Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

Glossary

Apoptosis – The process of programmed cell death, whereby the cell proceeds through a highly regulated series of morphological changes resulting in the controlled disassembly of the affected cell.

Bax, Bak, Bad, and Bid – Proapoptotic proteins. Bcl-2 – B-cell CLL/lymphoma 2, an antiapoptotic protein.

Excitotoxicity – The process by which raised levels of neurotransmitters trigger cell death.

Glaucoma – A major cause of blindness worldwide, resulting from the loss of retinal ganglion cells, with raised intraocular pressure as a major modifiable factor.

Neuroprotection – The use of therapeutic agents to prevent or reverse neuronal damage thereby retaining physiological function.

Neurotrophic factors – The growth factors that promote the growth, differentiation, and survival of neurons.

Retinal ganglion cells – The neurons that relay visual information from their cell soma located in the retina, through their axons which project along the optic nerve to the brain.

Introduction

Retinal ganglion cells (RGCs) relay visual information from their cell soma located in the retina, through their axons which project along the optic nerve to the brain. Their loss is associated with various optic neuropathies such as Leber hereditary optic neuropathy, optic neuritis, anterior ischemic optic neuropathy, and glaucoma. The most prominent of these is glaucoma, which is a major cause of irreversible blindness worldwide. In this article, glaucoma is used to highlight the challenges involved in unraveling the complexities of RGC apoptosis and neuronal cell death in order to facilitate more effective treatment strategies.

Apoptosis

fragmentation, oxidative damage, and autophagic degeneration, commonly proceeding through either extrinsic or intrinsic caspase-dependent pathways outlined in Figure 1. Identifying which aspect of apoptosis regulation is susceptible in glaucoma has important implications for the elucidation of pharmacological targets.

Apoptosis in Glaucoma

The exact mechanism triggering RGC apoptosis in glaucoma is still unidentified, although the major modifiable risk factor identified is that of raised intraocular pressure (IOP) and is commonly used to investigate apoptotic mechanisms. Mitochondria play a fundamental role in RGC apoptosis, with raised hydrostatic pressure inducing apoptosis that is at least partially dependent on mitochondria. More specifically, Bax, a regulator of membrane permeability, has been suggested to be essential in triggering apoptosis, with RGCs-expressing mutant Bax showing complete resistance to raised hydrostatic pressure, even after axonal loss.

While the primary site of damage in RGCs appears to be the axon, leading to axonal degeneration, and a positive correlation between raised IOP and axon loss has been established, it does not inevitably lead to cell soma death. The contribution of axonal degeneration and secondary challenges to the cell soma are evaluated below.

Diagnosis and Measuring Glaucoma

Progression

Traditionally, glaucoma is screened by monitoring changes in IOP using tonometry, a technique lacking sensitivity in glaucoma detection, as damage to the RGCs can occur in the absence of raised IOP. This lack of sensitivity sparked the development and introduction of alternative screening methods. Examples of these are standard automated perimetry to measure visual-field loss, optical coherence tomography, allowing the quantification of the retinal nerve fiber layer (RNFL) thickness, and disk tomography to assess any structural damage to the optic nerve head (ONH). The major drawback common to these methods is the inability to detect the disease before considerable damage to the retina has occurred. It is estimated that death of 50% of the RGCs occurs before there is a significant-enough visual loss to diagnose glaucoma. The development of detection of apoptosing retinal cell (DARC), a method of detecting RGC apoptosis in glaucoma before the onset of visual loss, may be instrumental in early diagnosis and successful treatment

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of the disease as well as a high-throughput screening method for new neuroprotectants.

Current Treatments for Glaucoma

Once diagnosed, the current treatments for glaucoma concentrate on lowering the raised IOP. First-line pharmacological therapies include prostaglandin analogs which increase aqueous humor outflow and beta-blockers to reduce aqueous humor formation. Alpha-agonists to increase the uveoscleral outflow of aqueous humor and carbonic anhydrase inhibitors suppressing enzymes involved in aqueous humor production are used as second-line treatments, while thirdor fourth-line treatments rely on increasing trabecular outflow using

cholinergenic agonists and miotic agents. The disadvantages all the drugs share are the potential side effects and the requirement for topical administration up to four times daily. Furthermore, the incidence of poor compliance and persistence with the topical application is high and so the success of glaucoma management is limited. In addition, glaucoma appears to progress in many sufferers despite the continued use of pharmacological treatments. In these cases, surgery such as trabeculoplasty to reduce resistance to the outflow of aqueous humor by modifying the trabecular meshwork or trabeculotomy to remove part of the trabecular meshwork, allowing enhanced drainage of the aqueous humor, may prove successful in reducing IOP. As the degeneration of visual field in glaucoma is known to progress through RGC apoptosis, research is currently underway to develop neuroprotective treatments for glaucoma.

Neuroprotection

Neuroprotection is defined as the use of therapeutic agents to prevent or reverse neuronal damage, thereby retaining physiological function. For example, neuroprotective treatments are being researched for diseases which progress through the death of neurons of the central nervous system such as Alzheimer’s, Parkinson’s, and Huntington’s. The efficacy of these therapeutic agents in clinical trials has been somewhat controversial. A number of clinical trials performed on stroke patients testing different neuroprotective agents showed either little success or adverse side effects, while treating patients of spinal cord trauma with the neuroprotective agent methylprednisolone resulted in improved motor function. The variation in success of these agents is thought to be due to the mechanism of neuronal death in the different diseases. In strokes, neuronal injury occurs at the cell body, so irreversible cell death occurs instantly; therefore, treatment with a neuroprotective agent would be given too late. However, in spinal cord trauma, neuroprotective treatment may have the desired effect as injury occurs at the axon and death of the cell body results hours later. For this reason, the use of neuroprotection as a treatment for glaucoma, thought to be brought about by death of RGC axons, seems a promising option.

Research Models

Research into the eye disease uses models which are important tools allowing researchers to monitor the progression of the disease and test new therapies. In order for these models to provide informative and transferable data, similarity to the human disease and reproducibility is required. A number of different models are used in the study of the disease, all of which have their strengths and limitations.

In Vitro Glaucoma Models

Initial experiments for neuroprotection in glaucoma are likely to be carried out in vitro. A number of ocular cells have successfully been cultured and present a costeffective alternative to animal models for studying the effects of apoptosis and neuroprotection. Cell cultures have the advantage of allowing rapid screening of potential therapies and observation of direct effects on the cultured cells, under strict environmental control. Cell culture also provides a greater understanding of how compounds function at a cellular level; however, the response of the isolated cells, maintained in an artificial environment, may differ from that in situ. For this reason and the contentious use of immortalized cells to study apoptosis, a number of in vivo glaucoma models have been developed.

In Vivo Glaucoma Models

Pioneered in rat and more recently developed in monkey, the optic nerve crush is a well-calibrated and reproducible model of glaucoma. Damage to the optic nerve results in cell-body death and subsequent secondary injury to adjacent neurons, as seen in glaucoma. Alternatively, RGC apoptosis can be induced by raising the IOP and causing retinal ischemia, typically achieved through blockage of the aqueous humor outflow, including injection of hypertonic saline into the episcleral veins, cauterization of the episcleral veins, and laser photocoagulation of translimbus. As an alternative, excessive exposure to excitotoxins, such as glutamate or N-methyl-D-aspartic acid (NMDA) by intravitreal injections can be used to induce RGC apoptosis. A further model can be generated by laser coagulation at the retina where RGC apoptosis is induced adjacent to the site of laser contact. The DBA/J2 mouse is a genetically determined glaucoma model showing increased IOP, RGC apoptosis, optic nerve atrophy, and ONH cupping.

Mechanisms of Apoptosis and

Development of Neuroprotective Agents

Neurotrophic Factor Withdrawal

Deprivation of neurotrophic factors (NFs) induces apoptosis, and has been suggested to play a role in glaucoma as outlined in Figure 2. Raised IOP is proposed to lead to a blockade of anterograde and retrograde transports, preventing transport of NFs. The neuroprotective effects of neurotrophins (NTs), a family of NTFs, are thought to be mediated through the activation of phosphoinositide (PI)-

(3)-kinase, the inhibition of which is sufficient to block the survival effects of NT. PI(3)K phosphorylates and activates Akt which is known to target several key apoptosis regulators, including Bcl-2 antagonist of cell death (Bad) and cyclic adenosine monophosphate (cAMP) response element binding protein (CREB). The phosphorylation of Bad promotes its sequestration by the chaperone protein 14-3-3, while CREB is activated by Akt (also known as protein kinase B), leading to the upregulation of B-cell CLL/lymphoma 2 (Bcl-2). The actions of Akt have downstream consequences for mitochondrial function and both caspase activation and increased ROS production have been attributed to loss of NT-stimulated pathways.

NFs as Neuroprotective Agents

Brain-derived neurotrophic factor (BDNF) has been showed in a number of studies to have neuroprotective effects. Intravitreal injections of the NT administered to rats, following optic nerve transection and IOP-induced ischemia, increased the survival of RGCs. In addition to BDNF, ciliary neurotrophic factor (CNTF), glial-cell-line-derived neurotrophic factor (GDNF), and pigment-epithelium-derived

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growth factor (PEDF) have also demonstrated protection of RGCs in rat glaucoma models. The neuroprotective effects of the NFs are short-lived, with reduced survival of RGCs only weeks after a single injection. The longevity of the treatment has been addressed using a number of techniques. Injection of PEDF-peptide-loaded nanospheres into an ischemic rat model reduced RGC apoptosis over a longer period. The same technique was used to administer GDNF to the DBA/2J mouse glaucoma model and an ischemic rat model; again, increased survival of RGCs was observed over a longer period. An alternative method, meant to prolong the effects of the treatment, used an osmotic pump to administer NFs to an axotomized rat model. The technique was successful in reducing RGC apoptosis but most cells were dead within 1 month.

As an alternative to prolonged administration of NTs, researchers in this field have also looked toward gene transfer as a way of maintaining the required level of the NFs for long-term RGC survival. Transformation of a rat glaucoma model with the BDNF gene extended the life of the axotomized RGCs for a similar period. This work was furthered in the same model by injecting BDNF while simultaneously expressing the gene encoding the BDNF receptor, TrkB, known to show reduced expression in glaucoma models. The results showed increased survival of 76% of RGCs over a prolonged period.

Excitotoxicity

Glutamate is a neurotransmitter reported in raised concentrations in the vitreous of glaucoma patients and in animal models. RGCs are known to be highly susceptible to cell death through not only glutamate excitotoxicity and but also treatment with the glutamate analog, NMDA. This has lead to glutamate excitotoxicity being proposed as both a mechanism of primary insult upon RGCs and as a secondary insult following the death of RGCs and the release of further excitory amino acids and glutamate. At elevated levels, glutamate triggers the excessive activation of ionotrophic receptors, such as NMDA receptors, resulting in a subsequent influx of Ca2+ ions and an increase in oxidative stress leading to cell death as depicted in Figure 3.

Ca2+ is shown to increase mitochondrial permeability to ions and solutes and thought to regulate the mitochondrial permeability transition pore (mPTP), which is associated with the release of proapoptotic factors such as cytochrome C (Cyt C). Ca2+ has also been proposed to activate nitrous oxide synthase (NOS), leading to the generation of pathological quantities of nitric oxide (NO) and the subsequent production of reactive NO free radicals capable of triggering cell death, the full implications of which are discussed later.

Calpains are a family of cytoplasmic-calcium-activated cysteine proteases. Axotomy, ocular hypertension, and

14-3-3

NMDA excitotoxicity all demonstrated calpain activity with inhibition showing reduced RGC loss. Increased Ca2+ in the retina correlates with increased calpain activity, which can activate the caspase cascade as well as lead to the phosphorylation of spectrin, Tau, and p35, and cleavage of known calpain substrates, including the autoinhibition domain of calcineurin (CaN).

In addition to activation by calpains, CaN has also been shown to undergo activating cleavage by caspases and interestingly raised IOP, although the mechanism of the latter is unknown. Sustained Ca2+ has also been shown to lead to increased levels of CaN, which induces apoptosis through the dephosphorylation of Bad, releasing the proapoptotic protein from sequestration by the chaperon protein 14-3-3. Bad is then able to translocate from the cytosol to the mitochondria, allowing it to heterodimerize with Bcl-2 and B-cell leukemia XL (Bcl-XL), leading to the release of Cyt C and the triggering of apoptosis. A role for CaN is further supported by the observation that dosing of cells with glutamate led to the translocation of a green fluorescent protein (GFP) fusion protein of Bad from the cytosol to the mitochondria, and the prevention of this in cells containing an inactive mutant form of CaN.

As mentioned previously, Bad phosphorylation is regulated by NT and treatment with NT is sufficient to confer

resistance to excitotoxic insult. This suggests that excitotoxicity rather than working alone could act in conjunction with withdrawal of NFs to facilitate RGC apoptosis. Recent findings, however, have thrown doubt upon the hypothesis of glutamate excitotoxicity. Crucially, the raised glutamate levels observed in humans and animals have failed to be reproduced in follow-up studies. In addition, NMDA was able to induce apoptosis in the absence of Bax, a proapoptotic factor, suggested to be essential in apoptosis of RGCs in glaucoma. An alternative explanation for increased intracellular calcium and the subsequent triggering of apoptotic pathways is the activation of stress-activated channels by raised hydrostatic pressure.

NMDA-Antagonists and Neuroprotective Agents

This breakthrough of excess glutamate resulting in RGC apoptosis has led to considerable interest in the use of NMDA-receptor antagonists as neuroprotective therapies, many of which have been identified and characterized. MK801, a noncompetitive NMDA-receptor blocker, has been shown to protect RGCs from apoptosis in a number of experimental models including increased IOP and following intravitreal injections of NMDA in

rats. Despite showing promise as a successful neuroprotectant in the animal models, MK801 was never used in clinical trials due to its adverse side effects of inducing vacuole formation in neurons and neuronal necrosis.

Other NMDA-receptor antagonists were also studied for use as potential neuroprotectants. For example, dextromethorphan was shown to aid the recovery of retinal activity in rabbits suffering from IOP-induced ischemia and flupirtine had a similar effect on the same model. Riluzole, an agent used for treatment of amyotrophic lateral sclerosis, reduced neuronal death following retinal ischemia brought about by raised IOP in rats. The most promising neuroprotective agent to date, memantine, is similar to MK801 but lacks the neurotoxic effects. Successfully used in the treatment of Alzheimer’s disease (AD) and Parkinson’s disease, memantine has also been shown to protect neurons from apoptosis in many different glaucoma models. Disappointingly, a recent second phase III clinical trial of memantine has revealed the compound to have no positive effect on visual-field deterioration in glaucoma patients.

Reactive Oxygen Species

Oxidative stress leading to neuronal apoptosis is an early event, occurring within hours of raised IOP, both in vitro and in vivo. Following atoxomy of RGCs, there is both an

increase in reactive oxygen species (ROS) and cell death. Inhibition of ROS demonstrated a 50% reduction in RGC loss as did increasing expression of superoxide dismutase (SOD), known to be reduced in rat models with raised IOP.

The principal mechanism for ROS production is disruption of mitochondrial function, shown in Figure 4, leading to loss of mitochondrial membrane potential and subsequent release of death factors. In addition to raised IOP, light exposure has been suggested as a risk factor in glaucoma, through the increased generation of ROS. In addition, ROS can mediate apoptosis by reacting directly with various molecules including DNA, proteins, and lipids.

Mitochondrial Dysfunction and ROS Generation

ROS generation, particularly through the inhibition of complexes I and IV of the electron transport chain, has been implicated in various mechanisms of apoptosis, with treatment of antioxidants preventing apoptosis by inhibiting ROS generation and Cyt C release. A wide variety of antioxidants have been suggested as neuroprotectants in glaucoma, including vitamin E, 3-methyl-1,2-cyclopentanedione (MCP), and melatonin. Derivatives of catechin have been suggested to be potent antioxidants, and intravitreal coaddition of epigallocatechin was shown to attenuate the

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retinal damage caused by treatment with the NO donor, sodium nitroprusside.

Ubiquinone (CoQ 10), a member of the electron transport chain, has been demonstrated to prevent lipid peroxidation and DNA damage. Furthermore, in recent tests in rats, the intraocular administration of CoQ 10 afforded neuroprotective effects.

Antioxidants as Neuroprotective Agents

The most promising antioxidant treatment to date is that of orally administered Gingko biloba extract, which contains a number of substances shown to be effective in preventing mitochondrial damage through oxidative stress. Visual improvement has also been demonstrated in a doublemasked long-term placebo-controlled study, with efficacy and safety reports suggesting a daily dose of 120 mg to be sufficient.

Protein Misfolding

The observation that AD patients demonstrated RGC loss typically associated with glaucomatous changes, including

optic neuropathy and visual impairment, suggested possible mechanistic similarities. Supportive of mechanistic similarities between Alzheimer’s and glaucoma is the observations of abnormal and phosphorylated Tau, a major plaque component, in the retina of glaucoma patients. b-Amyloid (Ab), generated by the abnormal processing of amyloid precursor protein (APP), is another major component in Alzheimer’s plaques. APP and Ab are present in RGCs following elevation of IOP in rat models and in the DBA/2J mouse model. It has been shown in vivo that Ab is neurodegenerative and that disruption of these APP processing pathways is sufficient to reduce RGC apoptosis in glaucoma models in rats.

In AD, mutations altering the function of APP processing proteins lead to increased Ab production and, subsequently, increased apoptosis. A complementary mechanism, shown in Figure 5, has been implicated for glaucoma whereby normally rapid anterograde transport is blocked resulting in increased somal concentrations of APP. This has been proposed to trigger the abnormal processing of APP by caspase-3 in RGCs in glaucoma models generating increased Ab in the retina and is supported by observations of decreased vitreal Ab (suggesting increased deposition) in glaucoma patients.

mPTP

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Ab is believed to induce apoptosis through elevated intracellular calcium and increased oxidative stress, as seen in AD, where oxidative damage is observed before significant plaque formation. Increased oxidative damage is indicative of mitochondrial dysfunction and both APP and Ab have been shown to target mitochondria. At raised levels associated with glaucoma, APP has been shown to interact with mitochondria clogging them and preventing normal function. Ab has been implicated in the inhibition of ketoglutarate dehydrogenase and complex IV of the electron transport chain, inhibition of both of which leads to increased ROS generation. This process could constitute a positive-feedback loop accelerating cell death, as the presence of ROS has been shown to facilitate Ab production. Following mitochondrial dysfunction, apoptosis has ultimately been suggested to be mediated through the initiation of a caspase cascade.

Reduction of Misfolded Proteins in

Neuroprotection

Recent in vivo studies have shown that a reduction in RGC apoptosis can be achieved by targeting the Ab

pathway. Inhibition of b-secretase, responsible for Ab production, reduced plaque formation and RGC apoptosis. Increased plaque removal and inhibition of plaque formation had a similar effect. All of these mechanisms were shown to be effective in reducing RGC apoptosis, with the greatest benefit seen through the use of combination therapy, which resulted in a maximal reduction in RGC apoptosis of greater than 80%.

Glial–Neuronal Interactions

Glial cells, comprised of astrocytes, microglia, and Mu¨ller cells, act as support cells for neurons, maintaining their regular function by providing both neurotrophins and sustenance while removing toxic neurotransmitters and ions. Glaucoma induces dramatic changes in the glia gene expression patterns, potentially converting them from supportive to neurotoxic, as summarized in Figure 6. In the scope of this article, the impact of distinct types of glial cells are not differentiated, but evaluated as a whole.

Glial cell activation has been demonstrated in the glaucomatous optic nerve, in the retina of glaucoma

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patients and in glaucoma models showing altered levels of secretion. Increased secretion of NT appears to represent an attempted neuroprotective function, supporting damaged neuronal cells, while other changes, such as decreased secretion of interleukin (IL)-6 and increased secretion of NO and tumor necrosis factor (TNF)-a, which have been implicated in RGC apoptosis.

TNF-a, a trigger for extrinsic apoptosis, activates caspase-8 leading to activation of the caspase cascade, Bid activation, the loss of mitochondrial membrane potential, and the subsequent release of cell-death mediators such as apoptosis-inducing factor (AIF) and Cyt C. Increased secretion of TNF-a by glia has been shown to correlate with increased disease severity in glaucoma patients and is further accompanied by the upregulation of TNF receptor 1 (TNF-R1) in glial cells and in RGCs and their axons in glaucoma patients. TNF-a also represents a possible genetic component for glaucoma with the TNF-a-308 gene polymorphism identified in glaucoma patients suggesting a role for TNF-a signaling.

Glial cell secretion of NO, potentially stimulated by TNF-a , is raised in glaucoma with elevated NO concentrations shown to trigger axonal degradation and cell death in RGCs. There are three isoforms of NOS: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). Initially, iNOS received the most attention as it is induced under various stress conditions. It was suggested to be raised in the ONH of glaucoma patients and also shown to be raised in glaucoma rat model utilizing cauterization, where treatment with inhibitors was sufficient to abate apoptosis.

However, validation of this work in alternative models failed to produce correlating results, with both a murine model and a rat glaucoma model of IOP raised through intravitreal injections, failing to show the involvement of NOS or a reduction in RGC apoptosis by NOS inhibitors. It appears that iNOS upregulation could be due to secondary factors caused by the cauterization process and not raised IOP. While the role of iNOS in glaucoma has been called into doubt, it does not undermine the potential importance of NO signaling. nNOS and eNOS have been showed to be expressed at low levels in normal eyes in glial cells in the ONH, but shows dramatically raised levels in the ONH of glaucoma sufferers.

The main mechanism, through which NO triggers cell death, appears to be disruption of mitochondrial function. An interaction between NO and the mPTP directly facilitates the release of Cyt C and AIF, with mPTP inhibitors abating NO-induced neuronal apoptosis. Caspase-3 activation, however, has been shown without initial loss of mitochondrial membrane potential, suggesting activation may result from blockade of the electron transport chain and subsequent increased levels of ROS. Loss of mitochondrial membrane potential can be triggered as a downstream event, following prolonged inactivation of the electron transport chain.

NO binds complex IV of the electron transport chain, also known as cytochrome oxidase, reducing the enzyme’s affinity for oxygen. Prolonged exposure to NO or peroxynitrite (OONO ), a highly destructive molecule formed from ROS and NO, can lead to blockade of complex I of the electron transport chain. This facilitates further ROS production and OONO synthesis, which are capable of extensively modifying cellular components leading to apoptosis, while antioxidant treatment in NO-induced apoptosis was shown to abate caspase activation.

Disruption of mitochondrial respiration has also been suggested to facilitate the disruption of Ca2+, associating NO with excitotoxicity-induced apoptosis. This link was strengthened by observations that nNOS-deficient mice were resistant to NMDA-induced RGC death. NO induced modest increases in the amplitude of Ca2+ channels and the induction of the Ca2+-mediated death effectors, the calpains.

Extracellular Matrix Degradation

Glaucomatous changes include extensive remodeling of the extracellular matrix (ECM), altering the levels of the various ECM components, including collagen I and IV, matrix metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), transforming growth factor beta 2 (TGF-b2), and laminin.

MMP expression in particular has received attention, with expression in glaucoma patients being raised in comparison to normal patients. The condition of the ECM both regulates and is regulated by the expression and release of MMPs, with the subsequent reduced levels of laminin contributing to cell death, and significant RGC loss in the retina. Supportive evidence from the ability of TIMP-1 to abate neuronal apoptosis, as well as observations that MMP-9-deficient mice demonstrate reduced laminin degradation and increased resistance to neural trauma, further highlights the potential importance of MMPs in glaucoma.

It was recently demonstrated in vivo that raised IOP induced remodeling of the ECM within the retina. Raised IOP was shown to correlate to decreases in laminin and TGF-b2 and increased MMP-9, TIMP-1, and RGC apoptosis. Loss of survival signals from the ECM is thought to induce a specific form of apoptosis called anoikis.

Whether glaucomatous changes are initiated at the retina or ONH is still a matter of contention. Astrocytes in the ONH have been shown to be activated by raised IOP and produce MMPs that are able to remodel the ECM, possibly resulting in axonal compression and thereby facilitating apoptotic mechanisms associated with the blockade of anterograde and retrograde transport.

In addition to raised IOP, potential apoptotic mechanisms such as Ab, NO, ROS, excitotoxicity, and TNF-a have all been implicated in triggering an increase in

MMP-9, suggesting that MMP-9 may represent an important downstream executor of apoptosis rather than a primary affecter.

Neuroprotective Vaccine

Studies using rat and mouse models of glaucoma have suggested a potential role for autoimmunity in the protection of neurons from secondary damage. Following the initial insult, nonspecific T-lymphocytes have been shown to accumulate at the primary lesion site. While nonspecific T-lymphocytes did not exhibit neuroprotection, effects were observed upon injection with myelin basic protein (MBP)-specific T-lymphocytes or immunization with MBP. Unfortunately, injections of anti-MBP-specific T-lymphocytes and MBP immunization induced the paralytic condition – experimental autoimmune encephalomyelitis (EAE).

Copolymer 1 (Cop 1), a synthetic peptide based on MBP, was discovered to suppress EAE and shown in clinical trials to be beneficial to patients with multiple sclerosis. Further studies have revealed that immunization with Cop 1 increases RGC survival following optic nerve crush, glutamate injections, and increased IOP in rats. This research shows that there is potential for the development of a glaucoma vaccine.

Summary

The primary mechanism of RGC loss in glaucoma is through apoptosis, which can be triggered through a variety of mechanisms both intrinsic and extrinsic. Apoptotic signaling shows a large level of redundancy with extensive cross talk. This redundancy poses a major problem in terms of neuroprotection with inhibition of one apoptotic pathway merely delaying apoptosis before mediation through an alternative pathway or the triggering of necrosis. It is still unclear as to the primary mechanism of apoptosis induction in glaucoma, although an increased understanding would aid a more effective development of neuroprotective strategies. Fundamental to the successful development of neuroprotective strategies is targeting the apoptotic pathway upstream and at multiple points to maximize effectiveness.

The studies of neuroprotective agents on glaucoma models carried out to date have shown great promise with many different agents demonstrating efficacy in a large number of models, summarized in Table 1. Unfortunately, translation of this research from animals through to clinical trials has exposed complications with side effects and inefficacy, hindering progression. However, due to the complicated nature and a number of different overlapping pathways inducing apoptosis, the identification of a multitude of potential neuroprotectants has been possible. One major problem still faced in the analysis of these compounds is the difficulties monitoring efficacy in vivo.

Table 1 Comparison of the models used to test the different neuroprotective agents

It is hoped that the development of the novel DARC technique for early diagnosis of glaucoma may also be a valuable tool in the analysis of potential neuroprotectants.

See also: Information Processing: Ganglion Cells; IOP and Damage of ON Axons.

Further Reading

Cheung, W., Guo, L., and Cordeiro, M. F. (2008). Neuroprotection in glaucoma: Drug-based approaches. Optometry and Vision Science 85(6): 406–416.

Cordeiro, M. F., Guo, L., Luong, V., et al. (2004). Real-time imaging of single nerve cell apoptosis in retinal neurodegeneration. Proceedings of the National Academy of Sciences of the United States of America

101(36): 13352–13356.