Retinal ischemia leads to apoptosis which is ameliorated by aurintricarboxylic acid

PDGFRB upregulation contributes to retinal damages in the rat model of retinal ischemia-reperfusion

Retinal ischemic disease is a major type of retinal diseases causing vision loss. Identifying the molecular mechanisms mediating the retinal ischemia-reperfusion (RIR) is the key to targeted intervention. In this study, we performed RNA-seq analysis of the retinal tissues of a retinal ischemia-reperfusion model of Sprague-Dawley (SD) rats, followed by differential gene expression analysis, gene ontology (GO) enrichment analysis, and protein-protein interaction (PPI) analysis. After studying we found that: The major biological processes affected after RIR was the regulation of vascular development. PPI analysis unveiled a regulatory module in which Platelet Derived Growth Factor Receptor Beta (PDGFRB) was upregulated. In the RIR cell model of human retinal microvascular endothelial cells (HRCEC) induced by oxygen-glucose deprivation/reperfusion (OGD/R), silencing PDGFRB at least partially rescued the detrimental effect on cell proliferation and in vitro angiogenic ability. In the rat model of RIR, the administration of PDGFR inhibitor alleviated the damages in the retinal microvascular system. Besides, we further demonstrated the protective effect of procyanidin against RIR induced damages in both the cell and animal model by dampening the overexpression of PDGFRB. Together, our data indicate that the upregulation of PDGFRB contributes to RIR-induced damages in retinal microvascular system, which provides a targetable strategy for therapeutic intervention.

Aflibercept for the Treatment of Macular Edema Secondary to Idiopathic Retinal Vasculitis, Aneurysms, and Neuroretinitis Syndrome

We report a case of idiopathic retinal vasculitis, aneurysms, and neuroretinitis (IRVAN) syndrome in a patient whose cystoid macular edema (CME) was successfully treated with aflibercept and pan-retinal photocoagulation (PRP). A 56-year-old male was sent to our uveitis service for further evaluation after a fluorescein angiogram revealed symmetric retinal ischemia for 360 degrees in both eyes. A fundus examination revealed an aneurysm, neuroretinitis, and occlusive vasculitis, all consistent with a diagnosis of IRVAN syndrome. An optical coherence tomography examination revealed CME of the left eye. A chest X-ray revealed minimally prominent interstitial markings. The patient had a positive QuantiFERON-TB Gold test and was treated for tuberculosis with a one-year course of isoniazid and pyrimethamine. A further workup for other infectious and autoimmune etiologies was negative. The initial treatment consisted of bilateral PRP of the areas of peripheral ischemia, treatment for which was provided in a fragmented fashion over the course of seven months. Soon after the diagnosis, he received treatment with two intravitreal injections of aflibercept (2 mg/0.5 mL), one month apart, to the left eye. Subsequently, four months following the presentation, he developed CME in the right eye, which was treated with a single intravitreal injection of aflibercept (2 mg/0.5 mL). At his last follow-up visit, four years after the initial presentation, the patient remained asymptomatic with 20/20 visual acuity in both eyes and no evidence of CME recurrence. Our case suggests that aflibercept may serve as an adjuvant to the standard treatment with PRP, especially in cases that present with associated macular edema.

Reperfusion of retinal ischemia in retinal occlusive vasculitis with nicotinic acid and infliximab in Adamantiades-Behçet's disease

To describe a case of ischemic retinal vasculitis in Adamantiades-Behçet disease (ABD) that demonstrated significant resolution of retinal ischemia following treatment with nicotinic acid and infliximab.Observations: A 12-year-old male with a history of recurrent oral ulcers, fevers, and failure to thrive was admitted to the hospital with fever, oral and perirectal mucositis, and poor oral intake one month before presentation to uveitis clinic. He was suspected to have ABD and was treated with three doses of intravenous (IV) methylprednisolone (30 mg/kg/day) which led to improvement in his systemic symptoms. One week after admission, he complained of decreased vision in both eyes (OU), during which he was found to have anterior uveitis in OU and was referred to the Uveitis Clinic. Upon examination, his visual acuity was 20/80 in OU. Intraocular pressures were within normal limits. Anterior chamber evaluation revealed 0.5+ cells and 1.5+ flare in OU. Posterior examination revealed pale optic nerve, sclerosis and vascular sheathing of retinal arteries, and collateral vessels in OU. Fluorescein angiography (FA) showed optic disc leakage and widespread retinal ischemia in OU. The patient was diagnosed with retinal occlusive vasculitis associated with ABD. He was initially treated with infliximab (5 mg/kg), systemic methylprednisolone, and mycophenolate mofetil. Three months later, his BCVA improved to 20/70 OU with slight improvement of retinal ischemia on FA. Nicotinic acid was added to his treatment regimen. Due to logistic challenges, he did not receive infliximab treatment during the subsequent three months. However, three months after beginning nicotinic acid therapy, FA revealed significant improvement of his retinal ischemia OU. Conclusion: To our knowledge, the index report is the first to show that nicotinic acid may improve retinal ischemia in vaso-occlusive retinal vasculitis and be an integral part of the treatment regimen of this sight-threatening condition.

A Short Duration Transient Ischemia Induces Apoptosis in Retinal Layers: An Experimental Study in Rabbits

Purpose

To investigate retinal cell apoptosis in an experimental transient, short duration ocular ischemia model.

Methods

An experimental ischemia model, which simulates creating temporary high intraocular pressure to control intraocular bleeding during pars plana vitrectomy, was set up. Rabbits were randomly divided into three groups. Group 1 was the control group. In Group 2, intraocular pressure was increased to 97 mmHg for 5 minutes. In Group 3, intraocular pressure was increased to 97 mmHg for 10 minutes. After 24 hours, terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick-end labeling assay was used to detect retinal apoptosis in rabbit eyes. Only nuclear staining in retinal cells was counted.

Results

Groups with 5 minutes and 10 minutes of ischemia showed significantly higher amount of ganglion cell layer apoptosis when compared with the control group (p<0.05). Light microscopy and standard hematoxylin-eosin did not show any significant damage in the retina cells.

Conclusions

Apoptotic cell death in the retinal cell layers occurs in temporary ischemia-reperfusion as early as 5 and 10 minutes duration.

Brimonidine's Neuroprotective Effects against Transient Ischaemia-Induced Retinal Ganglion Cell Death

Purpose

Brimonidine is a lowering pressure agent currently used in glaucoma. This chronic degenerative condition is characterised by neuronal death, and an agent which offers neuroprotection may slow down or impede the progression of neuronal cell death.

Methods

The effects of brimonidine (BMD) on the short- and long-term survival of retinal ganglion cells (RGCs) after transient retinal ischaemia are reported here using a rat model. The fluorescent tracer Fluorogold (FG) was applied to both superior colliculi to retrogradely label RGCs. A ninety-minute period of ischaemia was induced and densities of surviving RGCs were estimated over time by counting FG-labelled RGCs in 12 standard regions of each retina.

Results

Seven days after inducing transient ischaemia, there was loss of approximately half of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD prevented ischaemia-induced RGC death.

Conclusions

These results indicate that optimal neuroprotective effects against the early loss of RGCs are seen with 0.1% or 0.5% BMD. Ischaemia-induced RGC loss continued between day 7 and day 21 in the vehicle treated groups and amounted to approximately 25% of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD was also effective in reducing the slow loss of RGCs.

BAX and BCL-2 polymorphisms, as predictors of proliferative vitreoretinopathy development in patients suffering retinal detachment: the Retina 4 project

PURPOSE

To compare the distribution of BCL-2 -938C>A (rs2279115) and BAX -248G>A (rs4645878) genotypes among European subjects undergoing rhegmatogenous retinal detachment (RRD) surgery in relation to the further development of proliferative vitreoretinopathy (PVR).

METHODS

A case-control gene association study, as a part of Retina 4 project, was designed. rs2279115 and rs4645878 polymorphisms were analysed in 555 samples from patients with RRD (134 with PVR secondary to surgery). Proportions of genotypes and AA homozygous groups of BCL-2 and BAX polymorphisms between subsamples were analysed in two phases. Genotypic and allelic frequencies were compared in global sample and in subsamples.

RESULTS

BAX: Differences were observed in the genotype frequencies and in AA carriers between controls and cases in the global series. The odds ratio (OR) of A carriers in the global sample was 1.7 (95% CI: 1.23-2.51). Proportions of genotypes in Spain + Portugal were significant different. The OR of A carriers from Spain and Portugal was 1.8 (95% CI: 1.11-2.95). BCL-2: No significant differences were observed in genotype frequencies. However, proportions of genotypes in Spain + Portugal were significant. A protective effect (OR: 0.6 95% CI: 0.43-0.96) was found in A carriers from Spain and Portugal.

CONCLUSIONS

Results suggest that A allele of rs4645878 could be a biomarker of high risk of developing PVR in patients undergoing RD surgery. The possible role of BCL-2 (inhibitor of necroptosis pathway) as a possible new target in PVR prophylaxis should be investigated.

Retinal ganglion cell (RGC) programmed necrosis contributes to ischemia-reperfusion-induced retinal damage

Retinal ischemia-reperfusion (IR) injury remains a common cause of blindness and has a final pathway of retinal ganglion cell (RGC) death by apoptosis and necrosis. RGC apoptosis was intensively studied in IR injury, while RGC necrosis did not receive nearly enough consideration since it was viewed as an accidental and unregulated cellular event. However, there is evidence that necrosis, like apoptosis, can be implemented by a programmed mechanism. In this study, we tested the role of RGC programmed necrosis (necroptosis) in IR-induced retinal injury. We employed the mouse model of retinal IR injury for in vivo experiments. The oxygen and glucose deprivation (OGD) model was used as an IR model in vitro. Primary RGCs were isolated by an immunopanning technique. Necrostatin 1 (Nec1) was used to inhibit necroptosis in in vitro and in vivo experiments. The changes in gene expression were assessed by quantitative RT-PCR. The distribution of proteins in the retina and in RGC cultures was evaluated by immunohistochemistry and immunocytochemistry, respectively. Our data suggest that proteins (Ripk1 and Ripk3), which initiate necroptosis, were present in normal and ischemic RGCs. Treatment with Nec1 significantly reduced retinal damage after IR. Increased RGC survival and reduced RGC necrosis following OGD were observed in Nec1-treated cultures. We found significantly reduced expression of genes coding pro-inflammatory markers Il1b, Ccl5, Cxcl10, Nos2 and Cybb in Nec1-treated ischemic retinas. Thus, our findings suggest that RGC necroptosis contributes to retinal damage after IR through direct loss of cells and induction of associated inflammatory responses.

Ischemic retinal vasculitis and its management

Ischemic retinal vasculitis is an inflammation of retinal blood vessels associated with vascular occlusion and subsequent retinal hypoperfusion. It can cause visual loss secondary to macular ischemia, macular edema, and neovascularization leading to vitreous hemorrhage, fibrovascular proliferation, and tractional retinal detachment. Ischemic retinal vasculitis can be idiopathic or secondary to systemic disease such as in Behçet's disease, sarcoidosis, tuberculosis, multiple sclerosis, and systemic lupus erythematosus. Corticosteroids with or without immunosuppressive medication are the mainstay treatment in retinal vasculitis together with laser photocoagulation of retinal ischemic areas. Intravitreal injections of bevacizumab are used to treat neovascularization secondary to systemic lupus erythematosus but should be timed with retinal laser photocoagulation to prevent further progression of retinal ischemia. Antitumor necrosis factor agents have shown promising results in controlling refractory retinal vasculitis excluding multiple sclerosis. Interferon has been useful to control inflammation and induce neovascular regression in retinal vasculitis secondary to Behçet's disease and multiple sclerosis. The long term effect of these management strategies in preventing the progression of retinal ischemia and preserving vision is not well understood and needs to be further studied.

Rho/Rho-associated kinase pathway in glaucoma (Review)

The Rho/ROCK pathway plays important roles in the modulation of the cytoskeletal integrity of cells, the synthesis of extracellular matrix components in the aqueous humor outflow tissue and the permeability of Schlemm's canal endothelial cells. The activation of the Rho/ROCK pathway results in trabecular meshwork (TM) contraction, and the inhibition of this pathway would provoke relaxation of TM with subsequent increase in outflow facility and, thereby, decrease intraocular pressure (IOP). ROCK inhibitors also serve as potent anti‑scarring agents via inhibition of transdifferentiation of tenon fibroblasts into myofibroblasts. Furthermore, the RhoA/ROCK pathway is involved in optic nerve neuroprotection. Inactivation of Rho/ROCK signaling increase ocular blood flow, improve retinal ganglion cell (RGC) survival and promote RGC axon regeneration. Considering the IOP modulation, potent bleb anti-scarring effect and neuroprotective properties of ROCK inhibitors, the Rho/ROCK pathway is an attractive target for anti-glaucoma therapy, and it may be used for human therapy in the near future.

Alpha B-crystallin improved survival of retinal ganglion cells in a rat model of acute ocular hypertension

Increased endogenous αB-crystallin protein levels have been shown to reduce cell apoptosis, although the effects of exogenous αB-crystallin protein remain poorly understood. The present study established an acute ocular hypertension model in the right eye of Sprague-Dawley rats. Fluorogold retrograde tracing and immunofluorescence methods showed that the number of retinal ganglion cells decreased in the right eyes and caspase-3 expression increased following acute ocular hypertension. Intravitreal injection of αB-crystallin in the right eye increased the number of retinal ganglion cells and reduced caspase-3 expression. Results demonstrated that exogenous αB-crystallin protein inhibited caspase-3 expression and improved retinal ganglion cell survival following acute ocular hypertension.

Neuroprotective effect of small interfering RNA targeted to caspase-3 on rat retinal ganglion cell loss induced by ischemia and reperfusion injury

PURPOSE

To investigate neuroprotective effects of siRNA targeted to caspase-3 against ischemia and reperfusion (I/R) injury in rat eyes.

METHODS

Retinal ischemia was induced in Wistar rats by increasing the intraocular pressure (IOP) to 110 mmHg for 120 min. To examine the effect of siRNA on rat caspase-3, siRNA was injected into the vitreous cavity 24 h prior to induction of retinal ischemia. Eyes were removed at 2, 7 or 14 days later, and then analyzed for the number of retinal ganglion cells (RGCs), the retinal thickness and the amount of apoptosis of the retinal neural cells (as demonstrated by the TUNEL assay). The amount of caspase-3 mRNA was analyzed by rt-PCR. Differences between groups were evaluated by an unpaired t test.

RESULTS

The numbers of RGCs in the saline and non-silencing siRNA controls were reduced significantly at 2 and 7 days after the I/R injury. RGCs were significantly retained in eyes pretreated with siRNA targeted to caspase-3 as compared to the control eyes at 2 days after the I/R injury. Inner retinal thickness in the control eyes was significantly thinner as compared to the treated eyes at 2 and 7 days after the I/R injury. After siRNA treatment, the amount of caspase-3 mRNA was significantly lower when compared to the saline control group.

CONCLUSIONS

The injection of siRNA targeted to caspase-3 into the vitreous cavity of rat eyes may block caspase-3, and may thus be able to prevent retinal cell death associated with ischemic injury. As inhibition of the apoptosis pathway may provide a neuroprotective effect, examination of new strategies for treating these disorders needs to be undertaken.

Endogenous erythropoietin protects neuroretinal function in ischemic retinopathy

Because retinal ischemia is a common cause of vision loss, we sought to determine the effects of ischemia on neuroretinal function and survival in murine oxygen-induced retinopathy (OIR) and to define the role of endogenous erythropoietin (EPO) in this model. OIR is a reproducible model of ischemia-induced retinal neovascularization; it is used commonly to develop antiangiogenic strategies. We investigated the effects of ischemia in murine OIR on retinal function and neurodegeneration by electroretinography and detailed morphology. OIR was associated with significant neuroretinal dysfunction, with reduced photopic and scotopic ERG responses and reduced b-wave/a-wave ratios consistent with specific inner-retinal dysfunction. OIR resulted in significantly increased apoptosis and atrophy of the inner retina in areas of ischemia. EPO deficiency in heterozygous Epo-Tag transgenic mice was associated with more profound retinal dysfunction after OIR, indicated by a significantly greater suppression of ERG amplitudes, but had no measurable effect on the extent of retinal ischemia, preretinal neovascularization, or neuroretinal degeneration in OIR. Systemic administration of recombinant EPO protected EPO-deficient mice against this additional suppression, but EPO supplementation in wild-type animals with OIR did not rescue neuroretinal dysfunction or degeneration. Murine OIR offers a valuable model of ischemic neuroretinal dysfunction and degeneration in which to investigate adaptive tissue responses and evaluate novel therapeutic approaches. Endogenous EPO can protect neuroretinal function in ischemic retinopathy.

Deleterious role of anti-high mobility group box 1 monoclonal antibody in retinal ischemia-reperfusion injury

PURPOSE

To investigate the effect of anti-high mobility group box 1 (HMGB1) monoclonal antibody (mAb) against ischemia-reperfusion injury in the rat retina.

MATERIALS AND METHODS

Retinal ischemia was induced by increasing and then maintaining intraocular pressure at 130 mmHg for 45 min. An intraperitoneal injection of anti-HMGB1 mAb was administered 30 min before ischemia. Retinal damage was evaluated at 7 days after the ischemia. Immunohistochemistry and image analysis were used to measure changes in the levels of reactive oxygen species (ROS) and the localization of anti-HMGB1 mAb. Dark-adapted full-field electroretinography (ERG) was also performed.

RESULTS

Pretreatment with anti-HMGB1 mAb significantly enhanced the ischemic injury of the retina. HMGB1 expression increased at 6-12 h after ischemia in the retina. After the ischemia, production of ROS was detected in retinal cells. However, pretreatment with anti-HMGB1 mAb increased the production of ROS. On the seventh postoperative day, the amplitudes of both the ERG a- and b-waves were significantly higher in the vehicle group than in the groups pretreated with anti-HMGB1 mAb.

CONCLUSIONS

The current in vivo model of retinal injury demonstrated that anti-HMGB1 mAb plays a large deleterious role in ischemia-reperfusion injury. In order to develop neuroprotective therapeutic strategies for acute retinal ischemic disorders, further studies on anti-HMGB1 mAb function are needed.

A novel calpain inhibitor for treatment of transient retinal ischemia in the rat

After an acute ischemia/reperfusion of the rat retina, the activation of cytotoxic proteases, including calpain, results in necrosis and apoptosis of retinal ganglion cells resulting in their degeneration. Using a systemically administered calpain inhibitor that crosses the blood-retinal barrier would provide for novel systemic intervention that protects the retina from acute injury and loss of function. Herein, we study a novel calpain peptide inhibitor, cysteic-leucyl-argininal (CYLA), in an in-vivo rat model of retinal ischemia to determine functional protection using electroretinography. The CYLA prodrug was administered intraperitoneally before and/or after ischemia-reperfusion at concentrations of 20-40 mg/kg. We found that administering 20 mg/kg of CYLA only after ischemia provides significant preservation of retinal function.

Opioid receptor activation: suppression of ischemia/reperfusion-induced production of TNF-α in the retina

PURPOSE

The detrimental role of TNF-α in ischemia-induced tissue damage is known. The authors study examined whether opioid receptor activation alters TNF-α levels in the postischemic retina.

METHODS

Retinal ischemia was induced by raising the intraocular pressure above systolic blood pressure (155-160 mm Hg) for 45 minutes. Rats were pretreated with the opioid receptor agonist morphine (1 mg/kg; intraperitoneally) before injury. Selected animals were pretreated with the opioid antagonist naloxone (3 mg/kg; intraperitoneally). Human optic nerve head (ONH) astrocytes and rat microglial cells were treated with morphine (0.1-1 μM) for 24 hours and then treated with 10 μg/mL or 30 ng/mL lipopolysaccharide (LPS), respectively. TNF-α was measured by ELISA. Opioid receptor subtypes in astrocytes and microglia were determined by Western blot analysis.

RESULTS

There was a time-dependent increase in TNF-α production; the maximum production occurred at 4 hours after ischemia and localized to the inner retinal regions. Ischemia-induced TNF-α production was significantly inhibited by morphine. In astrocytes and microglia, LPS triggered a robust increase in the release of TNF-α, which was significantly inhibited (P < 0.05) by morphine. Naloxone reversed the morphine-induced suppression of TNF-α production in vivo and in vitro. Both ONH astrocytes and microglial cells expressed δ-, κ-, and μ-opioid receptor subtypes.

CONCLUSIONS

These data provide evidence that the production of TNF-α after ischemia/reperfusion injury is an early event and that opioid receptor activation reduces the production of TNF-α. Immunohistochemistry data and in vitro studies provide evidence that ONH astrocytes and microglial cells are the primary sources for the TNF-α production under ischemic/inflammatory conditions. Activation of one or more opioid receptors can reduce ischemic/reperfusion injury by the suppression of TNF-α production.

Calcineurin mediates AKT dephosphorylation in the ischemic rat retina

Calcineurin (CaN) is a calcium/calmodulin-dependent protein phosphatase that has an important role in ischemia-induced apoptosis. The serine/threonine kinase, Akt, which is also known as protein kinase B, has an important role in the cell death/survival pathways. Akt is activated by its phosphorylation, which is positively regulated by phosphatidylinositol 3-kinase (PI3K) and negatively regulated by a class of protein phosphatases (PPs) in tissue. However, the relationship between CaN and Akt after transient ischemia remains unclear. In the present study, we investigated whether CaN is involved in neuronal cell apoptosis and Akt dephosphorylation that occur during ischemic injury. We examined the interdependence between CaN and Akt/protein kinase B (PKB) in the rat retina after transient ischemia. After ischemic damage, we detected changes in levels of CaN, Akt and Bad in rats in the presence or absence FK506, CaN inhibitor. Our results show that CaN cleavage reduced Akt phosphorylation at Thr308 and Ser473, and led to apoptosis via dephosphorylation of the proapoptotic Bcl-2 family member Bad. After treatment with FK506, Akt and Bad dephosphorylation was greatly reduced. The total number of TUNEL-positive neurons was reduced by intravitreal injection of FK506 after transient ischemia. These results indicate that CaN cleavage negatively regulates Akt phosphorylation and is involved in retinal cell apoptosis after transient ischemia.

Label-free calcium imaging in ischemic retinal tissue by TOF-SIMS

The distribution and movement of elemental ions in biologic tissues is critical for many cellular processes. In contrast to chemical techniques for imaging the intracellular distribution of ions, however, techniques for imaging the distribution of ions across tissues are not well developed. We used time-of-flight secondary ion mass spectrometry (TOF-SIMS) to obtain nonlabeled high-resolution analytic images of ion distribution in ischemic retinal tissues. Marked changes in Ca(2+) distribution, compared with other fundamental ions, such as Na(+), K(+), and Mg(2+), were detected during the progression of ischemia. Furthermore, the Ca(2+) redistribution pattern correlated closely with TUNEL-positive (positive for terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick end-labeling) cell death in ischemic retinas. After treatment with a calcium chelator, Ca(2+) ion redistribution was delayed, resulting in a decrease in TUNEL-positive cells. These results indicate that ischemia-induced Ca(2+) redistribution within retinal tissues is associated with the order of apoptotic cell death, which possibly explains the different susceptibility of various types of retinal cells to ischemia. Thus, the TOF-SIMS technique provides a tool for the study of intercellular communication by Ca(2+) ion movement.

Epigallocatechin-3-gallate reduces retinal ischemia/reperfusion injury by attenuating neuronal nitric oxide synthase expression and activity

Retinal ischemia/reperfusion (IR) injury causes profound tissue damage, especially retinal ganglion cell death. The aims of the study were twofold: (1) to investigate the benefits of epigallocatechin-3-gallate (EGCG), the major catechin found in tea, after IR challenge, and (2) to elucidate the mechanism of EGCG inhibition of nitric oxide synthase (NOS) expression. Wistar female rats were divided into four groups: normal control, EGCG with sham operation, retinal IR, and EGCG with IR groups. EGCG (50mg/kg) was administered by intraperitoneal injection 30 min before the experiment. IR injury to a rat's retina was induced by raising intraocular pressure to 150 mmHg for 60 min. With EGCG pretreatment, retinal ganglion cell death from IR was reduced by approximately 10% 3 days afterward. EGCG significantly downregulated IR-induced glial fibrillary acidic protein expression. EGCG treatment also reduced TUNEL-positive cells after IR in the inner retina as well as IR-induced lipid peroxidation. Histological analyses showed fewer neuronal NOS and nicotinamide adenine dinucleotide phosphate diaphorase-positive cells in the retina after IR with EGCG administration. Therefore, EGCG is effective in protecting retinal ganglion cells from IR challenge by ameliorating retinal nitrosactive stress and by regulating cell death through apoptotic pathways.

Rational basis for the development of coenzyme Q10 as a neurotherapeutic agent for retinal protection

Glaucoma is a worldwide leading cause of irreversible vision loss characterized by progressive death of retinal ganglion cells (RGCs). In the course of glaucoma, RGC death may be the consequence of energy impairment that triggers secondary excitotoxicity and free radical generation. There is substantial evidence also that a number of free radical scavengers and/or agents that improve mitochondrial function may be useful as therapies to ameliorate cell death in various neurological disorders including glaucoma. Coenzyme Q10 (CoQ10), an essential cofactor of the electron transport chain, has been reported to afford neuroprotection in neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, and its protective effect has been attributed in part to its free radical scavenger ability and to a specific regulation of the mitochondrial permeability transition pore. Using an established animal model of transient retinal ischemia, we have conclusively identified a role for abnormal elevation of extracellular glutamate in the mechanisms underlying RGC death that occurs, at least in part, via activation of the apoptotic program. Under these experimental conditions, N-methyl-D-aspartate (NMDA) and non-NMDA subtype of glutamate receptor antagonists, nitric oxide synthase inhibitors, and CoQ10 afford retinal protection supporting an important role for excitotoxicity in the mechanisms underlying RGC death.

Y-27632, a Rho-associated protein kinase inhibitor, attenuates neuronal cell death after transient retinal ischemia

PURPOSE

Transient retinal ischemia induces the death of retinal neuronal cells. Postischemic damage is associated with the infiltration of leukocytes into the neural tissue through vascular endothelia. The current study aimed to investigate whether this damage was attenuated by the inhibition of Rho/ROCK (Rho kinases) signaling, recently shown to play a critical role in the transendothelial migration of leukocytes.

METHODS

Y-27632, a selective inhibitor of ROCK, was injected intravitreally into rat eyes with transient retinal ischemia. Cell loss of the ganglion cell layer (GCL) and thinning of the inner plexiform layer (IPL) with and without the administration of Y-27632 were evaluated by histological analysis, TUNEL assay and retrograde labeling of retinal ganglion cells (RGCs). To examine the attenuation of leukocyte infiltration in postischemic retinas with the administration of Y-27632, silver nitrate staining and immunohistochemistry using an anti-LCA antibody were performed.

RESULTS

Cell loss of the GCL and thinning of the IPL were significantly attenuated when 100 nmol Y-27632 was administered within three hours of the induction of ischemia. TUNEL assay and retrograde labeling of RGCs showed a decreased number of apoptotic cells and an increased number of RGCs in Y-27632-injected retinas. Moreover, silver nitrate staining and immunohistochemical analysis using an anti-LCA antibody showed that Y-27632 injection dramatically inhibited leukocyte infiltration and endothelial disarrangement.

CONCLUSIONS

Our data suggest that inhibition of Rho/ROCK signaling offers neuroprotective therapy against postischemic neural damage, by regulating leukocyte infiltration in the neural tissue.

Role of extracellular signal-regulated kinase in glutamate-stimulated apoptosis of rat retinal ganglion cells

PURPOSE

To investigate the involvement of the extracellular signal-regulated kinase (ERK) signaling pathway after intravitrevous injection of glutamate in rat retina.

METHODS

Three groups of five Sprague-Dawley rats each were studied. Group I was a normal control group, intravitreal saline injections. In Group II, one eye received an intravitreal glutamate injection (375 nmol, dissolved in saline) while the contralateral eye served as control. In Group III, intravitreal PD98059 (100 micro mol, an inhibitor of ERK) injections were administered 1 hr before glutamate injections. Seven days after injections, phosphorylated (activated) ERK in retina was localized by immunohistochemistry and fluorescent double labeling of retinal cryosections. Specific ERK blockade was documented to assess the functional significance of activated ERK. TUNEL staining was performed to assess apoptotic cell death.

RESULTS

Expression of phosphorylated ERK in rat retina was observed in the inner nuclear layer, the outer nuclear layer, and the nerve fiber layer after 3 days intravitreous injection of glutamate, increasing significantly after 7 days. Double immunofluorescence labling demonstrated that the increased retinal immunostaining for phospho-ERK was predominantly localized to the retinal Müller cells after 7 days intravitreous injection of glutamate. Moreover, blocking activation of ERK significantly improved the number of TUNEL-positive cells in the eyes receiving intravitreal PD98059 injections compared with the eyes receiving glutamate injections.

CONCLUSIONS

The ERK pathway is involved in signal transduction in the retina after excessive stimulation by glutamate, which may contribute to the antiapoptotic role in retinal ganglion cell death induced by glutamate.

EDG receptors as a potential therapeutic target in retinal ischemia-reperfusion injury

LPA (lysophosphatidic acid) specific endothelial differentiation gene (EDG) receptors have been implicated in various anti-apoptotic pathways. Ischemia of the brain and retina causes neuronal apoptosis, which raises the possibility that EDG receptors participate in anti-apoptotic signaling in ischemic injury. We examined the expression of EDG receptors in a model of retinal ischemia-reperfusion injury and also tested LXR-1035, a novel analogue of LPA, in the rat following global retinal ischemic injury. Rats were subjected to 45 or 60 min of raised intraocular pressure. Animals were sacrificed at 24 h post-ischemia and retinal tissue was stained for EDG receptors. In separate experiments, animals were randomized to receive LXR or saline vehicle by intravitreal injection 24 h prior to ischemia. The degree of retinal damage was assessed morphologically by measuring the thickness of the inner retinal layers as well as functionally by electroretinography (ERG). We found that the normal retina has a baseline expression of the LPA receptors, EDG-2 and EDG-4, which are significantly upregulated in the inner layers in response to ischemia. Animals pretreated with LXR-1035 had dose-dependent, significant reductions in histopathologic damage and significant improvement in functional deficits compared with corresponding vehicle-controls, after 45 and 60 min of ischemia. These results suggest that LPA receptor signaling may play an important role in neuroprotection in retinal ischemia-reperfusion injury.

Oxidative stress in glaucomatous neurodegeneration: mechanisms and consequences

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. Although ROS are essential participants in cell signaling and regulation, when their cellular production overwhelms the intrinsic antioxidant capacity, damage to cellular macromolecules such as DNA, proteins, and lipids ensues. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of neurodegenerative diseases. Growing evidence supports the involvement of oxidative stress as a common component of glaucomatous neurodegeneration in different subcellular compartments of retinal ganglion cells (RGCs). Besides the evidence of direct cytotoxic consequences leading to RGC death, it also seems highly possible that ROS are involved in signaling RGC death by acting as a second messenger and/or modulating protein function by redox modifications of downstream effectors through enzymatic oxidation of specific amino acid residues. Different studies provide cumulating evidence, which supports the association of ROS with different aspects of the neurodegenerative process. Oxidative protein modifications during glaucomatous neurodegeneration increase neuronal susceptibility to damage and also lead to glial dysfunction. Oxidative stress-induced dysfunction of glial cells may contribute to spreading neuronal damage by secondary degeneration. Oxidative stress also promotes the accumulation of advanced glycation end products in glaucomatous tissues. In addition, oxidative stress takes part in the activation of immune response during glaucomatous neurodegeneration, as ROS stimulate the antigen presenting ability of glial cells and also function as co-stimulatory molecules during antigen presentation. By discussing current evidence, this review provides a broad perspective on cellular mechanisms and potential consequences of oxidative stress in glaucoma.

Neuroprotection of insulin against oxidative stress-induced apoptosis in cultured retinal neurons: involvement of phosphoinositide 3-kinase/Akt signal pathway

In order to investigate the neuroprotection of insulin in retinal neurons, we used retinal neuronal culture as a model system to study the protective effects of insulin against H2O2-induced cytotoxicity and apoptotic death. Primary retinal neuronal cultures were grown from retinas of 0-2-day old Sprague-Dawley rats. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay. Apoptotic cell death was evaluated by the TdT-mediated digoxigenin-dUTP nick-end labeling assay, and by DNA laddering analysis. Phosphoinositide 3-kinase (PI3K) activity was measured using phosphoinositide 4,5-bisphophate and [gamma-32P]ATP as substrate. Western blot analysis with anti-phospho-Akt (pS473) antibody was performed to examine the level of phosphorylated Akt. We observed that treatment with 100 microM H2O2 for 24 h significantly decreased cell viability and induced apoptotic death of retinal neurons, and that pretreatment with 10 nM insulin significantly inhibited or attenuated H2O2-induced cytotoxicity and apoptosis. Pretreatment with LY294002, a specific PI3K inhibitor, abolished the cytoprotective effect of insulin. Insulin also strongly activated both PI3K and the downstream effector Akt. These results suggest that insulin protects retinal neurons from oxidative stress-induced apoptosis and that the PI3K/Akt signal pathway is involved in insulin-mediated retinal neuroprotection.

XIAP-mediated neuroprotection in retinal ischemia

Retinal ischemia results in the loss of vision in a number of ocular diseases including acute glaucoma, diabetic retinopathy, hypertensive retinopathy and retinal vascular occlusion. Recent studies have shown that most of the neuronal death that leads to loss of vision results from apoptosis. XIAP-mediated gene therapy has been shown to protect a number of neuronal types from apoptosis but has never been assessed in retinal neurons following ischemic-induced cell death. We injected an adeno-associated viral vector expressing XIAP or GFP into rat eyes and 6 weeks later, rendered them ischemic by raising intraocular pressure. Functional analysis revealed that XIAP-treated eyes retained larger b-wave amplitudes than GFP-treated eyes up to 4 weeks post-ischemia. The number of cells in the inner nuclear layer (INL) and the thickness of the inner retina were significantly preserved in XIAP-treated eyes compared to GFP-treated eyes. Similarly, there was no significant reduction in optic nerve axon numbers in XIAP-treated eyes. There were also significantly fewer TUNEL (TdT-dUTP terminal nick end labeling) positive cells in the INL of XIAP-treated retinas at 24 h post-ischemia. Thus, XIAP-mediated gene therapy imparts both functional and structural protection to the retina after a transient ischemic episode.

Involvement of Insulin/Phosphoinositide 3-Kinase/Akt Signal Pathway in 17β-Estradiol-mediated Neuroprotection

In the present study, we tested the hypothesis that 17beta-estradiol (betaE2) is a neuroprotectant in the retina, using two experimental approaches: 1) hydrogen peroxide (H(2)O(2))-induced retinal neuron degeneration in vitro, and 2) light-induced photoreceptor degeneration in vivo. We demonstrated that both betaE2 and 17alpha-estradiol (alphaE2) significantly protected against H(2)O(2)-induced retinal neuron degeneration; however, progesterone had no effect. betaE2 transiently increased the phosphoinositide 3-kinase (PI3K) activity, when phosphoinositide 4,5-bisphosphate and [(32)gammaATP] were used as substrate. Phospho-Akt levels were also transiently increased by betaE2 treatment. Addition of the estrogen receptor antagonist tamoxifen did not reverse the protective effect of betaE2, whereas the PI3K inhibitor LY294002 inhibited the protective effect of betaE2, suggesting that betaE2 mediates its effect through some PI3K-dependent pathway, independent of the estrogen receptor. Pull-down experiments with glutathione S-transferase fused to the N-Src homology 2 domain of p85, the regulatory subunit of PI3K, indicated that betaE2 and alphaE2, but not progesterone, identified phosphorylated insulin receptor beta-subunit (IRbeta) as a binding partner. Pretreatment with insulin receptor inhibitor, HNMPA, inhibited IRbeta activation of PI3K. Systemic administration of betaE2 significantly protected the structure and function of rat retinas against light-induced photoreceptor cell degeneration and inhibited photoreceptor apoptosis. In addition, systemic administration of betaE2 activated retinal IRbeta, but not the insulin-like growth factor receptor-1, and produced a transient increase in PI3K activity and phosphorylation of Akt in rat retinas. The results show that estrogen has retinal neuroprotective properties in vivo and in vitro and suggest that the insulin receptor/PI3K/Akt signaling pathway is involved in estrogen-mediated retinal neuroprotection.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Betaxolol, a beta1-adrenoceptor antagonist, protects a transient ischemic injury of the retina

In the present study, we investigated the protective effects of the topical beta-adrenoceptor antagonist Betoptic((R)) (0.25% betaxolol) in the rat retina following the ischemic injury induced by a transient increase of intraocular pressure (IOP). Like other areas of the central nervous system, the retina is highly vulnerable to ischemic-induced injury. Ischemia was induced in the rat retina by raising the IOP above the systolic blood pressure for 60min. After an ischemia/reperfusion, the thickness of the retinal layers and the immunoreactivities of choline acetyltransferase (ChAT), gamma-amino butyric acid (GABA) and tyrosine hydroxylase (TH) were examined. After a reperfusion period of 7 days, the thickness of both the inner plexiform layer and inner nuclear layer was much decreased. After a reperfusion period of 14-28 days, the thickness of the outer nuclear layer decreased markedly. Moreover, the ChAT and TH immunoreactivity had almost completely disappeared in the retinas after 7 days, while GABA immunoreactivity remained for 28 days. These results suggest that the inner retinal layers are more susceptible to ischemic-induced injury than the outer retinal layer.Histological examination demonstrated protective effects of betaxolol on ischemic-induced retinal damage, which was more substantial in the inner retinal layer. When two drops of betaxolol, once before ischemic injury and twice daily for 28 days after ischemia, were continuously administered, the reductions in the retinal ChAT, GABA and TH immunoreactivities were significantly attenuated. The present study suggests that topically applied betaxolol is an efficient neuroprotective agent and prevents the retinal cell damage induced by ischemic injury in rats.

Loss of retinal ganglion cells following retinal ischemia: the role of inducible nitric oxide synthase

Following experimental, transient, retinal ischemia in the rat, there is loss of retinal neurons, which occurs over several weeks. Retinal ganglion cells (RGCs) are particularly susceptible and there is early, massive degeneration of these neurons after ischemia. We have determined the early mechanisms by which RGCs are killed following ischemia. Retinal ischemia/reperfusion was produced in rats by transient unilateral elevation of intraocular pressure above systolic blood pressure. Retinas were studied by immunohistochemistry for the presence of inducible nitric oxide synthase (NOS-2) at several time points post-ischemia and specific cell types were identified. Rats were also treated orally with L -N(6) -(1-iminoethyl)lysine 5-tetrazole amide (SC-51), a prodrug of an inhibitor of NOS-2 or with aminoguanidine (AG) for a period of 14 days. Retrograde labelling with Fluoro-Gold quantitated the loss of RGCs. NOS-2 was not present in the normal retina and was not present in the eyes that were contralateral to the ischemic eyes. Within 24hr after ischemia, polymorphonuclear leukocytes containing NOS-2 had entered the ganglion cell layer and surrounded RGCs. Within 5 days after ischemia, NOS-2 was present in many inner retina cells and in invading monocytes in the vitreous. Between 7 and 14 days post-ischemia, there were few hematogenous cells in the retina but NOS-2 was sparsely detectable in microglia and other cells of the inner retina. Two weeks after ischemia, rat eyes lost approximately 50% of the RGCs. Treatment with AG for 14 days following ischemia was partially neuroprotective; approximately 28% of the RGCs were lost. Treatment with SC-51 for 14 days following ischemia almost completely prevented the loss of RGCs. Thus, within 24hr following ischemia, polymorphonuclear leukocytes containing NOS-2 attack and kill neurons in the ganglion cell layer. For 2 weeks after ischemia, NOS-2 appears transiently in the retina in several different cell types at different times. Continuous pharmacological treatment with inhibitors of NOS-2 activity during the 2 weeks post-ischemia period provides significant neuroprotection against the loss of RGCs.

Erythropoietin administration protects retinal neurons from acute ischemia-reperfusion injury

Erythropoietin (EPO) plays an important role in the brain's response to neuronal injury. Systemic administration of recombinant human EPO (rhEPO) protects neurons from injury after middle cerebral artery occlusion, traumatic brain injury, neuroinflammation, and excitotoxicity. Protection is in part mediated by antiapoptotic mechanisms. We conducted parallel studies of rhEPO in a model of transient global retinal ischemia induced by raising intraocular pressure, which is a clinically relevant model for retinal diseases. We observed abundant expression of EPO receptor (EPO-R) throughout the ischemic retina. Neutralization of endogenous EPO with soluble EPO-R exacerbated ischemic injury, which supports a crucial role for an endogenous EPO/EPO-R system in the survival and recovery of neurons after an ischemic insult. Systemic administration of rhEPO before or immediately after retinal ischemia not only reduced histopathological damage but also promoted functional recovery as assessed by electroretinography. Exogenous EPO also significantly diminished terminal deoxynucleotidyltransferase-mediated dUTP end labeling labeling of neurons in the ischemic retina, implying an antiapoptotic mechanism of action. These results further establish EPO as a neuroprotective agent in acute neuronal ischemic injury.

Retinal ganglion cell death after acute retinal ischemia is an ongoing process whose severity and duration depends on the duration of the insult

In adult Sprague-Dawley rats we have investigated retinal ganglion cell survival after transient intervals of retinal ischemia of 30, 45, 60, 90 or 120 min duration, induced by ligature of the ophthalmic vessels. Animals were killed 5, 7, 14, 21, 30, 60, 90 or 180 days later and densities of surviving retinal ganglion cells were estimated in retinal whole mounts by counting cells labelled with diAsp. This dye was applied, 3 days prior to death, to the ocular stump of the intraorbitally transected optic nerve. We found that retinal ganglion cell loss after retinal ischemia proceeds for different lengths of time. All the ischemic intervals induced loss of retinal ganglion cells whose severity and duration was related to the length of the ischemic interval. Following 30 or 45 min of ischemia, cell loss lasted 14 days and caused the death of 46 or 50%, respectively, of the population of retinal ganglion cells. Sixty, 90 or 120 min of retinal ischemia were followed by a period of cell loss that lasted up to 90 days and caused the death of 75%, 87% or 99%, respectively, of the population of retinal ganglion cells. We conclude that retinal ganglion cell loss after retinal ischemia is an ongoing process that may last up to 3 months after the injury and that its severity and duration are determined by the ischemic interval.

Prospects for relevant glaucoma models with retinal ganglion cell damage in the rodent eye

Retinal ganglion cell (RGC) death is the end result of practically all diseases of the optic nerve, including glaucomatous optic neuropathy. Understanding the factors determining susceptibility of the retina or the optic nerve to glaucomatous damage, and the means to prevent it, requires good animal models. Here we review the different, current models in rodents that have been used to study RGC damage, discuss their value, and their adequacy as models for human glaucoma.

Neuroprotective effects of brimonidine against transient ischemia-induced retinal ganglion cell death: a dose response in vivo study

The purpose of this study was to investigate the dose-response effects of topically administered brimonidine (BMD) on retinal ganglion cell (RGC) survival, short and long periods of time after transient retinal ischemia. In adult Sprague-Dawley rats, RGCs were retrogradely labeled with the fluorescent tracer fluorogold (FG) applied to both superior colliculi. Seven days later, the left ophthalmic vessels were ligated for 90 min. One hr prior to retinal ischemia, two 5 microl drops of saline alone or saline containing 0.0001, 0.001, 0.01 or 0.1% BMD were instilled on the left eye. Rats were processed 7, 14 or 21 days later and densities of surviving RGCs were estimated by counting FG-labeled RGCs in 12 standard regions of each retina. The following have been found. (1) Seven days after 90 min of transient ischemia there is loss of approximately 46% of the RGC population. (2) topical pre-treatment with BMD prevents ischemia-induced RGC death in a dose-dependent manner. Administration of 0.0001% BMD resulted in the loss of approximately 37% of the RGC population and had no significant neuroprotective effects. Administration of higher concentrations of BMD (0.001 or 0.01%) resulted in the survival of 76 or 90%, respectively, of the RGC population, and 0.1% BMD fully prevented RGC death in the first 7 days after ischemia. (3) Between 7 and 21 days after ischemia there was an additional slow cell loss of approximately 25% of the RGC population. Pre-treatment with 0.1% BMD also reduced significantly this slow cell death. These results indicate that the neuroprotective effects of BMD, when administered topically, are dose-dependent and that the 0.1% concentration achieves optimal neuroprotective effects against the early loss of RGCs. Furthermore, this concentration is also effective to diminish the protracted loss of RGCs that occurs with time after transient ischemia.

Retinal Ganglion Cell Death Induced by Retinal Ischemia

We have investigated in adult Sprague-Dawley rats the neuroprotective effects of two alpha-2-selective agonists [AGN 191,103 (AGN) and brimonidine tartrate (BMD)] on retinal ganglion cell (RGC) survival after transient retinal ischemia. RGCs were labelled with Fluorogold (FG) applied to both superior colliculi. Seven days later, 90 min of retinal ischemia were induced in the left eyes by ligature of the ophthalmic vessels (LOV). In one group of animals, vehicle or AGN (0.01 mg/kg) were administered systemically 1 hr before ischemia. In another group of animals, two 5 microl drops of vehicle, AGN (0.05%) or BMD (0.1%) were administered topically in the left eye 1 hr before ischemia. The animals were processed 7 or 21 days later. RGC survival was estimated by counting FG-labelled cells in 12 standard areas of each retina. In control retinas of systemically pretreated animals, mean densities of labelled RGCs were 2372 +/- 49 cells/mm(2) (mean +/- SEM; n = 6). In experimental retinas of systemically pretreated animals, mean RGC densities had decreased 7 days after ischemia to 53% (n = 6) or 81% (n = 6) of control in the groups treated with vehicle or AGN, respectively. Twenty-one days after ischemia, mean RGC densities had decreased to 38% (n = 6) or 79% (n = 6) of control in the groups treated with vehicle or AGN, respectively. In control retinas of topically pretreated animals, mean densities of labelled RGCs were 2208 +/- 29 cells/mm(2) (n = 6). In experimental retinas of topically pretreated animals, mean RGC densities had decreased 7 days after ischemia to 54% (n = 6), 95% (n = 6) or 96% (n = 6) of control in the groups treated with vehicle, AGN or BMD, respectively. These results indicate that pretreatment with a single systemic or topical dose of AGN or BMD can prevent completely the early rapid phase of RGC loss and abolish the delayed RGC loss observed after 90 min of retinal ischemia induced by ligature of the ophthalmic vessels.

Cell-specific caspase expression by different neuronal phenotypes in transient retinal ischemia

Emerging evidence supports an important role for caspases in neuronal death following ischemia-reperfusion injury. This study assessed whether cell specific caspases participate in neuronal degeneration and whether caspase inhibition provides neuroprotection following transient retinal ischemia. We utilized a model of transient global retinal ischemia. The spatial and temporal pattern of the active forms of caspase 1, 2 and 3 expression was determined in retinal neurons following ischemic injury. Double-labeling with cell-specific markers identified which cells were expressing different caspases. In separate experiments, animals received various caspase inhibitors before the induction of ischemia. Sixty minutes of ischemia resulted in a delayed, selective neuronal death of the inner retinal layers at 7 days. Expression of caspase 1 was not detected at any time point. Maximal expression of caspase 2 was found at 24 h primarily in the inner nuclear and ganglion cell layers of the retina and localized to ganglion and amacrine neurons. Caspase 3 also peaked at 24 h in both the inner nuclear and outer nuclear layers and was predominantly expressed in photoreceptor cells and to a lesser extent in amacrine neurons. The pan caspase inhibitor, Boc-aspartyl fmk, or an antisense oligonucleotide inhibitor of caspase 2 led to significant histopathologic and functional improvement (electroretinogram) at 7 days. No protection was found with the caspase 1 selective inhibitor, Y-vad fmk. These observations suggest that ischemia-reperfusion injury activates different caspases depending on the neuronal phenotype in the retina and caspase inhibition leads to both histologic preservation and functional improvement. Caspases 2 and 3 may act in parallel in amacrine neurons following ischemia-reperfusion. These results in the retina may shed light on differential caspase specificity in global cerebral ischemia.

Functional and morphologic comparison of two methods to produce transient retinal ischemia in the rat

OBJECTIVES

Much of our knowledge of the pathophysiology of retinal ischemic injury is from a multitude of studies that use in vitro or in vivo animal models of retinal ischemia followed by reperfusion. The objective of this study was to compare histopathologic and electrophysiologic (electroretinography) parameters using two different models of transient retinal ischemia: high intraocular pressure (HIOP) and suture ligation of the optic nerve (SL).

METHODS

Transient retinal ischemia was induced using the HIOP model or the SL model in the Sprague-Dawley rat for either 30 or 60 minutes. Histopathologic outcome was determined at 1 and 7 days after ischemia. In addition, electroretinography (ERG) was performed at 2 hours, I day, 3 days, and 7 days after ischemia.

RESULTS

At 1 and 7 days after 30 minutes of ischemia, there were no significant histopathologic abnormalities in the retina with either model, except for a slight decrease of the cell count in the ganglion cell layer (GCL) with the SL method. After 60 minutes of ischemia, there was significant thinning of the inner retina. There was a significant early dropout of cells at 1 day in the inner nuclear layer (INL) in the HIOP method compared to the SL method where the dropout was delayed and gradually progressive. Dropout of cells in the GCL was early (I day) and gradually progressive in both models but more severe in HIOP than SL. There was a significant decrease in the ERG b-wave amplitudes as early as 2 hours after both 30 and 60 minutes of ischemia compared to preischemic baselines.

CONCLUSIONS

The degree of retinal injury after transient retinal ischemia was more severe at 1 day after reperfusion in the HIOP method compared to the SL method but was similar at 7 days in both models. Furthermore, our data suggests that functional assessment of ischemic damage by electroretinography may be a more sensitive parameter than conventional histopathologic quantification. The timing of either measurement relative to the ischemic stimulus is critical because histologic measurements performed too early after ischemia may underestimate the degree of injury.

Restoration of the retinofugal pathway

In a relatively short period of time covering the last 2 decades, regeneration of retinofugal axons has become one of most prominent experimental models in restorative neurobiology. There is now a significant knowledge both on the mechanisms governing retinal ganglion cell responses to transection of the optic nerve, and the subsequent cell-cell interactions accumulating in death of the neurons. In addition, retinofugal axons served as an excellent model to examine whether, and to conclude that these axons have remarkable abilities for re-growth. This last issue was of invaluable importance, because axons could regenerate in vivo, into peripheral nerve grafts, and last but not least within the white matter of the cut optic nerve. As it stands to date, the extremely complex aspects of axonal regeneration will probably be understood within the retinofugal pathway. Final elucidation of this delicate system will essentially lead to some revision of our knowledge concerning neurotraumatology and CNS-repair.

Aurintricarboxylic acid promotes survival and regeneration of axotomised retinal ganglion cells in vivo

Aurintricarboxylic acid (ATA) has been used as an anti-apoptotic drug to counteract ischemic or cytotoxic injury to neurons. We investigated whether ATA has a neuroprotective effect on axotomized, adult retinal ganglion cells (RGC) as a model for traumatic neuronal cell death. A solution of ATA was injected into the vitreous body of rat eyes whose optic nerves had been cut. In controls, 14% of RGC survived 14 days after axotomy, whereas 44% of RGC survived after a single injection of ATA solution, and 59% survived when the injection was repeated after 7 days. A single injection of ATA 1 day after axotomy rescued 58% of RGC. However, injection of ATA 4 days after axotomy did not influence the survival of RGC, indicating that crucial, irreversible cascades of death are initiated prior to this point in time. The TUNEL technique was used to visualise apoptotic ganglion cells and revealed that 4 days after axotomy their number was significantly less in retinas whose optic nerves were axotomized and treated with ATA, than those of controls. As a consequence of neuroprotection, more RGC were recruited to regenerate into a peripheral nerve graft used to replace the cut optic nerve. In this paradigm, ATA-treated RGC extended significantly more axons within the graft than control RGC. This number could be increased by a second injection of ATA 7 days after axotomy. These data show that ATA is not only able to delay post-traumatic neuronal death but also enhances the extent of axonal regeneration in vivo.

Death and neuroprotection of retinal ganglion cells after different types of injury

In adult Sprague-Dawley rats, retinal ganglion cell survival was investigated after intraorbital optic nerve section and after transient ischemia of the retina induced by elevation of the intraocular pressure or by selective ligature of the ophthalmic vessels. The thickness of the inner nuclear and inner plexiform layers was also assessed after transient periods (120 min) of retinal ischemia induced by selective ligature of the ophthalmic vessels. In addition, we have also investigated the neuroprotective effects of different substances in these paradigms. The intraocular injection of brain-derived neurotrophic factor increased RGC survival after retinal ischemia induced by elevation of the intraocular pressure or by selective ligature of the ophthalmic vessels. The caspase-inhibitor Z-DEVD increased retinal ganglion cell survival after optic nerve section and also after 90 min of retinal ischemia induced by selective ligature of the ophthalmic vessels. The peptide Bcl-2 did not increase retinal ganglion cell survival after optic nerve section but increased retinal ganglion cell survival after 60 or 90 min of retinal ischemia induced by selective ligature of the ophthalmic vessels. Finally, BDNF, nifedipine, naloxone and bcl-2 prevented in part the decrease in thickness of the inner nuclear layer and inner plexiform layer induced by selective ligature of the ophthalmic vessels. Our results suggest that retinal ganglion cell loss induced by different types of injury, may be prevented by substances with neuroprotective effects, by altering steps of the cascade of events leading to cell death.

Blue light induced apoptosis in rat retina

PURPOSE

To explore cell death in blue light induced retinal damage.

METHODS

Sprague-Dawley rats reared under cyclic light were exposed continuously to diffuse blue light (400-480 nm) at 0.64 W/m2 for 3 or 6 h after 22 h of dark adaptation. The rats were kept in darkness and killed immediately, 8, 16 and 24 h following light exposure. The retinal damage by the blue light was examined with a transmission electron microscope. The cell death was characterised by in situ terminal dUTP nick end labelling (TUNEL) and gel electrophoresis.

RESULTS

During the 24 h following light exposure, photoreceptor cell death was characterised by progressive condensation and margination of the chromatin, shrinkage or convolution and fragmentation of the nucleus, condensation of the cytoplasm, and formation of apoptotic bodies along with rapid removal of dying cells from damaged areas in the absence of inflammatory response. The TUNEL-positive nuclei were scattered individually in the outer nuclear layer just after light exposure. A wave of massive TUNEL labelling of photoreceptor nuclei peaked at 8-16 h and dropped at 24 h following light exposure. The distribution of TUNEL-positive nuclei was located predominantly at the upper temporal region of the retina, which was the most sensitive area to the damage caused by blue light. Furthermore, the multiples of internucleosomal cleavage of 180-200 base pairs were demonstrated at corresponding time points.

CONCLUSION

Photoreceptor cell apoptosis is seen early after the retina is damaged by blue light.

Dynamics of phosphorothioate oligonucleotides in normal and laser photocoagulated retina

AIMS

To investigate the distribution, persistence, and stability of fluorescently labelled phosphorothioate oligonucleotides (PS-ODNs) in normal and laser photocoagulated retina following intravitreal injection in the rat.

METHODS

Fluorescently labelled PS-ODNs were injected intravitreally into pigmented eyes at doses of 0.5-10.0 nmol in 2.0 microl solution. The dynamics of PS-ODNs was evaluated by fluorescent microscopy of cryosections and flat mounted retinal pigment epithelium (RPE)-choroid-sclera. Genescan analysis was used to assess the integrity of PS-ODNs in the retina after injection. The dynamics of PS-ODNs was also evaluated in the retina following krypton laser photocoagulation with a protocol producing choroidal neovascularisation (CNV).

RESULTS

Following intravitreal injection the PS-ODNs demonstrated dose and time dependent distribution and persistence in the retina, where they accessed all neural layers. However, they preferentially accumulated in the RPE layer, demonstrated as bright granules in the cytoplasm of the cells. Injections of 5.0 and 7.5 nmol of PS-ODNs exhibited strong fluorescence in the retina for 6 weeks after injection. Genescan analysis demonstrated that the PS-ODNs remained almost completely intact for at least 12 weeks. Following laser treatment, the PS-ODNs were concentrated in the regions of laser photocoagulation and retained high intensity for at least 8 weeks after injection, particularly localised to macrophages, RPE, and the local choroidal tissue.

CONCLUSIONS

These results indicate that PS-ODNs are stable and accessible to most neural layers of the retina, and they preferentially accumulate in the RPE layer following intravitreal injection. The successful delivery of PS-ODNs into normal and laser photocoagulated retina suggests that PS-ODNs may have potential in the development of therapy for attenuating retinal degenerations and CNV.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

Aurintricarboxylic acid prevents GLUR2 mRNA down-regulation and delayed neurodegeneration in hippocampal CA1 neurons of gerbil after global ischemia

Aurintricarboxylic acid (ATA), an inhibitor of endonuclease activity and other protein-nucleic acid interactions, blocks apoptosis in several cell types and prevents delayed death of hippocampal pyramidal CA1 neurons induced by transient global ischemia. Global ischemia in rats and gerbils induces down-regulation of GluR2 mRNA and increased alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced Ca2+ influx in CA1 before neurodegeneration. This result and neuroprotection by antagonists of AMPA receptors suggests that formation of AMPA receptors lacking GluR2, and therefore Ca2+ permeable, leads to excessive Ca2+ influx in response to endogenous glutamate; the resulting delayed neuronal death in CA1 exhibits many characteristics of apoptosis. In this study, we examined the effects of ATA on expression of mRNAs encoding glutamate receptor subunits in gerbil hippocampus after global ischemia. Administration of ATA by injection into the right cerebral ventricle 1 h before (but not 6 h after) bilateral carotid occlusion prevented the ischemia-induced decrease in GluR2 mRNA expression and the delayed neurodegeneration. These findings suggest that ATA is neuroprotective in ischemia by blocking the transcriptional changes leading to down-regulation of GluR2, rather than by simply blocking endonucleases, which presumably act later after Ca2+ influx initiates apoptosis. Maintaining formation of Ca2+ impermeable, GluR2 containing AMPA receptors could prevent delayed death of CA1 neurons after transient global ischemia, and block of GluR2 down-regulation may provide a further strategy for neuroprotection.