Caspase-independent component of retinal ganglion cell death, in vitro

PURPOSE: Although in vitro and in vivo models demonstrate caspase activation in retinal ganglion cells (RGCs) undergoing apoptosis, the caspase-independent component of RGC death is unclear. Identification of the precise mechanisms of cell death in these distinct neurons is essential for the development of effective neuroprotective strategies in glaucoma. Because TNF-alpha and hypoxia have been implicated in RGC death during glaucomatous optic nerve degeneration, this study was conducted to determine whether RGCs survive exposure to TNF-alpha or hypoxia in the presence of caspase inhibitor treatment, and whether mitochondrial dysfunction is involved in RGC death induced by these glaucomatous stimuli. METHODS: Primary cultures of rat RGCs were exposed to TNF-alpha or hypoxia for up to 48 hours. The temporal relationship of RGC death with the loss of mitochondrial membrane potential and the release of cell death mediators, including cytochrome c and apoptosis-inducing factor (AIF), was studied in the absence and presence of specific inhibitors of caspases. In addition, treatment with a free-radical scavenger, 4-hydroxytetramethylpiperidine-1-oxyl (tempol; 5 mM), was used in some experiments. Cell viability was assessed using calcein assay, and annexin V binding combined with propidium iodide staining was used for the distinction of apoptotic and necrotic cells. Caspase-3-like protease activity was measured using a fluorometric assay, and for the in situ detection of caspase activity, immunocytochemistry was performed with a cleavage-site-specific antibody. The time course of alterations in the mitochondrial membrane potential and the release of cell death mediators in individual cells undergoing cell death were assessed with a fluorescent tracer and subsequent immunocytochemistry. In addition, a fluorescent dye, dihydroethidium was used to assess the generation of reactive oxygen species (ROS). RESULTS: Findings of this study revealed that the loss of mitochondrial membrane potential and the release of cell death mediators accompanied RGC death induced by TNF-alpha or hypoxia. Although caspase inhibitor treatment temporarily decreased the rate of apoptosis, caspase inhibition was not adequate to block RGC death if the mitochondrial membrane potential was lost and mitochondrial mediators were released. Despite the inhibited caspase activity, survival rate was less than 70% after a 48-hour incubation with death stimuli, and both apoptotic and necrotic cells were detectable in these cultures. When combined with caspase inhibition, tempol reduced the production of ROS and provided an additional 20% increase in RGC survival. CONCLUSIONS: Based on these novel findings, RGC death induced by TNF-alpha or hypoxia involves a caspase-independent component, and reducing the free-radical generation provides additional protection of RGCs temporarily saved by caspase inhibition. Therefore, neuroprotective strategies in glaucoma should include tools to improve the ability of these neurons to survive the cytotoxic consequences of mitochondrial dysfunction.     

The emerging role of proteases in retinal ganglion cell death

Retinal ganglion cell (RGC) death is an important issue in Primary Open Angle-Glaucoma (POAG) in terms of both vision loss and health care costs. Yet, the pathophysiology underlying RGC death in glaucoma is unclear. A growing body of evidence indicates that proteases that modulate the extracellular matrix (ECM) milieu in the retina, either directly or indirectly, play an important role in dictating the fate of RGCs. Recent evidence indicates that proteases, in addition to ECM-remodeling, have broader functional roles in glutamate receptor processing and predisposing RGCs to secondary damage. This review is focused on discussing the role of two groups of proteases, the matrix metalloproteinases (MMPs) and the plasminogen activators (PAs), in RGC death. In a long-run, a better understanding of the mechanisms involved in the regulation of proteases may lead to the development of adjunctive treatment options to attenuate RGC death and improve vision loss in glaucoma.     

The molecular basis of retinal ganglion cell death in glaucoma

Glaucoma is a group of diseases characterized by progressive optic nerve degeneration that results in visual field loss and irreversible blindness. A crucial element in the pathophysiology of all forms of glaucoma is the death of retinal ganglion cells (RGCs), a population of CNS neurons with their soma in the inner retina and axons in the optic nerve. Strategies that delay or halt RGC loss have been recognized as potentially beneficial to preserve vision in glaucoma; however, the success of these approaches depends on an in-depth understanding of the mechanisms that lead to RGC dysfunction and death. In recent years, there has been an exponential increase in valuable information regarding the molecular basis of RGC death stemming from animal models of acute and chronic optic nerve injury as well as experimental glaucoma. The emerging landscape is complex and points at a variety of molecular signals - acting alone or in cooperation - to promote RGC death. These include: axonal transport failure, neurotrophic factor deprivation, toxic pro-neurotrophins, activation of intrinsic and extrinsic apoptotic signals, mitochondrial dysfunction, excitotoxic damage, oxidative stress, misbehaving reactive glia and loss of synaptic connectivity. Collectively, this body of work has considerably updated and expanded our view of how RGCs might die in glaucoma and has revealed novel, potential targets for neuroprotection.     

Mouse models of retinal ganglion cell death and glaucoma

Once considered too difficult to use for glaucoma studies, mice are now becoming a powerful tool in the research of the molecular and pathological events associated with this disease. Often adapting technologies first developed in rats, ganglion cell death in mice can be induced using acute models and chronic models of experimental glaucoma. Similarly, elevated IOP has been reported in transgenic animals carrying defects in targeted genes. Also, one group of mice, from the DBA/2 line of inbred animals, develops a spontaneous optic neuropathy with many features of human glaucoma that is associated with IOP elevation caused by an anterior chamber pigmentary disease. The advent of mice for glaucoma research is already having a significant impact on our understanding of this disease, principally because of the access to genetic manipulation technology and genetics already well established for these animals.     

小胶质细胞参与视网膜神经节细胞死亡机制的实验研究

目的探讨小胶质细胞活化在急性高眼压引起的视网膜缺血再灌注损伤视网膜神经节细胞(RGC)死亡中的作用及其机制。方法实验研究。取C57BL/6小鼠原代RGC与小鼠小胶质细胞BV2细胞共培养或单独培养,建立体外氧糖剥夺/再灌注(OGD/R)模型模拟体内视网膜缺血再灌注损伤(复氧时间分别设为6 h、24 h、36 h、48 h),使用小胶质细胞特异性离子钙接头蛋白(iba1)免疫荧光染色评估BV2细胞活化程度;采用活细胞计数试剂盒检测RGC的细胞活性;应用细胞凋亡检测试剂盒检测RGC凋亡率;通过半定量逆转录PCR、Western印迹、细胞免疫荧光检测BV2细胞中Toll样受体-4(TLR4)与Nod样受体家族含pyrin结构域蛋白3(NLRP3)的mRNA及蛋白水平;应用半胱氨酸天冬氨酸蛋白酶-8(caspase-8)染色试剂盒检测BV2细胞中caspase-8活性;酶联免疫吸附试验检测BV2细胞上清液中白细胞介素1β(IL-1β)含量;应用TLR4小干扰RNA(siRNA)转染和caspase-8抑制剂阻断相应通路,对比TLR4、NLRP3的表达、caspase-8的活性变化、IL-1β含量的差异以及共培养RGC的细胞活性变化。采用方差分析进行统计学分析。结果RGC与BV2细胞共培养下,细胞免疫荧光检测显示OGD/R模型中BV2细胞iba1表达增多,BV2细胞显著激活。RGC与BV2细胞共培养下,OGD/R模型中RGC的凋亡率(71.1%±3.2%)高于RGC单独培养下OGD/R模型中RGC的凋亡率(35.1%±1.8%),差异有统计学意义(t=10.10,P<0.01)。细胞免疫荧光检测显示BV2细胞单独培养下,OGD/R模型中BV2细胞TLR4、NLRP3表达显著增加,其mRNA水平均随复氧时间延长显著上升,差异均有统计学意义(F=64.45,72.74;均P<0.01),且在OGD/R复氧24 h时达到峰值(TLR4 mRNA与对照的相对比值为2.83±0.23,NLRP3 mRNA与对照的相对比值为3.12±0.27);caspase-8的活性也随复氧时间延长显著增加,差异有统计学意义(F=93.57,P<0.01),且在OGD/R复氧24 h时达到峰值(与对照的相对比值为2.92±0.31)。应用TLR4 siRNA转染BV2细胞后其caspase-8的活性明显受到抑制,应用caspase-8抑制剂并不影响BV2细胞中TLR4的表达上调,而OGD/R作用下BV2细胞分泌的成熟IL-1β在caspase-8抑制剂干预下显著减少(由3.52±0.55降低为1.39±0.37,t=7.19,P<0.01),同时NLRP3的表达量在caspase-8抑制剂干预下显著降低(由2.79±0.23降低至1.37±0.19,t=9.37,P<0.01)。OGD/R模型中,与TLR4 siRNA转染BV2细胞共培养的RGC细胞活性为74.5%±1.2%,与经caspase-8抑制剂处理BV2细胞共培养的RGC细胞活性为62.8%±1.5%,均高于与对照BV2细胞共培养的RGC细胞活性(36.7%±0.3%),差异均有统计学意义(t=11.60,6.83;均P<0.01)。结论视网膜缺血再灌注损伤促使小胶质细胞活化,激活TLR4-caspase-8-NLRP3炎性反应小体信号通路,介导RGC死亡。     

Retinal ganglion cell death and optic nerve degeneration by genetic ablation in adult mice

Despite the magnitude of the problem, no effective treatments exist to prevent retinal ganglion cell (RGC) death and optic nerve degeneration from occurring in diseases affecting the human eye. Animal models currently available for developing treatment strategies suffer from cumbersome procedures required to induce RGC death or rely on mutations that induce defects in developing retinas rather than in mature retinas of adults. Our objective was to develop a robust genetically engineered adult mouse model for RGC loss and optic nerve degeneration based on genetic ablation. To achieve this, we took advantage of Pou4f2 (Brn3b), a gene activated immediately as RGCs begin to differentiate and expressed throughout life. We generated adult mice whose genomes harbored a conditional Pou4f2 allele containing a floxed-lacZ-stop-diphtheria toxin A cassette and a CAGG-Cre-ER™ transgene. In this bigenic model, Cre recombinase is fused to a modified estrogen nuclear receptor in which the estrogen-binding domain binds preferentially to the estrogen agonist tamoxifen rather than to endogenous estradiol. Upon binding to the estrogen-binding domain, tamoxifen derepresses Cre recombinase, leading to the efficient genomic deletion of the floxed-lacZ-stop DNA sequence and expression of diphtheria toxin A. Tamoxifen administered to adult mice at different ages by intraperitoneal injection led to rapid RGC loss, reactive gliosis, progressive degradation of the optic nerve over a period of several months, and visual impairment. Perhaps more reflective of human disease, partial loss of RGCs was achieved by modulating the tamoxifen treatment. Especially relevant for RGC death and optic nerve degeneration in human retinal pathologies, RGC-ablated retinas maintained their structural integrity, and other retinal neurons and their connections in the inner and outer plexiform layers appeared unaffected by RGC ablation. These events are hallmarks of progressive optic nerve degeneration observed in human retinal pathologies and demonstrate the validity of this model for use in developing stem cell therapies for replacing dead RGCs with healthy ones.     

Age-dependent susceptibility of the retinal ganglion cell layer to cell death

PURPOSE: The purpose of this study was to determine the susceptibility of the retinal ganglion cell layer (GCL) to apoptosis after optic nerve transection and excitotoxic stimulus and to investigate the regulation of apoptosis in the GCL during development. The authors also sought to determine the role played by caspases in cell death and their expression during development. METHODS: TdT-mediated dUTP nick end labeling (TUNEL) was used to identify cells undergoing apoptosis during mouse retinal development from postnatal day (P)3 to P5 and in retinal explant sections under various conditions. The expression of active caspases was determined by immunohistochemistry (IHC) using an antibody that detects the cleaved large subunit. IHC was also used to detect the expression levels of procaspase-3, procaspase-9, and Apaf-1 in P6 and P60 whole eye sections. Retinal ganglion cells at ages P6 and P60 were purified by immunopanning, total RNA was extracted, and mRNA levels of the above proteins were determined by semiquantitative PCR. RESULTS: After optic nerve transection, a significant number of TUNEL-positive cells were seen 24 hours after lesion in P6 retinas. This death was caspase dependent, as shown by IHC and caspase inhibition with zVAD-fmk. In contrast, adult GCL was resistant to apoptosis under these conditions. Similarly, after excitotoxic stimulus, the GCL of the P6 retinas underwent apoptosis at 6 hours and was caspase dependent, whereas adult GCL was resistant. Developmental apoptosis in the GCL between P2 and P6 was shown to involve caspase-3 and caspase-9. Significant downregulation of Apaf-1 and caspase-3 was detected in the P60 GCL at both the mRNA and the protein levels. CONCLUSIONS: Adult GCL is more resistant to apoptosis than neonatal GCL after ON transection and excitotoxic stimulus. The expression of caspase-3 and Apaf-1 is significantly reduced in adult GCL. The authors suggest that age-dependent susceptibility to apoptosis may be caused by this reduced expression.     

Caspase inhibitors protect against NMDA-mediated retinal ganglion cell death

Background: Apoptosis is a major mechanism of cell death in glutamate‐induced excitotoxicity and caspases as the executors of apoptosis play an important role in the development of various central nervous system and eye diseases. We studied the involvement of certain caspases in excitotoxic retinal ganglion cell death, which was experimentally induced in Brown Norway Rats by application of the glutamate receptor agonist N‐methyl‐D‐aspartate (NMDA). Methods: Animals were injected intravitreally with one of six caspase inhibitors (against caspases 1, 3, 4, 6, 8 and 9). Seven hours later, NMDA or phosphate‐buffered saline as a control was injected intravitreally into the respective eyes. The neuroprotective potential against NMDA toxicity was assessed by retinal ganglion cell quantification. Additionally, wholemount TUNEL was performed. Results: Statistical analysis revealed significant neuroprotective effects for the inhibitors of caspases 3, 6, 8 and 9, but not for those of caspases 1 and 4. The inhibitors of caspases 6 and 9 showed greater neuroprotective potential than those of caspases 3 and 8, although cell death was not entirely averted in any case. Results of ganglion cell counts were confirmed for the most pronounced treatment groups using wholemount TUNEL. Conclusion: Excitotoxic retinal ganglion cell death after NMDA injection is mediated mainly through apoptosis, whereby extrinsic as well as intrinsic pathways of caspase activation play a role.     

Experimental detection of retinal ganglion cell damage in vivo

In vivo detection of retinal ganglion cell (RGC) damage should have experimental and clinical relevance. A number of experimental models have been recently described to visualize RGCs in vivo. With retrograde injection of fluorescent tracers into the superior colliculus, lateral geniculate body, or optic nerve, RGCs can be detected in vivo with confocal laser scanning microscopy, fluorescent microscopy, or confocal scanning laser ophthalmoscopy. Although the resolution of these imaging techniques is limited to detecting only the cell bodies, the addition of adaptive optics has allowed in vivo visualization of axonal and dendritic processes. An ideal experimental model for detection of RGC damage should be non-invasive and reproducible. The introduction of a strain of transgenic mice that express fluorescent proteins under the control of Thy-1 promoter sequence has offered a non-invasive approach to detect RGCs. Long- term serial monitoring of RGCs over a year has been shown possible with this technique. In vivo imaging of RGCs could provide crucial information to investigating the mechanisms of neurodegenerative diseases and evaluating the treatment response of neuroprotective agents.     

A key role for calpains in retinal ganglion cell death

PURPOSE: The purpose of this study was to examine the importance of calpains in retinal ganglion cell (RGC) apoptosis and the protection afforded by calpain inhibitors against cell death. METHODS: Two different models of RGC apoptosis were used, namely the RGC-5 cell line after either intracellular calcium influx or serum withdrawal and retinal explant culture involving optic nerve axotomy. Flow cytometry analysis with Annexin V/PI staining was used to identify RGC-5 cells undergoing apoptosis after treatment. TdT-mediated dUTP nick end labeling (TUNEL) was used to identify cells undergoing apoptosis in retinal explant sections under various conditions. Serial sectioning was used to isolate the cell population of the ganglion cell layer (GCL). Western blotting was used to demonstrate calpain cleavage and activity by detecting cleaved substrates. RESULTS: In the RGC-5 cell line, the authors reported the activation of mu-calpain and m-calpain after serum starvation and calcium ionophore treatment, with concurrent cleavage of known calpain substrates. They found that the inhibition of calpains leads to the protection of cells from apoptosis. In the second model, after a serial sectioning method to isolate the cells of the ganglion cell layer (GCL) on a retinal explant paradigm, protein analysis indicated the activation of calpains after axotomy, with concomitant cleavage of calpain substrates. The authors found that inhibition of calpains significantly protected cells in the GCL from cell death. CONCLUSIONS: These results suggest that calpains are crucial for apoptosis in RGCs after calcium influx, serum starvation, and optic nerve injury.     

Disruption of the complement cascade delays retinal ganglion cell death following retinal ischemia-reperfusion

Recent reports have indicated that components of the complement cascade are synthesized during the degeneration of retinal ganglion cells (RGC) in glaucoma. While complement deposition in the retina may simply serve to aid phagocytosis of damaged RGC, activation of the complement cascade can also contribute to neuronal loss in neurodegenerative diseases. This study was designed to determine if disruption of the complement cascade affects RGC survival in a murine model of retinal ischemia-reperfusion (I/R) injury. We induced retinal ischemia in the eyes of normal mice and mice with a targeted disruption of the complement component 3 (C3) gene. Tissue was harvested 7 and 21 days after induction of I/R and retinal complement synthesis was determined by quantitative PCR and immunohistochemical methods. RGC death and associated axon loss was evaluated through histological examination of the optic nerve and retina. Our data show that retinal I/R induces the expression and deposition of complement components. C3 deficient mice clearly exhibited reduced optic nerve damage and substantial preservation of RGC 1 week after I/R when compared to normal animals (p = 0.005). Three weeks after the ischemic event C3 deficient mice retained more RGC cell bodies although the degree of optic nerve damage was similar between both groups. These findings demonstrate that inhibition of the complement cascade delays optic nerve axonal and RGC degeneration in retinal I/R. It appears that injured RGC are targeted and actively destroyed through complement mediated processes. These results may have implications for the pathophysiology and clinical management of ischemic retinal conditions.     

Retinal ganglion cell numbers and delayed retinal ganglion cell death in the P23H rat retina

The P23H-1 rat strain carries a rhodopsin mutation frequently found in retinitis pigmentosa patients. We investigated the progressive degeneration of the inner retina in this strain, focussing on retinal ganglion cells (RGCs) fate. Our data show that photoreceptor death commences in the ventral retina, spreading to the whole retina as the rat ages. Quantification of the total number of RGCs identified by Fluorogold tracing and Brn3a expression, disclosed that the population of RGCs in young P23H rats is significantly smaller than in its homologous SD strain. In the mutant strain, there is also RGC loss with age: RGCs show their first symptoms of degeneration at P180, as revealed by an abnormal expression of cytoskeletal proteins which, at P365, translates into a significant loss of RGCs, that may ultimately be caused by displaced inner retinal vessels that drag and strangulate their axons. RGC axonal compression begins also in the ventral retina and spreads from there causing RGC loss through the whole retinal surface. These decaying processes are common to several models of photoreceptor loss, but show some differences between inherited and light-induced photoreceptor degeneration and should therefore be studied to a better understanding of photoreceptor degeneration and when developing therapies for these diseases.     

Retinal ganglion cell numbers and delayed retinal ganglion cell death in the P23H rat retina

The P23H-1 rat strain carries a rhodopsin mutation frequently found in retinitis pigmentosa patients. We investigated the progressive degeneration of the inner retina in this strain, focussing on retinal ganglion cells (RGCs) fate. Our data show that photoreceptor death commences in the ventral retina, spreading to the whole retina as the rat ages. Quantification of the total number of RGCs identified by Fluorogold tracing and Brn3a expression, disclosed that the population of RGCs in young P23H rats is significantly smaller than in its homologous SD strain. In the mutant strain, there is also RGC loss with age: RGCs show their first symptoms of degeneration at P180, as revealed by an abnormal expression of cytoskeletal proteins which, at P365, translates into a significant loss of RGCs, that may ultimately be caused by displaced inner retinal vessels that drag and strangulate their axons. RGC axonal compression begins also in the ventral retina and spreads from there causing RGC loss through the whole retinal surface. These decaying processes are common to several models of photoreceptor loss, but show some differences between inherited and light-induced photoreceptor degeneration and should therefore be studied to a better understanding of photoreceptor degeneration and when developing therapies for these diseases.     

Increased spontaneous retinal ganglion cell activity in rd mice after neural retinal transplantation

PURPOSE: To study the functional success of neural retinal transplantation by means of retinal surface ganglion cell recordings. METHODS: Eight-week-old C3H/HeJ (rd/rd) retinal degeneration mice received transplants (subretinal) in one eye only of neural retinal tissue isolated from newborn normal C57/BL6J mice. Four weeks after transplantation, ganglion cell responses were recorded directly from the retinal surface over the transplant, with a differential bipolar surface electrode. Measurements were performed, both with and without light stimulation. Similar recordings were performed in nontransplant areas of the transplant-recipient eyes, and in age-matched sham-treated and untreated control eyes. After the recordings, the eyes were processed for light and transmission electron microscopy. RESULTS: Histologic examination showed that in some areas, transplanted cells were organized into small sheets and differentiated into photoreceptors with outer segments in intimate contact with the host RPE. No light-driven ganglion cell responses were recordable from the transplant-recipient or control eyes. However, the spontaneous ganglion cell activity was higher in the transplant areas (mean: 10.8 +/-12.0 spikes/1.6 sec) compared with nontransplant areas of these recipient eyes (mean: 2.4 +/- 5.7spikes/1.6 sec; P < 0.001), sham-treated eyes (mean: 2.5 +/- 4.8 spikes/1.6 sec; P < 0.001), and the untreated control eyes (mean: 2.2 +/- 4.4 spikes/1.6 sec; P < 0.001). CONCLUSIONS: Subretinal transplantation of neural retinal tissue results in a local increase of spontaneous ganglion cell activity. The increased activity may be due to the release of neurochemically active substances as a result of the presence of the transplant. Although light responses were not recordable, the technique of retinal surface ganglion cell recording may be useful for assessing the functional success of transplantation.     

Light-driven retinal ganglion cell responses in blind rd mice after neural retinal transplantation

PURPOSE: Light-elicited retinal ganglion cell (RGC) responses after fetal neural retinal transplantation have not been demonstrated in animal or human subjects blind from outer retinal degeneration, despite apparent morphologic success. This study was designed to test the hypothesis that the functional success of retinal transplantation may be enhanced by using a young host retina (13 days old). METHODS:At postnatal day (P)13 C3H/HeJ (rd/rd) retinal degenerate mice received a subretinal transplant, in one eye only, of neural retinal tissue isolated from newborn normal C57/BL6J mice. Between 33 and 35 days after transplantation, local electroretinograms (ERGs) and ganglion cell responses were recorded directly from the retinal surface using a differential bipolar surface electrode. Measurements were performed both with and without light stimulation. Similar recordings were also performed in age-matched eyes subjected to sham transplantation, in control eyes that were not subjected to surgery, and in animals eyes that underwent transplantation at 8 weeks of age. After the recordings, the eyes were processed for light and transmission electron microscopy. RESULTS:Three of 10 mice showed bursts of ganglion cell action potentials (ON response only) as well as recordable intraocular ERGs over the transplant in response to 1-second and 200-msec light stimuli. Light-driven ganglion cell responses could not be recorded in areas outside the transplant in all transplant-recipient eyes, age-matched control eyes, and sham-transplantation eyes. Light responses also could not be recorded in animal eyes that received transplants at an older age (8 weeks). Electron microscopic examination confirmed the presence of photoreceptor outer segments in the areas affected by transplantation. CONCLUSIONS: This study demonstrates the presence of light-driven ganglion cell responses after subretinal transplantation in a retinal degenerate model. This finding may reflect functional integration of the transplant with the host, but a rescue effect on remaining host photoreceptors cannot be ruled out. The findings suggest, however, that modification of host parameters, such as host age, may be important approaches for improving the functional success of retinal transplantation.     

Superoxide signaling and cell death in retinal ganglion cell axotomy: effects of metallocorroles

Injury to retinal ganglion cell (RGC) axons within the optic nerve causes apoptosis of the soma. We previously demonstrated that in vivo axotomy causes elevation of superoxide anion within the RGC soma, and that this occurs 1-2 days before annexin-V positivity, a marker of apoptosis. Pegylated superoxide dismutase delivery to the RGC prevents the superoxide elevation and rescues the soma. Together, these results imply that superoxide is an upstream signal for apoptosis after axonal injury in RGCs. We then studied metallocorroles, potent superoxide dismutase mimetics, which we had shown to be neuroprotective in vitro and superoxide scavengers in vivo for RGCs. RGCs were retrograde labeled with the fluorescent dye 4Di-10Asp, and then axotomized by intraorbital optic nerve transection. Iron(III) 2,17-bis-sulfonato-5,10,15-tris(pentafluorophenyl)corrole (Fe(tpfc)(SO(3)H)(2)) (Fe-corrole) was injected intravitreally. Longitudinal imaging of RGCs was performed and the number of surviving RGCs enumerated. There was significantly greater survival of labeled RGCs with Fe-corrole, but the degree of neuroprotection was relatively less than that predicted by their ability to scavenge superoxide-This implies an unexpected complexity in signaling of apoptosis by reactive oxygen species.