation signals. These include drugs that are caspase inhibitors, inhibitors of caspase-activated DNAses, and inhibitors of endonucleases (Fig. 1). These agents have been shown to delay neural cell death in model systems, but there is some question whether blocking components mediating the late decisional and degradative phases may not be too late a step for a clinically useful drug to prevent, as opposed to delay, the death of that cell. However, on the basis of the emerging role of activated caspases generating toxic peptides from normal neuronal proteins such as amyloid precursor protein (APP), blockade of caspases and other proteases associated with apoptosis could be an important way to prevent signals from dying RGC causing secondary RGC death (see later discussion on APP).

Mitochondria are most often viewed for their primary metabolic role of oxidative phosphorylation, but they also contribute to the control of intracellular Ca²⁺ levels and generate superoxide, both of which can influence the survival of neurons. However, their role in apoptosis signaling is emerging as a major target area for neuroprotection. There is an increasing body of evidence that mitochondria play a key decisional role in apoptosis initiated by the insults of ischemia, excitotoxins, and trophic insufficiency (shown in red) thought to be responsible for RGC death in glaucoma.⁵⁸ The scheme in Figure 1 illustrates a pro-apoptotic pathway for trophic insufficiency (shown in yellow), acting through the enzyme sphingomyelinase, ceramide, and a phosphatase, which interrupts the anti-apoptotic signal pathway for BDNF and other trophic/growth factors (shown in green) by preventing activation of Akt, a key step in the neurotrophin anti-apoptotic signal pathway. The active (phosphorylated) form of Akt, which is itself a kinase, converts a pro-apoptotic protein such as BAD or BAX into phosphorylated form (P-BAD/BAX), thus inactivating it. Also shown is a pathway for local ischemia and overactivation of glutamate receptors (ionotropic glutamate receptor [iGluR]) acting through p53 and pro-apoptotic protein BAX, or related proteins such as BIM. These pathways also affect the anti-apoptotic interaction of BCL-2–like proteins that have specific binding sites on mitochondria. (Not shown is a more direct alternative pathway for iGluR overactivation and ischemia, which by induced cell depolarization, elevation of intracellular Ca²⁺ levels, and metabolic stress may directly cause mitochondrial depolarization and activate mitochondrial apoptotic signaling.) Thus, the major noxious initiating stimuli thought to be relevant to glaucoma (shown in red) seem to converge on the mitochondrion.

APOPTOSIS SIGNALING BY MITOCHONDRIA

The mitochondrion is a double-membrane organelle with the components for oxidative phosphorylation (e.g., the electron transport chain, cytochrome C, adenosine triphosphate (ATP) synthase, and adenine nucleotide transporter) located within or associated with the inner membrane. Energy derived from the electron transport chain pumps protons and other ions from the mitochondrial matrix across the inner membrane, maintaining a pH gradient and an electrochemical potential difference, termed the mitochondrial potential (MP). The recent development of potentiometric dyes that accumulate in and provide a measure of MP has shown that metabolic stress as well as noxious stimuli, such as trophic withdrawal, causes a decrease in MP that precedes apoptotic degeneration of a cell. The decrease in MP under metabolic stress, which depletes ATP, is caused in part by closure of ATP-activated K⁺ channels. As important as metabolism to maintain MP is the regulation of MP by a large complex of proteins that associate together at contact points of the mitochondrial inner and outer membranes, termed the mitochondrial permeability transition pore complex (PTPC). The PTPC normally functions as a regulated pore that can effect highly selective transfer of small molecules, including small proteins, between the matrix (the mitochondrial intermembrane space) and the host cell's cytoplasm. However, a sustained abnormal nonselective high-conductance opening of the pore results in dissipation of MP and is a critical event in the release of pro-apoptotic degradative signaling proteins. In the rat chronic elevated IOP glaucoma model, a significant downward shift in the distribution of MP level in RGC mitochondria was found compared with the retina of the contralateral normotensive eye.⁵⁹ This preliminary result indicates that conclusions about the major role of mitochondria in apoptotic cell death obtained with cultured neurons may apply to apoptosis of RGCs in glaucoma. The release of signaling molecules from the mitochondria (pink, Fig. 1) includes cytochrome C and other caspase activators (Apaf-1), procaspase 9, and activators of the endonucleases that degrade DNA (apoptosis inducing factor [AIF]).

Several ligands have been found that maintain regulation of the PTPC in the molecule-selective and low-conductive or closed position, and prevent the release of apoptotic degradative signal proteins. These drugs include cyclosporine and its analogue FK506, which have anti-apoptotic activity in neural culture systems. Systemic treatment with FK506 is somewhat effective in reducing RGC loss in the in vivo optic nerve crush model in the rat.⁶⁰ Another agent that maintains MP, but by activating ATP-regulated K⁺ channels (gabapentin-lactam), has also recently been shown to prolong RGC survival in culture.⁶¹ Proof-of-principle experiments have yet to be done on chronic elevated IOP models to determine whether MP stabilizing agents such as cyclosporine, FK506, or K (ATP) channel activators can prevent or delay RGC death in these clinically more relevant cases.

BCL-FAMILY PROTEINS IN APOPTOSIS

Members of the BCL family of proteins are the major participants in regulation of PTPC opening. The association of the anti-apoptotic protein BCL-2 and/or BCL-X_L with the PTPC appears to facilitate closure and/or to prevent the conformational change of the pore into the high-conductance state, thus maintaining the MP and blocking release of apoptotic signal proteins. Conversely, PTPC binding of another member of this protein family, BAX dimer, increases nonselective mitochondrial permeability and is pro-apoptotic. As indicated in Figure 1, up-regulation of BAX, and/or a related protein BIM,⁶² appears to be a major signal for apoptosis initiated by lack of neurotrophic anti-apoptotic signaling in the retina or by excitotoxicity. A fifth member of this family, BAD, can bind to BCL-2/BCL-X_L and deactivate its antiapoptotic action in mitochondria. However, when BAD is phosphorylated through the neurotrophin receptor/PI3–kinase/Akt signal pathway (to P-BAD), its ability to block the anti-apoptotic actions of BCL-2/BCL-X_L is removed. These findings from studies in neural cultures indicate that members of the BCL protein family function as essential “gatekeepers” of PTPC pore opening and can control the decisional phase of apoptosis.

Almost all neural cells in the rat retina express BCL-X_L and some also express BCL-2.⁶³ BAX and BIM are also expressed and up-regulated in RGCs after optic nerve lesions.⁶⁴ Studies done mostly in mice have established the important role of some members of the BCL protein family in RGC survival/death signaling. The effect of gene knockout and overexpression of BCL-2 and for BAX have been studied in two types of RGC apoptosis: developmental programmed cell death (PCD) in the neonate and pathologic cell death after optic nerve axotomy, ischemia, or NMDA excitotoxity.²⁵ Developmental PCD in the mouse neonate is complete by 10 to 14 days after birth with more than 60% loss of RGC. BCL-2 deficiency had no effect on the rate and extent of RGC death in developmental PCD or after optic nerve axotomy in the adult. However, adult BCL-2 null mutants had 29% fewer RGC axons⁶⁵ and showed no change in the 10% to 15% of RGC surviving axotomy,⁶⁶ suggesting one minority adult subpopulation of RGC may require BCL-2 and a smaller subpopulation that is independent of it for long-term survival. In contrast, in mice overexpressing BCL-2 90% of the RGC in the newborn are maintained through the developmental PCD phase into adulthood. Axotomy in adult mice overexpressing BCL-2 also showed a major difference; 65% of RGC showed long-term survival compared with approximately 10% in normal mice.⁶⁷

With regard to BAX deficiency, developmental PCD was largely blocked with more than double the normal number of RGC surviving in the adult. BAX deficiency also prevented RGC death caused by optic nerve crush,⁶⁸ as well as in the glaucoma model with chronic high IOP in the DBA/2J mouse,¹³ but did not protect against NMDA-mediated excitotoxic death. BAX overexpression might be expected to potentiate apoptosis, but surprisingly was found not to affect developmental PCD of RGC or change the extent of RGC loss caused by optic nerve crush, axotomy, or excitotoxicity.

Overall these findings emphasize the important role that BCL-family proteins play in survival/death signaling in RGC and indicate the complexity of their interactions in apoptosis. The role of these proteins appears to differ in the apoptosis pathways for neonatal RGC in developmental PCD and for apoptotic RGC death in the adult caused by injury or other insult. Also, different experimental damaging stimuli appear to activate different pathways of cell death. In particular, there is a significant difference in pathways thought to be of major importance in glaucoma, namely, between optic nerve damage or ischemia, and experimental excitotoxicity with NMDA.

As indicated in Figure 1, one of the pathways for excitotoxicity by overactivity of iGluR and ischemic stress in the retina involves the p53 tumor suppressor protein, which is a transcription factor. The p53 protein is not required for developmental PCD, but in ischemia or oxyradical exposure, p53 expression increases in neurons.⁶⁹ Mice deficient in p53 are reported to be resistant to kainate neuronal damage,⁷⁰ but NMDA excitotoxicity in the retina of such mice still proceeds, but by an alternative pathway.⁷¹ Up-regulation of p53 increases expression of a number of genes involved in apoptosis, for example, BAX (see earlier discussion) and especially glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (Fig. 1). GAPDH has important metabolic functions and converts NAD⁺ to NADH, but it also has other activities, in particular a role in apoptosis signaling that is not completely understood.⁷² GAPDH is the major source of NADH to support mitochondrial oxidative phosphorylation. It occurs in the neural cytoplasm as a proenzyme in the form of a tetramer bound to RNA. An increase in cytoplasmic NAD⁺/NADH ratio signals metabolic demand and frees the inactive bound form to enzymatically active GAPDH. However, modification of GAPDH, for example, by oxidation or nitrosylation by nitric oxide, releases it from RNA binding in an enzymatically inactive form that interrupts the NADH supply. Thus the signaling by this non-enzyme form of GAPDH to participate in apoptosis is mediated in part by metabolic stress to mitochondria caused by NADH deficiency (e.g., interruption of electron transport, oxidative phosphorylation, and loss of MP). This signaling can be initiated by ischemia or elevated levels of oxyradicals and/or nitric oxide. Additional properties of enzymatically inactive GAPDH in neurons undergoing apoptosis is interference with the anti-apoptotic BCL-2–like proteins indicated in Figure 1, and nuclear accumulation. The role of localization of GAPDH to the nucleus as a possible transcription factor for proteins involved in apoptotic signaling is not known.

Deprenyl and related propargylamines, which were originally developed as inhibitors of monamine oxidase enzymes, have anti-apoptotic properties related to their interaction with GAPDH. The mechanism of this activity is thought to be that these drugs bind to GAPDH, which causes dissociation of the native tetramer into dimers. The dimeric forms of GAPDH remain active in glycolysis and NADPH synthesis, but it appears that their capacity to signal apoptosis and for nuclear binding is prevented by deprenyl binding. Whatever the mechanism for anti-apoptotic activity, the finding that deprenyl increases survival of RGCs in the rat optic nerve crush model may indicate a future role for propargylamines in glaucoma therapy.⁷³

GLUTAMATE EXCITOTOXICITY

An obvious candidate for a toxic endogenous secondary mediator is excessive glutamate—the major neurotransmitter input to RGCs.⁷⁴ Prolonged exposure to a high level of glutamate has been found to be toxic to virtually all neurons and results from the persistent activation of glutamate receptor-coupled ion channels. This causes prolonged depolarization of the cell by Na⁺ entry/K⁺ efflux through the open glutamate receptor-coupled channels, as well as excessive Ca²⁺ entry into the cell. Excitotoxic death of RGC uses the p53-dependent apoptotic pathway shown in Figure 1, but not exclusively so.⁷¹ Among the multiplicity of such glutamate receptors, the NMDA and kainate subtypes seem to be the most important in causing toxicity in the retina, but these excitotoxins kill other retinal neurons in addition to RGC. Each NMDA/kainate receptor complex has multiple sites for binding of ligands besides glutamate. These include an accessory binding site for glycine or D-serine acting as a co-agonist, a modulatory site that binds polyamines, and binding sites within the channel for drugs such as MK801 or memantine. In addition to these ligand-binding sites, the receptor/channel activity is controlled by divalent cations (Mg²⁺, Zn²⁺), by the redox state or nitrosylation of proteins composing the channel, and by phosphorylation/dephosphorylation sites regulated by protein kinase/phosphoprotein phosphatase enzymes. All of these are potential sites for drugs that might be useful to control excitotoxicity if it is definitively shown to be involved in glaucomatous retinal pathology. Several such agents that have been tested for proof-of-principle in model systems may have application to block or delay RGC death, and one of them (memantine) is in clinical trial.

The loss of ionic homeostasis by prolonged iGluR (Fig. 1) overactivation leads to depletion of ATP, which is consumed in the effort to restore ionic balance, and to a severe metabolic stress to the cell. Metabolic stress and Ca²⁺ overload, whether caused by glutamate/aspartate excitotoxicity or by ischemia, can initiate production of additional toxic mediators as well as activate intracellular mitochondrial apoptotic signal transduction pathways (as discussed previously). A specific example of this is the association of NMDA receptors with postsynaptic density protein-95, which couples NMDA receptor activation to potential nitric oxide toxicity by acting as a cosignal with Ca²⁺ for activation of neuronal NOS.⁷⁵ This may account for the finding that knockout mice that do not express neuronal NOS were found resistant to RGC death by NMDA excitotoxicity.⁷⁶ This indicates that it is actually nitric oxide that mediates experimental excitotoxic death of RGC in the mouse retina (nitric oxide toxic and protective actions are discussed further next).

The evidence that glutamate excitotoxicity could play a role in clinical glaucoma and anterograde death of RGCs is circumstantial and also controversial. Elevated IOP in the eyes of some animal models decreases glutamate transporters⁷⁷ and in some subjects with glaucoma is associated with increased glutamate levels relative to other amino acids in the vitreous.⁷⁸ Elevation of glutamate levels was also found in the aqueous humor after optic nerve crush in the rat.⁷⁹ Antagonists of iGluR, particularly NMDA receptor blockers, such as MK801 and memantine, appear to reduce the loss of RGCs in rodent optic nerve crush, ischemia-reperfusion, and chronic high IOP models.^80–82

Despite these promising results from model systems, many significant questions relating to a role for glutamate excitotoxicity in clinical glaucoma remain. For example, it is unclear whether the modest elevations of vitreous glutamate reported in some studies reflect toxic concentrations at the level of RGCs or whether this is merely an epiphenomenon resulting from a stress-induced minor perturbation or adjustment of the rapid turnover rate of the glutamate/glutamine system mediating normal visual function. If one accepts that excitotoxic levels of glutamate do in fact cause secondary RGC death in the glaucomatous retina, this raises two important questions. The first is the cause for the excessive glutamate level, that is, whether there is an abnormally high release from nerve terminals synapsing on RGCs or whether the release is at normal levels, but the uptake and metabolism of glutamate by cells surrounding RGCs are impaired. If either of these suppositions is correct, then it implies that glaucoma initiates pathology in retinal cells other than RGCs, either excessive glutamate signaling by neurons such as the bipolar cells and/or especially in glial cells that take up and metabolize glutamate, for example, in Muller cells. Muller cells respond to almost any retinal stress by changes in gene expression and functional activities possibly involved in glutamate metabolism, but the significance of this in glaucoma is still largely unknown.

Neuronally released glutamate is cleared through high-affinity glutamate transporters by rapid uptake, mainly into glial cells and their processes that surround every glutamatergic synapse, as Muller cells do with respect to RGCs, but also into neurons. The transporters are activated by extracellular glutamate and use the electrochemical gradient of Na⁺ as the driving force for uptake, resulting in tight coupling of the uptake of one glutamate with approximately three Na⁺ ions and the efflux of one K⁺ ion. Glial cells maintain this ability to rapidly terminate glutamatergic transmission by metabolizing the glutamate to glutamine and restoring the Na⁺ and K⁺ gradients through the Na⁺/K⁺-ATPase; enzyme activities that together consume the two ATP molecules provided by glycolysis of one glucose molecule to lactate. Thus, any impairment of glucose uptake or metabolism that depletes ATP in glial cells, such as ischemia, has the potential to facilitate excitotoxicity to neurons at normal levels of glutamate neurotransmission by decreasing the efficiency of glutamate uptake, thus raising and prolonging its extracellular level. The possibility that elevated endogenous glutamate can cause excitotoxic loss of RGC has been demonstrated in the rat eye by intravitreal injection of inhibitors of the transporter.⁸³ Additional support for this may be a study done in the primate eye,⁴¹ which showed that RGCs in glaucomatous monkey eyes were preserved in focal areas where visual input to the RGCs was removed by prior laser ablation of the photoreceptors in that area. One interpretation of this finding is that removing the source of glutamate neurotransmission to RGCs prevents RGC death that results from chronic high IOP. However an alternative interpretation discussed previously is that the laser damage induces a local endogenous protective response analogous to that for other forms of preconditioning.

A second question arising from the hypothesis that glutamate excitotoxicity causes a secondary apoptotic death in apparently normal RGCs that are unaffected by the primary pathology relates to whether such cells are in fact normal or have an abnormal sensitivity to glutamate excitotoxicity. The latter supposition implies that RGCs undergoing the primary glaucomatous pathology or their associated glial cells may release diffusible mediators that make neighboring RGCs susceptible to damage by normal glutamate levels. Such nonprotein mediators could include peroxynitrite formed from nitric oxide, or high levels of reactive oxyradicals resulting from the mitochondrial response to metabolic stress (see earlier discussions).

PROTEIN MEDIATORS OF RETINAL GANGLION CELL DEATH

Toxic mediators of secondary RGC death can also be derived from endogenous proteins or peptides. One such factor secreted by injured RGC has recently been identified as semaphorin-3, which was thought to function only in development as a chemorepellent guidance molecule involved in directing growing axons to their target. Semaphorin-3 is up-regulated in the RGC layer in ocular hypertensive rabbits⁸⁴ and is transiently highly induced throughout the rat retina shortly after optic nerve transection. A peptide derived from semaphorin-3A causes more than 50% loss of RGC when injected into normal rat eyes, and the loss of RGCs after optic nerve axotomy can be completely prevented by intravitreal injection of a semaphorin-3 neutralizing antibody.⁸⁵

Clinical and experimental studies indicate that β-amyloid might also play a role in RGC death, as is thought to be the case in the death of neurons in Alzheimer's disease.⁸⁶ The β-amyloid peptide is produced from a normal neuronal precursor protein, APP, which is expressed in RGCs.⁸⁷ The β-amyloid peptide can be generated by specific cleavage of APP by the protease β-secretase (also termed presenilin-1) and further cleaved to additional pro-apoptotic peptides by activated caspases—the same proteases involved in apoptotic degradation. After optic nerve section or chronic high IOP in the rat, RGCs undergoing apoptosis show activation of caspase-3 and/or 8 and also generate the diffusible β-amyloid that is itself a potent inducer of secondary neuron death.⁸⁸ Some APP is also expressed in Muller cells that stain for presenilin-1 as well.⁸⁹ Therefore, stressed Muller cells could also be a source of β-amyloid in a glaucomatous retina. Stressed retinas express additional extracellular protein mediators that can cause secondary apoptotic cell death. TNF-α is a signal protein of this type (Fig. 1) that like nitric oxide can have both destructive and protective actions (see below).

The preceding brief description of some of the possible mediators of secondary RGC death indicates various drug classes that may have potential therapeutic value for glaucoma. These would include glutamate receptor antagonists or agonists at inhibitory γ-aminobutyric acid (GABA) receptors to counteract excitotoxicity; NOS inhibitors; antioxidants and free radical traps to minimize damage by oxyradicals; and protease inhibitors to block generation of neurotoxic peptides.

MEDIATORS WITH DESTRUCTIVE AND PROTECTIVE ACTIONS

TNF-α is an important cytokine present in many pathologic conditions, and it may play several roles in the glaucomatous retina. TNF-α is produced mainly by glial cells, particularly when stress-activated, and has two known receptors: TNF-R1 directly associated with pro-apoptotic signaling and TNF-R2 associated with anti-apoptotic activity. In addition, TNF-α can activate cells to release further potentially neurotoxic substances, such as nitric oxide, in the human glaucomatous optic nerve head.⁹⁰ Up-regulation of TNF-α in the glial cells of the inner retinal layer and of TNF-R1 in the RGC was found in glaucomatous human retinas,⁹¹ suggesting that TNF-α may be involved in secondary death of RGC by acting directly on the cell body.⁹² Figure 1 indicates the direct pro-apoptotic pathway for TNF-R1 and caspase-8, which does not involve mitochondrial signaling. However, in another experimental study TNF-α was reported to prevent secondary death of RGC after axotomy of the optic nerve in vivo.⁹³ Experiments on mixed glial/RGC cultures showed that TNF-α decreased cellular outward K⁺ currents, which was associated with increased anti-apoptosis signaling by the PI3-k/Akt pathway in the RGC. These findings suggest neuroprotection of RGC by TNF-R2 receptors. The availability of mice deficient in TNF-α, or either of its receptors, has added further complexity to the role(s) of this cytokine in the retina. In the mouse ischemia-reperfusion model, TNF-α deficiency did not affect the extent of RGC loss, whereas absence of TNF-R1 greatly reduced loss.⁹⁴ A similar decrease in RGC loss was also found for the optic nerve crush model in TNF-R1–deficient mice.⁹⁵ Absence of TNF-R2 led to enhancement of RGC loss. These experimental findings on TNF-α leave open the question on the role this cytokine plays at the RGC level in clinical glaucoma.

As discussed previously, nitric oxide (NO) is thought to be destructive in the glaucomatous retina, but it also has the potential to be neuroprotective at the RGC level. On the one hand it appears to participate in RGC death caused by excitotoxic activation of NMDA receptors, but it can also block excitotoxicity by S-nitrosylation of iGluR receptors. Other experiments indicate additional neuroprotective actions of NO,⁹⁶ including inactivation of caspases, protein kinase G activation of PI3-k, and increases in expression of genes for protective proteins, heme oxygenase, BCL-2, and HSP-70. Understanding the role(s) of NO in glaucoma is further complicated by a recent report on DBA/2 glaucomatous mice in which the absence of NOS-2 gene expression did not affect the progression of glaucomatous damage to the optic nerve.⁹⁷ On the other hand NOS-2 inhibitors are reported to protect RGC in the chronic high IOP and ischemia models in the rat.⁹⁸ These conflicting findings might be explained by the hypotheses that in the absence of NOS-2 the death of RGC in glaucoma proceeds by an alternative pathway or that NOS-2 inhibitors act by mechanisms other than NOS-2 inhibition.

The immune system can also have different roles in glaucoma. As discussed in the section on neuroprotective responses to preconditioning, induction of protective immunity by auto-antigen reactive T cells may protect the retina against secondary degeneration of RGC. The protective response seems to require immunization by an antigen that is also present at or near the site of damage, but this also has the risk of causing a destructive autoimmune response at the same locus. The possibility that autoimmune mechanisms have a pathogenic role in some glaucoma cases arose from initial observations that many patients with normal tension glaucoma had elevated serum titers of antibodies cross-reactive to optic nerve and retina antigens. Among these are autoantibodies to neuroprotective HSPs, such as HSP-60 and HSP-27, which are up-regulated in glaucomatous eyes.⁹⁹ The antibodies to HSP-27 are toxic to RGC in culture, but it is not known whether this direct pro-apoptotic effect on RGC occurs in vivo. Immune-mediated retinal pathology may also be caused by resident microglial cells that are capable of an immune response. Activated microglia, similar to the other glial cells in the retina, can release cytokines, such as TNF-α and other factors that are potentially neurotoxic.

Neuroprotective therapies aimed at the glaucomatous retina will most likely focus on agents that protect RGCs and optic nerve axons from the initiating and extracellular signals for RGC death, and secondarily on agents that interrupt the intracellular signal cascade for RGC apoptosis at the decisional or earlier phase (Fig. 1). The main focus should be to intercept, block, or counteract signals leading to damage of RGC axons or somata because current knowledge about regeneration is not sufficiently advanced for development of drugs targeted to this component of neuroprotection. In addition to agents already mentioned in the foregoing part of this chapter, recent studies on retinal neurons in culture or in acute in vivo models have provided additional leads for potential neuroprotective drugs in glaucoma. These include citicholine,¹⁰⁰ the tetracycline antibiotic minocycline,¹⁰¹ lithium chloride,¹⁰² arachidonic acid,¹⁰³ inhibitors of cyclo-oxygenase-2,¹⁰⁴ calpain and other protease inhibitors,¹⁰⁵ gingko biloba extract,¹⁰⁶ and endogenous antioxidants thioredoxin,¹⁰⁷ hemopexin,¹⁰⁸ and metallothionein.¹⁰⁹

Despite all these possibilities, most of which are only partially effective in preserving RGC for a short period, there is slow progress in moving forward from proof-of-principle to a clinically useful neuroprotective agent for glaucoma therapy. There are several reasons for this, including the difficulties of performing in vivo experimental drug studies in chronic elevated IOP animal models and also in devising clinical studies for retinal neuroprotection. Another reason for slow development of clinical neuroprotectants may relate to the “one agent, one effect” concept for experimental drug studies. This research approach has greatly increased our understanding of cellular responses and intracellular signal cascades relating to neuron death or survival. However, for a process as complex as neuron apoptosis, which has multiple signals and alternative/default pathways, no single agent is likely to be highly effective. It may be that the path to a truly effective neuroprotective glaucoma therapy is to study a cocktail of several agents, each of which affects a different signal cascade leading to cell survival or to apoptosis. The same considerations probably will apply to the gene-therapy approach.

With regard to drugs in current ophthalmic clinical use that, on the basis of model system studies, already appear to have some neuroprotective activity on the retina/optic nerve, namely, brimonidine and betaxolol, it will be a challenge to demonstrate in a clinical study that these agents actually have a direct neuroprotective component that is independent of their IOP-lowering effect. Furthermore, for these established drugs and any new agents that are to be used topically, it will be important to determine whether a sufficient quantity of drug can reach the retina or optic nerve head to exert a measurable protective effect on RGCs after administration to the surface of the eye. Unfortunately, almost all the pharmacokinetic factors affecting topically applied ophthalmic drugs work against penetration to the posterior pole. These include losses caused by the dynamics of the tear film, penetration through the cornea, drug dilution and clearance from the anterior chamber, absorption into and clearance by the vasculature of the anterior uvea, diffusion into and dilution by the vitreous, and vascular clearance from the retina.

Getting a topically applied drug to the retina will be difficult to achieve for any combined ocular hypotensive + neuroprotective agent developed in the future. Thus, a great need exists for innovative approaches to increase access of topical agents to the retina, because the alternative approach of systemic chronic treatment for glaucoma brings with it another set of problems. Among these new approaches are a means to greatly increase drug penetration across barriers of cornea, conjunctiva, and sclera, and a means to improve the amount of drug retained by tissues in the vicinity of the retina. Possible examples of such strategies are to link drugs to peptoid molecular transporters modeled on part of the TAT sequence of the acquired immune deficiency syndrome virus¹¹⁰ and/or increasing the binding of drug to melanin in the retinal pigment epithelium. Pharmacodynamic factors relating to topical application will therefore play a decisive role in determining whether any future single drug or a cocktail that is purely neuroprotective (based on proof-of-principle in model systems and no effect on IOP) can become an effective sole therapy for glaucoma.

Chronic systemic therapy is the alternative to topical administration of neuroprotective agents for glaucoma. Proof-of-principle for some degree of neuroprotection in model systems has already been established for five such drugs that are in clinical use for other conditions and that have no significant IOP effect, namely, phenytoin for epilepsy, memantine and riluzole for Alzheimer's disease, lithium for bipolar disorder, and deprenyl for Parkinson's disease. It may be worthwhile to study these patient groups to establish the usefulness of these agents for glaucoma by evaluating their effectiveness, each presumably with a different neuroprotective mechanism, to lower the incidence of glaucomatous neuropathy. An added benefit of such studies could be to firmly establish whether the neuroprotective effects found in model systems of RGC death extrapolate to clinical glaucoma. However, even in such retrospective studies it might be difficult to determine whether these agents can slow progression of glaucoma, because their effect might be quite small when given singly. Also when individuals in the study population develop elevated IOP or show visual field loss or optic nerve cupping, they will have to be treated with a currently accepted agent to lower IOP and be excluded from the study. For the same reasons, a long-term clinical trial of a novel purely neuroprotective agent administered to patients with glaucoma on a chronic systemic basis will require extensive justification, and this has the additional difficulty of assessing the incidence and severity of side effects of a new systemic drug in relation to the ophthalmic benefit of treatment.

On the basis of these considerations, for the near term, neuroprotective agents for glaucoma therapy will most probably be drugs that either lower IOP or improve vascular perfusion in the retina/optic nerve head, combined with a proven neuroprotective action. The drugs should be topically active and will have been shown to reach the retina and optic nerve head in amounts sufficient to exert some neuroprotective effect, even if less than they have in model systems of RGC loss. It may be possible to develop agents to be given on a chronic systemic basis that are purely neuroprotective, that are more effective than current treatments for glaucoma, and that have an acceptably low incidence of side effects. However, this should be considered a long-term future goal, along with gene therapies for the retina introduced into the eye. It may take a long time to realize the goal of direct retinal neuroprotective therapy for glaucoma based on the disappointing progress, despite an enormous effort by basic science researchers and the pharmaceutical industry during the past decade to develop truly effective therapies for the other major neurodegenerative diseases.

1. Sheldon WG, Warbritton AR, Bucci TJ, Turturro A: Glaucoma in food-restricted and ad libitum-fed DBA/2NNIA mice. Lab Anim Sci 45:508, 1995

2. John SW, Smith RS, Savinova OV, et al: Essential iris atrophy, pigment dispersion, and glaucoma in DBA/2J mice. Invest Ophthalmol Vis Sci 39:951, 1998

3. Thanos S, Naskar R: Correlation between retinal ganglion cell death and chronically developing inherited glaucoma in a new rat mutant. Exp Eye Res 79:119, 2004

4. Agar A, Yip SS, Hill MA, Coroneo MT: Pressure related apoptosis in neuronal cell lines. J Neurosci Res 60:495, 2000

5. Flammer J: The vascular concept of glaucoma. Surv Ophthalmol 38(Suppl):S3, 1994

6. Prasanna G, Hulet C, Desai D, et al: Effect of elevated intraocular pressure on endothelin-1 in a rat model of glaucoma. Pharmacol Res 51:41, 2005

7. Flammer J, Haefliger IO, Orgul S, et al: Vascular dysregulation: A principal risk factor for glaucoma. J Glaucoma 8:212, 1999

8. Hernandez MR, Pena JD. The optic nerve head in glaucomatous optic neuropathy. Arch Ophthalmol 115:389, 1997

9. Neufeld AH, Hernandez MR, Gonzalez M: Nitric oxide synthase in the human glaucomatous optic nerve head. Arch Ophthalmol 115:497, 1997

10. Finn JT, Weil M, Archer F, et al: Evidence that Wallerian degeneration and localized axon degeneration induced by local neurotrophin deprivation do not involve caspases. J Neurosci 20:1333, 2000

11. Glass JD, Brushart TM, George EB, Griffin JWF: Prolonged survival of transected nerve fibres in C57BL/Ola mice is an intrinsic characteristic of the axon. J Neurocytol 22:311, 1993

12. Araki T, Sasaki Y, Milbrandt J: Increased nuclear NAD biosynthesis and SIRT1 activation prevent axonal degeneration. Science 305:1010, 2004

13. Libby RT, Savinova OV, Barter J, et al: BAX is necessary for glaucomatous RGC somal but not axonal degeneration in DBA/2J mice. Invest Ophthalmol Vis Sci 45:S2293, 2004

14. Osborne NN, Cazevieille C, Carvalho AL, et al: In vivo and in vitro experiments show that betaxolol is a retinal neuroprotective agent. Brain Res 751:113, 1997

15. Fern R, Ransom BR, Stys PK, et al: Pharmacological protection of CNS white matter during anoxia: Actions of phenytoin, carbamazepine and diazepam. J Pharmacol Exp Ther 266:1549, 1993

16. Netland PA, Chaturvedi N, Dreyer EB: Calcium channel blockers in the management of low-tension and open-angle glaucoma. Am J Ophthalmol 115:608, 1993

17. Sawada A, Kitazawa Y, Yamamoto T, et al: Prevention of visual field defect progression with brovincamine in eyes with normal-tension glaucoma. Ophthalmology 103:283, 1996

18. Hernandez MR, Agapova OA, Yang P, et al: Differential gene expression in astrocytes from human normal and glaucomatous optic nerve head analyzed by cDNA microarray. Glia 38:45, 2002

19. Neufeld AH, Kawai S, Das S, et al: Loss of retinal ganglion cells following retinal ischemia: The role of inducible nitric oxide synthase. Exp Eye Res 75:521, 2002

20. Chintala SK, Zhang X, Austin JS, Fini ME: Deficiency in matrix metalloproteinase gelatinase B (MMP-9) protects against retinal ganglion cell death after optic nerve ligation. J Biol Chem 277:47461, 2002

21. Zhang X, Chaudhry A, Chintala SK: Inhibition of plasminogen activation protects against ganglion cell loss in a mouse model of retinal damage. Mol Vis 9:238, 2003

22. Ren L, Mali R, Yang X, et al: Matrix metalloproteinases contribute to retinal degeneration in a murine model of glaucoma. Invest Ophthalmol Vis Sci 46:S1265, 2005

23. Tezel D, Hernandez MR, Wax M: Immunostaining of heat shock proteins in the retina and optic nerve head of human glaucomatous eyes. Arch Ophthalmol 118:511, 2000

24. Isenmann S, Kretz A, Cellerino A: Molecular determinants of retinal ganglion cell development, survival, and regeneration. Prog Retin Eye Res 22:483, 2003

25. Mansour-Robaey S, Clarke DB, Wang YC, et al: Effects of ocular injury and administration of brain-derived neurotrophic factor on survival and regrowth of axotomized retinal ganglion cells. Proc Natl Acad Sci U S A 91:1632, 1994

26. Martin KR, Quigley HA, Zack DJ, et al: Gene therapy with brain-derived neurotrophic factor as a protection: Retinal ganglion cells in a rat glaucoma model. Invest Ophthalmol Vis Sci 44:4357, 2003

27. Pugazhenthi S, Nesterova A, Sable C, et al: Akt/protein kinase B upregulates Bcl-2 expression through cAMP response element-binding protein. J Biol Chem 275:10761, 2000

28. Cheon EW, Kim YE, Cho YY, et al: Betaxolol, a β₁-adrenoceptor antagonist, protects a transient ischemic injury of the retina. Exp Eye Res 75:591, 2002

29. Wood JP, DeSantis L, Chao HM, Osborne NN: Topically applied betaxolol attenuates ischaemia-induced effects to the rat retina and stimulates BDNF mRNA. Exp Eye Res 72:79, 2001

30. Agarwal N, Martin E, Krishnamoorthy RR, et al: Levobetaxolol-induced up-regulation of retinal bFGF and CNTF mRNAs and preservation of retinal function against a photic-induced retinopathy. Exp Eye Res 74:445, 2002

31. Cui Q, Lu Q, So K-F, et al: CNTF, not other trophic factors, promotes axonal regeneration of axotomized retinal ganglion cells in adult hamsters. Invest Ophthalmol Vis Sci 40:760, 1999

32. Zhang C, Takahashi K, Lam TT, et al: Effects of basic fibroblast growth factor in retinal ischemia. Invest Ophthalmol Vis Sci 35:3163, 1994

33. Kermer P, Klocker N, Labes M, et al: Insulin-like growth factor-1 protects axotomized rat retinal ganglion cells from secondary death via PI3-k–dependent Akt phosphorylation and inhibition of caspase-3 in vivo. J Neurosci 20:2, 2000

34. Wen R, Chang T, Li Y, et al: Alpha-2-adrenergic agonists induce basic fibroblast growth factor expression in photoreceptors in vivo and ameliorate light-damage. J Neurosci 16:5986, 1996

35. Yoles E, Wheeler LA, Schwartz M: Alpha2-adrenoreceptor agonists are neuroprotective in a rat model of optic nerve degeneration. Invest Ophthalmol Vis Sci 40:65, 1999

36. Lafuente MP, Villegas-Perez MP, Mayor S, et al: Neuroprotective effects of brimonidine against transient ischemia-induced retinal ganglion cell death: A dose response in vivo study. Exp Eye Res 74:181, 2002

37. WoldeMussie E, Ruiz G, Wijono M, Wheeler LA: Neuroprotection of retinal ganglion cells by brimonidine in rats with laser-induced chronic ocular hypertension. Invest Ophthalmol Vis Sci 42:2849, 2001

38. Gao H, Qiao X, Hefti F, et al: Elevated mRNA expression of brain-derived neurotrophic factor in retinal ganglion cell layer after optic nerve injury. Invest Ophthalmol Vis Sci 38:1840, 1997

39. Fischer D, Pavladis M, Thanos S: Cataractogenic lens injury prevents traumatic ganglion cell death and promotes axonal regeneration both in vivo and in culture. Invest Ophthalmol Vis Sci 41:3943, 2000

40. Fischer D, Heiduschka P, Thanos S: Lens-injury-stimulated axonal regeneration throughout the optic pathway of adult rats. Exp Neurol 172:257, 2001

41. Nork MT, Poulsen GL, Nickells RW, et al: Protection of ganglion cells in experimental glaucoma by laser photocoagulation. Arch Ophthalmol 118:1242, 2000

42. Zhang C, Rosenbaum DM, Shaikh AR, et al: Ischemic preconditioning attenuates apoptotic cell death in the rat retina. Invest Ophthalmol Vis Sci 43:3059, 2002

43. Bakalash S, Kessler A, Mizrahi T, Nussenblatt R, Schwartz M: Antigenic specificity of immunoprotective therapeutic vaccination for glaucoma. Invest Ophthalmol Vis Sci 44:3374, 2003

44. Schwartz M: Vaccination for glaucoma: Dream or reality? Brain Res Bull 62:481, 2004

45. Zhang M, Chan CC, Vistica B, et al: Copolymer 1 inhibits experimental autoimmune uveoretinitis. J Neuroimmunol 103:189, 2000

46. Ziemssen T, Kumpfel T, Klinkert WE, Neuhaus O, Hohlfeld R: Glatiramer acetate-specific T-helper 1- and 2-type cell lines produce BDNF: Implications for multiple sclerosis therapy. Brain-derived neurotrophic factor. Brain 125:2381, 2002

47. Weishaupt JH, Rohde G, Polking E, et al: Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells. Invest Ophthalmol Vis Sci 45:1514, 2004

48. Ozaki H, Yu AY, Della N, et al: Hypoxia inducible factor-1alpha is increased in ischemic retina: Temporal and spatial correlation with VEGF expression. Invest Ophthalmol Vis Sci 40:182, 1999

49. Sakai M, Sakai H, Nakamura Y, Fukuchi T, Sawaguchi S: Immunolocalization of heat shock proteins in the retina of normal monkey eyes and monkey eyes with laser-induced glaucoma. Jpn J Ophthalmol 47:42, 2003

50. Tezel G, Hernandez R, Wax MB: Immunostaining of heat shock proteins in the retina and optic nerve head of normal and glaucomatous eyes. Arch Ophthalmol 118:511, 2000

51. Park KH, Cozier F, Ong OC, Caprioli J: Induction of heat shock protein 72 protects retinal ganglion cells in a rat glaucoma model. Invest Ophthalmol Vis Sci 42:1522, 2001

52. Caprioli J, Ishii Y, Kwong JM: Retinal ganglion cell protection with geranylgeranylacetone, a heat shock protein inducer, in a rat glaucoma model. Trans Am Ophthalmol Soc 101:39, 2003

53. Yokoyama A, Oshitari T, Negishi H, et al: Protection of retinal ganglion cells from ischemia-reperfusion injury by electrically applied Hsp27. Invest Ophthalmol Vis Sci 42:3283, 2001

54. Hargitai J, Lewis H, Boros I, et al: Bimoclomol, a heat shock protein co-inducer, acts by the prolonged activation of heat shock factor-1. Biochem Biophys Res Commun 307:689, 2003

55. Kieran D, Kalmar B, Dick JR, et al: Treatment with arimoclomol, a coinducer of heat shock proteins, delays disease progression in ALS mice. Nat Med 10:402, 2004

56. Levkovitch-Verbin H, Quigley HA, Kerrigan-Baumrind LA, et al: Optic nerve transection in monkeys may result in secondary degeneration of retinal ganglion cells. Invest Ophthalmol Vis Sci 42:975, 2001

57. Quigley HA, Nickells RW, Kerrigan LA, et al: Retinal ganglion cell death in experimental glaucoma and after axotomy occurs by apoptosis. Invest Ophthalmol Vis Sci 36:774, 1995

58. Tatton WG, Chalmers-Redman RME, Podos SM, et al: Maintaining mitochondrial membrane impermeability: An opportunity for new therapy in glaucoma? Surv Ophthalmol 45:5277, 2001

59. Mittag T, Danias J, Pohorenec G, et al: Retinal damage after 3 to 4 months of elevated intraocular pressure in a rat glaucoma model. Invest Ophthalmol Vis Sci 41:3451, 2000

60. Freeman EE, Grosskreutz CL: The effects of FK506 on retinal ganglion cells after optic nerve crush. Invest Ophthalmol Vis Sci 41:1111, 2000

61. Pielen A, Kirsch M, Hofmann HD, Feuerstein TJ, Lagreze WA: Retinal ganglion cell survival is enhanced by gabapentin-lactam in vitro: Evidence for involvement of mitochondrial KATP channels. Graefes Arch Clin Exp Ophthalmol 242:240, 2004

62. Napankangas U, Lindqvist N, Lindholm D, Hallbook F: Rat retinal ganglion cells upregulate the pro-apoptotic BH3-only protein Bim after optic nerve transection. Brain Res Mol Brain Res 120:30, 2003

63. Levin LA, Schlamp CL, Spieldoch RL, et al: Identification of the bcl-2 family of genes in the rat retina. Invest Ophthalmol Vis Sci 38:2545, 1997

64. Missing reference. Please provide

65. Cellerino A, Michaelidis T, Barski JJ, et al: Retinal ganglion cell loss after the period of naturally occurring cell death in bcl-2-/- mice. Neuroreport 10:1091, 1999

66. Dietz GP, Kilic E, Bahr M, Isenmann S: Bcl-2 is not required in retinal ganglion cells surviving optic nerve axotomy. Neuroreport 12:3353, 2001

67. Cenni MC, Bonfanti L, Martinou JC, et al: Long-term survival of retinal ganglion cells following optic nerve section in adult bcl-2 transgenic mice. Eur J Neurosci 8:1735, 1996

68. Li Y, Schlamp CL, Poulsen KP, Nickells RW: Bax-dependent and independent pathways of retinal ganglion cell death induced by different damaging stimuli. Exp Eye Res 71:209, 2000

69. Rosenbaum DM, Rosenbaum PS, Gupta H, et al: The role of the p53 protein in the selective vulnerability of the inner retina to transient ischemia. Invest Ophthalmol Vis Sci 39:2132, 1998

70. Morrison RS, Wenzel HJ, Kinoshita Y, et al: Loss of the p53 tumor suppressor gene protects neurons from kainate induced cell death. J Neurosci 16:1337, 1996

71. Li Y, Schlamp CL, Poulsen GL, et al: p53 regulates apoptotic retinal ganglion cell death induced by N-methyl-D-aspartate. Mol Vis 8:341, 2002

72. Tatton WG, Chalmers-Redman RME, Elstner M, et al: Glyceraldehyde-3-phosphate dehydrogenase in neurodegeneration and apoptosis signaling. J Neural Transm 60(Suppl):77, 2000

73. Buys YM, Trope GE, Tatton WG: (–)-Deprenyl increases the survival of rat retinal ganglion cells after optic nerve crush. Curr Eye Res 14:119, 1995

74. Sucher NJ, Lipton SA, Dreyer EB: Molecular basis of glutamate toxicity in retinal ganglion cells. Vis Res 37:3483, 1997

75. Sattler R, Xiong Z, Lu WY et al: Specific coupling of NMDA receptor activation to nitric oxide neurotoxicity by PSD-95 protein. Science 284:1845, 1999

76. Vorwerk CK, Hyman BT, Miller JW, et al: The role of neuronal and endothelial nitric oxide synthase in retinal excitotoxicity. Invest Ophthalmol Vis Sci 38:2038, 1997

77. Martin KR, Levkovitch-Verbin H, Valenta D, et al: Retinal glutamate transporter changes in experimental glaucoma and after optic nerve transection in the rat. Invest Ophthalmol Vis Sci 43:2236, 2002

78. Dreyer EB, Zurakowski D, Schumer RA, et al: Elevated glutamate levels in the vitreous body of humans and monkeys with glaucoma. Arch Ophthalmol 114:299, 1996

79. Yoles E, Schwartz M: Elevation of intraocular glutamate levels in rats with partial lesion of the optic nerve. Arch Ophthalmol 116:906, 1998

80. Yoles E, Muller S, Schwartz M: NMDA-receptor antagonist protects neurons from secondary degeneration after partial optic nerve crush. J Neurotrauma 14:665, 1997 (erratum in J Neurotrauma 16:345, 1999)

81. El-Asrar AM, Morse PH, Maimone D, et al: MK-801 protects retinal neurons from hypoxia and the toxicity of glutamate and aspartate. Invest Ophthalmology Vis Sci 33:3463, 1992

82. Chaudhary P, Ahmed F, Sharma SC: MK801: A neuroprotectant in rat hypertensive eyes. Brain Res 792:154, 1998

83. Vorwerk CK, Naskar R, Schuettauf F, et al: Depression of retinal glutamate transporter function leads to elevated intravitreal glutamate levels and ganglion cell death. Invest Ophthalmol Vis Sci 41:3615, 2000

84. Solomon AS, Kimron M, Holdengreber V, et al: Up-regulation of semaphorin expression in retina of glaucomatous rabbits. Graefes Arch Clin Exp Ophthalmol 241:673, 2003

85. Shirvan A, Kimron M, Holdengreber V, et al: Anti-semaphorin 3A antibodies rescue retinal ganglion cells from cell death following optic nerve axotomy. J Biol Chem 277:49799, 2002

86. Bayer AU, Keller ON, Ferrari F, Maag KP: Association of glaucoma with neurodegenerative diseases with apoptotic cell death: Alzheimer's disease and Parkinson's disease. Am J Ophthalmol 133:135, 2002

87. Morin PJ, Abraham CR, Amaratunga A, et al: Amyloid precursor protein is synthesized by retinal ganglion cells, rapidly transported to the optic nerve plasma membrane and nerve terminals, and metabolized. J Neurochem 61:464, 1993

88. McKinnon SJ, Lehman DM, Kerrigan-Baumrind LA, et al: Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension. Invest Ophthalmol Vis Sci 43:1077, 2002

89. Chen S-T, Gentleman SM, Garey LJ, et al: Distribution of β-amyloid precursor and B-cell lymphoma protooncogene proteins in the rat retina after optic nerve transection or vascular lesion. J Neuropathol Exp Neurol 55:1073, 1996

90. Yuan L, Neufeld AH: Tumor necrosis factor-alpha: A potentially neurodestructive cytokine produced by glia in the human glaucomatous optic nerve head. Glia 32:42, 2000

91. Tezel G, Li LY, Patil RV, Wax MB: TNF-alpha and TNF-alpha receptor-1 in the retina of normal and glaucomatous eyes. Invest Ophthalmol Vis Sci 42:1787, 2001

92. Tezel G, Yang X, Yang J, Wax MB: Role of tumor necrosis factor receptor-1 in the death of retinal ganglion cells following optic nerve crush injury in mice. Brain Res 996:202, 2004

93. Diem R, Meyer R, Weishaupt JH, Bahr M: Reduction of potassium currents and phosphatidylinositol 3-kinase-dependent AKT phosphorylation by tumor necrosis factor-(alpha) rescues axotomized retinal ganglion cells from retrograde cell death in vivo. J Neurosci 21:2058, 2001

94. Fontaine V, Mohand-Said S, Hanoteau N, et al: Neurodegenerative and neuroprotective effects of tumor necrosis factor (TNF) in retinal ischemia: Opposite roles of TNF receptor 1 and TNF receptor 2. J Neurosci 22:RC216, 2002

95. Tezel G, Yang X, Yang J, Wax MB: Role of tumor necrosis factor receptor-1 in the death of retinal ganglion cells following optic nerve crush injury in mice. Brain Res 996:202, 2004

96. Kang YC, Kim PK, Choi BM, et al: Regulation of programmed cell death in neuronal cells by nitric oxide. In Vivo 18:367, 2004

97. Libby RT, Smith RS, Savinova OV, Clark AF, John SWM: Inducible nitric oxide synthase (NOS2) is not required for glaucomatous optic nerve damage in DBA/2J mice. Invest Ophthalmol Vis Sci 44:S145, 2003

98. Neufeld AH: Pharmacologic neuroprotection with an inhibitor of nitric oxide synthase for the treatment of glaucoma. Brain Res Bull 62:455, 2004

99. Tezel G, Yang J, Wax MB: Heat shock proteins, immunity and glaucoma. Brain Res Bull 62:473, 2004

100. Grieb P, Rejdak R: Pharmacodynamics of citicoline relevant to the treatment of glaucoma. J Neurosci Res 67(2):143, 2002

101. Levkovitch-Verbin H, Habot-Wilner Z, Kalev-Landoy M, Melamed S: Minocycline is neuroprotective in glaucomatous and post optic nerve-transected rat eyes. Invest Ophthalmol Vis Sci 45:S1180, 2004

102. Huang X, Wu DY, Chen G, Manji H, Chen DF: Support of retinal ganglion cell survival and axon regeneration by lithium through a Bcl-2-dependent mechanism. Invest Ophthalmol Vis Sci 44:347, 2003

103. Kawasaki A, Han MH, Wei JY, et al: Protective effect of arachidonic acid on glutamate neurotoxicity in rat retinal ganglion cells. Invest Ophthalmol Vis Sci 2002; 43:1835, 2002

104. Ju WK, Kim KY, Neufeld AH: Increased activity of cyclooxygenase-2 signals early neurodegenerative events in the rat retina following transient ischemia. Exp Eye Res 77:137, 2003

105. Sakamoto YR, Nakajima TR, Fukiage CR, et al: Involvement of calpain isoforms in ischemia-reperfusion injury in rat retina. Curr Eye Res 21:571, 2000

106. Hirooka K, Tokuda M, Miyamoto O, et al: The Ginkgo biloba extract (EGb 761) provides a neuroprotective effect on retinal ganglion cells in a rat model of chronic glaucoma. Curr Eye Res 28:153, 2004

107. Shibuki H, Katai N, Yodoi J, Uchida K, Yoshimura N: Lipid peroxidation and peroxynitrite in retinal ischemia-reperfusion injury. Invest Ophthalmol Vis Sci 41:3607, 2000

108. Hunt RC, Hunt DM, Gaur N, Smith A: Hemopexin in the human retina: Protection of the retina against heme-mediated toxicity. J Cell Physiol 168:71, 1996

109. Chen L, Wu W, Dentchev T, Wong R, Dunaief JL: Increased metallothionein in light damaged mouse retinas. Exp Eye Res 79:287, 2004

110. Rothbard JB, Kreider E, VanDeusen CL, et al: Arginine-rich molecular transporters for drug delivery: Role of backbone spacing in cellular uptake. J Med Chem 45:3612, 2002