Expectations in the treatment of retinal diseases: neuroprotection

The potential neuroprotective effects of weekly treatment with glatiramer acetate in diabetic patients after panretinal photocoagulation

Purpose

Evaluation of the neuroprotective effect of weekly glatiramer acetate (GA) on retinal structure and function in diabetic patients who underwent panretinal photocoagulation (PRP).

Patients and methods

Patients with severe nonproliferative or early proliferative diabetic retinopathy and no previous laser treatment were randomly divided into two groups: (1) those who received four GA treatments and (2) those who received placebo treatment. The subcutaneous injections were administered 1 week prior to laser and weekly in the subsequent three sessions of PRP in both groups. All patients underwent a full ophthalmic examination (best-corrected logMAR visual acuity, slit lamp examination, applanation tonometry, fundus biomicroscopy and indirect fundus examination); functional examination (standard automated perimetry, electroretinography and frequency-doubling technology C-20 visual field) and anatomic examination (color photography, optical coherence tomography (OCT) and Heidelberg retinal tomography). The examinations were performed before the photocoagulation and repeated 1,3,6, and 12 months after treatment (in a double-masked manner). To compare the two groups, generalized estimating equation models were performed to account for the dependence between eyes of the same patient.

Results

Thirteen patients (23 eyes) were included in the study group and 13 patients (24 eyes) were included in the control group. OCT showed a statistically significant difference in retinal nerve fiber layer (RNFL) thickness in the inferior peripapillary region and average thickness with thinner measurements in the control group at 1-year post-PRP. Functional analysis demonstrated a difference between groups, but it did not reach statistical significance.

Conclusion

The results of this study suggest that weekly GA treatment has a potential neuro-protective effect on the RNFL following photocoagulation for diabetic retinopathy.

Neuroprotection in glaucoma

Glaucoma is a neurodegenerative disease characterized by loss of retinal ganglion cells and their axons. Recent evidence suggests that intraocular pressure (IOP) is only one of the many risk factors for this disease. Current treatment options for this disease have been limited to the reduction of IOP; however, it is clear now that the disease progression continues in many patients despite effective lowering of IOP. In the search for newer modalities in treating this disease, much data have emerged from experimental research the world over, suggesting various pathological processes involved in this disease and newer possible strategies to treat it. This review article looks into the current understanding of the pathophysiology of glaucoma, the importance of neuroprotection, the various possible pharmacological approaches for neuroprotection and evidence of current available medications.

Neurodegenerative diseases of the retina and potential for protection and recovery

Recent advances in our understanding of the mechanisms in the cascade of events resulting in retinal cell death in ocular pathologies like glaucoma, diabetic retinopathy and age-related macular degeneration led to the common descriptive term of neurodegenerative diseases of the retina. The final common pathophysiologic pathway of these diseases includes a particular form of metabolic stress, resulting in an insufficient supply of nutrients to the respective target structures (optic nerve head, retina). During metabolic stress, glutamate is released initiating the death of neurones containing ionotropic glutamate (N-methyl-D-aspartat, NMDA) receptors present on ganglion cells and a specific type of amacrine cells. Experimental studies demonstrate that several drugs reduce or prevent the death of retinal neurones deficient of nutrients. These agents generally block NMDA receptors to prevent the action of glutamate or halt the subsequent pathophysiologic cycle resulting in cell death. The major causes for cell death following activation of NMDA receptors are the influx of calcium and sodium into cells, the generation of free radicals linked to the formation of advanced glycation endproducts (AGEs) and/or advanced lipoxidation endproducts (ALEs) as well as defects in the mitochondrial respiratory chain. Substances preventing these cytotoxic events are considered to be potentially neuroprotective.

Ameliorative effect of PN-277 on laser-induced retinal damage

BACKGROUND

The retinal damage induced by laser photocoagulation increases considerably by the secondary degeneration process whereby tissues adjacent to the primary lesion are destroyed. As the neuroprotective effect of immunization by PN-277 was previously demonstrated in models of retina, optic nerve, brain, and spinal cord lesions, it may be used also for reducing retinal damage induced by laser. The aim of this study was to evaluate the neuroprotective effect of immunization with PN-277 in reducing the spread of laser-induced retinal damage.

METHODS

Standard argon laser lesions were created in 36 DA pigmented rats. Seven days before exposure to laser, the rats were divided into a test group (n = 18) that was pre-immunized with intraperitoneal injection of PN-277 and control group (n = 18) treated with saline. Histological and morphometrical evaluations of the retinal lesions were preformed 3, 20, and 60 days after the injury.

RESULTS

Significant ameliorative effect was demonstrated in the retinas of the pre-immunized animals 60 days after exposure to laser. The diameter of the lesion was 356 microm as compared with 406 microm (P < 0.01), the cell density of the photoreceptor cell bodies measured in the whole lesion was 72.4% of normal as compared with 64.5% (P = 0.01), and at the center of the lesion it was 57.3% of normal as compared with 38.2% (P < 0.01) (treated and control groups, respectively).

CONCLUSIONS

Immunization with PN-277 has an ameliorative effect in neural tissue such as the retina. This type of immunization may be of clinical significance in reducing laser-induced retinal injuries in humans.

Bioenergetic-based neuroprotection and glaucoma

Primary open-angle glaucoma (POAG) is a pressure-sensitive optic neuropathy which results in the death of retinal ganglion cells and causes associated loss of vision. Presently, the only accepted treatment strategy is to lower the intraocular pressure; however, for some patients this is insufficient to prevent progressive disease. Although the pathogenesis of POAG remains unclear, there is considerable evidence that energy failure at the optic nerve head may be involved. Neuroprotection, a strategy which directly enhances the survival of neurons, is desirable, but remains clinically elusive. One particular form of neuroprotection involves the notion of enhancing the energy supply of neurons. These 'bioenergetic' methods of neuroprotection have proven successful in animal models of other neurodegenerative diseases and conditions, including Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and traumatic brain injury, but have been relatively unexplored in glaucoma models. This review focuses on some of the potential approaches for bioenergetic neuroprotection in the retina, including increasing the energy buffering capacity of damaged cells, decreasing the permeability of the mitochondrial membrane pore and free radical scavenging.

Assessment of rat and mouse RGC apoptosis imaging in vivo with different scanning laser ophthalmoscopes

PURPOSE

We have recently described a novel way of imaging apoptosing retinal ganglion cells in vivo in the rat. This study investigated if this technique could be used in the mouse, and whether the Heidelberg Retina Angiograph II (HRAII) was appropriate.

METHODS

Retinal ganglion cell (RGC) death was induced by intravitreal injections in rat and mouse eyes using staurosporine. Fluorescent-labeled apoptosing cells were detected by imaging with both the HRAII and a prototype Zeiss confocal scanning laser ophthalmoscope (cSLO). Averaged in vivo images were analyzed and results compared with histologic analysis.

RESULTS

Fluorescent points (FPs) used as a measure of RGC apoptosis in vivo were detected in the mouse eye but only with the HRAII and not the Zeiss cSLO. The HRAII was able to detect 62% more FPs in rat than the Zeiss cSLO. Both cSLOs showed peak FP counts at the 5- to 10-microm range in rat and mouse. Maximal FP counts were detected in the superior and superior temporal regions in the rat, with no obvious pattern of distribution in the mouse. The HRAII was found to have more FP correspondence with histologically identified apoptosing RGCs.

CONCLUSIONS

To our knowledge, this is the first demonstration of visualized apoptosing RGC in vivo in a mouse. The improved image quality achieved with the HRAII compared with the Zeiss cSLO was validated by histology. This together with its enhanced maneuverability and the fact that it is already commercially available make the HRAII a potential tool for the early detection and diagnosis of glaucomatous disease in patients.

Cortisol promotes survival and regeneration of axotomised retinal ganglion cells and enhances effects of aurintricarboxylic acid

BACKGROUND

Neuroprotection is essential for repair processes after a traumatic insult in the central nervous system. We have demonstrated previously significant neuroprotective properties of the anti-apoptotic drug aurintricarboxylic acid in the model of axotomised retinal ganglion cells. Glucocorticoids are widely used to treat injuries of the nervous system. Due to the anti-inflammatory and microglia-inhibiting properties of glucocorticoids, we studied the neuroprotective effects of intravitreally administered cortisol after an optic nerve cut.

METHODS

Ninety-eight adult Sprague-Dawley rats were used in this study. The optic nerve was cut intra-orbitally. Either vehicle or compound solution was injected intravitreally. Fluorescent dye was put onto the optic nerve stump to label retinal ganglion cells retrogradely. Retinal whole mounts were prepared 2 weeks after axotomy, and surviving retinal ganglion cells were counted.

RESULTS

Two weeks after axotomy, up to 50+/-7% of all retinal ganglion cells survived if cortisol was injected into the eye compared with 17+/-5% survival if only vehicle solution was injected. The neuroprotective effects of aurintricarboxylic acid (43+/-5% survival) could be further enhanced if combined with cortisol (up to 61+/-5% survival). Regeneration of cut retinal ganglion cell axons into a peripheral nerve graft could also be enhanced by an intravitreal injection of cortisol (169+/-42 regenerating retinal ganglion cells per mm2 vs. 73+/-12 cells per mm2 after vehicle injection). The increase was not as high as with aurintricarboxylic acid (192+/-40 cells per mm2), although more retinal ganglion cells survived with cortisol. This indicates that neuronal survival alone is not sufficient for subsequent axonal regeneration. Nevertheless, regeneration could be markedly increased if aurintricarboxylic acid and cortisol were combined (308+/-72 cells per mm2).

CONCLUSIONS

Whereas aurintricarboxylic acid seems to act directly on lesioned retinal ganglion cells, cortisol seems to act on the glial environment, as indicated by microglial cell morphology and enhanced glial fibrillary acidic protein expression. The results show that both neuroprotection and regeneration can be enhanced by the combination of two simple compounds acting on different sites.

Optic nerve and neuroprotection strategies

BACKGROUND

Experimental studies have yielded a wealth of information related to the mechanism of ganglion cell death following injury either to the myelinated ganglion cell axon or to the ganglion cell body. However, no suitable animal models exist where injury can be directed to the optic nerve head region, particularly the unmyelinated ganglion cell axons. The process of relating the data from the various animal models to many different types of optic neuropathies in man must, therefore, be cautious.

RESULTS

Extensive studies on the isolated optic nerve have yielded valuable information on the way white matter is affected by ischaemia and how certain types of compounds can attenuate the process. Moreover, there are now persuasive data on how ganglion cell survival is affected when the ocular blood flow is reduced in various animal models. As a consequence, the molecular mechanisms involved in ganglion cell death are fairly well understood and various pharmacological agents have been shown to blunt the process when delivered before or shortly after the insult.

CONCLUSIONS

A battery of agents now exist that can blunt animal ganglion cell death irrespective of whether the insult was to the ganglion cell body or the myelinated axon. Whether this information can be applied for use in patients remains a matter of debate, and major obstacles need to be overcome before the laboratory studies may be applied clinically. These include the delivery of the pharmacological agents to the site of ganglion cell injury and side effects to the patients. Moreover, it is necessary to establish whether effective neuroprotection is only possible when the drug is administered at a defined time after injury to the ganglion cells. This information is essential in order to pursue the idea that a neuroprotective strategy can be applied to a disease like glaucoma, where ganglion cell death appears to occur at different times during the lifetime of the patient.

[Mechanisms of neuroprotection against glaucoma]

The goal of neuroprotection in glaucoma treatment is to employ agents that prevent or delay apoptosis of retinal ganglion cells (RGC) and facilitate regeneration of already damaged calls. The following contribution discusses the mechanisms of RGC death and current status of neuroprotective in vivo studies and investigations on cell cultures and animal models. Discussions on the etiopathogenesis of PCOAG center on elevated IOP and ocular disorders of vascular function. The mechanisms of axonal damage induced by ischemia are explained and the resultant possible neuroprotective effect mechanisms are discussed (Na(+) or Ca(2+) channel blockers, role of reactive astrocytes). Substitution of axonal survival factors and especially the role of BDNF are described. Glutamate excitotoxicity also plays a role in glaucomatous antegrade RGC death. Relevant questions and possible therapeutic approaches are discussed. The three phases of apoptosis cascade and the key role of mitochondria in the insult-induced apoptosis are considered as well as the still relatively unexplored possibilities of RGC regeneration. Finally, perspectives of neuroprotective treatment of PCOAG are presented.

[Characteristic features of optic nerve ganglion cells and approaches for neuroprotection. From intracellular to capillary processes and therapeutic considerations]

In many diseases associated with deterioration of the visual field and eyesight, optic nerve ganglion cells are at the highest risk. The clinical course of primary chronic open-angle glaucoma (PCOAG) is also determined by the degree of damage to these cells. Due to their anatomy, they are subject to extreme stress exerted by metabolic and microcirculatory forces. The interaction between hypoxia and metabolic stress leads to damage of the retinal ganglion cells. This is compounded by oxidative stress and age-dependent increase of advanced glycation end products. The following contribution gives consideration to approaches for delaying ganglion cell death in PCOAG, e.g., with neuroprotective agents. Furthermore, agents that reduce calcium influx into the cells could prevent cell destruction. Likewise, NMDA receptor antagonists could be effective; however, considerable side effects are to be feared. Antioxidants are also attributed with theoretical impact in combating PCOAG by preventing apoptosis. Finally, the ideal glaucoma medication should be well tolerated when taken orally, prevent destruction of retinal ganglion cells, and possess a low side effect profile.

Endogenous tissue type plasminogen activator facilitates NMDA-induced retinal damage

To investigate the role of tissue plasminogen activator (tPA) in retinal damage, tPA-deficient and wild-type mice were employed. Two different retinal neuron insult models were used in the present study. One is an excitotoxin-treated retinal model, created by direct intravitreal injection of glutamate analogs, NMDA or kainic acid (KA), and the other is an ischemia-reperfusion model induced by transient elevation of intraocular pressure. TdT-dUTP terminal nick-end labeling (TUNEL) method was used to examine the retinal cell nuclear damage. The number of TUNEL-positive cells in ganglion cell layer (GCL) and inner nuclear layer (INL) in tPA-deficient mice after low-, but not high-dose NMDA was significantly less compared to wild type. In contrast, neither intravitreal KA or transient ischemia produced significant difference in retinal damage in tPA vs. wild-type mice. These data show that tPA-deficient mice are resistant to retinal damage by intravitreal injection of NMDA, and indicate that tPA plays a role in the retinal cell damage induced by excitotoxins, especially NMDA.

Isch�mie und Hypoxie

In hypoxic or ischemic states, the receptors of the ganglion cells are overstimulated by release of neurotransmitters. Glutamate and GABA (gamma-aminobutyric acid) are the decisive neurotransmitters in the retina. It is presumed that the extent of cell death depends on the degree of depolarization, which in turn is determined by the amount of excitatory (glutamate) or inhibitory (GABA) receptors of the corresponding ganglion cell. The assumption is that the receptor profile of the individual ganglion cells determines the sensitivity of these cells to hypoxia or ischemia, i.e., the time up to cell death, and thus represents the underlying cause of the different rates of cell death in primary chronic open-angle glaucoma. Research on this receptor profile could be of pivotal importance for the approach to neuroprotective treatment of primary chronic open-angle glaucoma.

Potential role of glial cell line-derived neurotrophic factor receptors in Müller glial cells during light-induced retinal degeneration

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and their receptors (GFRalpha1, GFRalpha2 and Ret) play an important role in the survival of neurons in the central and peripheral nervous system. For example, GDNF as well as other trophic factors promotes photoreceptor survival during retinal degeneration. Recent studies have proposed that part of neurotophic rescue of photoreceptors may be indirect, mediated by interaction of the neurotrophic factors with other cell types, that in turn release secondary factors that act directly on photoreceptors. In the present study, we examined the GDNF receptor expression in control and light-damaged retina, and found that GFRalpha2 protein is upregulated in retina-specific Müller glial cells during photoreceptor degeneration. We also examined the effect of GDNF or NTN on cultured Müller cells. Exogenous GDNF increased brain-derived neurotrophic factor, basic fibroblast growth factor and GDNF, but not NTN mRNA production. On the other hand, NTN increased NTN, but not GDNF mRNA production in cultured Müller cells. These observations suggest that GDNF, NTN and their receptors are involved in the regulation of trophic factor production in retinal glial cells, and that functional glia-neuron network may utilize GDNF family for the protection of neural cells during retinal degeneration.

The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

beta-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the beta(1)-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with beta-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective beta-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three beta-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the beta-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic beta-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual beta-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

Some current ideas on the pathogenesis and the role of neuroprotection in glaucomatous optic neuropathy

The primary features of glaucomatous optic neuropathy are characteristic changes in the optic nerve head, a decrease in number of surviving ganglion cells and a reduction in vision. It is now generally accepted that a number of factors, including elevated intraocular pressure, could lead to the changes seen in the optic nerve head and to obtain a pharmacological means to treat the causes will vary from patient to patient. In contrast, a cascade of events have been proposed to explain how the changes in the optic nerve head may lead to the slow and differential death of ganglion cells in the disease. It is also proposed that drugs (neuroprotectants) influencing this cascade of events can attenuate ganglion cell death and lead to the treatment of all glaucoma patients.

Alpha-lipoic acid protects the retina against ischemia-reperfusion

The aim of this study was to examine whether the antioxidant alpha-lipoic acid protects retinal neurons from ischemia-reperfusion injury. Rats were injected intraperitoneally with either vehicle or alpha-lipoic acid (100 mg/kg) once daily for 11 days. On the third day, ischemia was delivered to the rat retina by raising the intraocular pressure above systolic blood pressure for 45 min. The electroretinogram was measured prior to ischemia and 5 days after reperfusion. Rats were killed 5 or 8 days after reperfusion and the retinas were processed for immunohistochemistry and for determination of mRNA levels by RT-PCR. Ischemia-reperfusion caused a significant reduction of the a- and b-wave amplitudes of the electroretinogram, a decrease in nitric oxide synthase and Thy-1 immunoreactivities, a decrease of retinal ganglion cell-specific mRNAs and an increase in bFGF and CNTF mRNA levels. All of these changes were clearly counteracted by alpha-lipoic acid. Moreover, in mixed rat retinal cultures, alpha-lipoic acid partially counteracted the loss of GABA-immunoreactive neurons induced by anoxia. The results of the study demonstrate that alpha-lipoic acid provides protection to the retina as a whole, and to ganglion cells in particular, from ischemia-reperfusion injuries. alpha-Lipoic acid also displayed negligible affinity for voltage-dependent sodium and calcium channels.