Glutamate level in optic nerve head is increased by artificial elevation of intraocular pressure in rabbits

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Functional role of peroxiredoxin 6 in the eye

Peroxiredoxin 6 (Prdx6) is the only mammalian 1-Cys member of the Prdx family, a group of enzymes which share the ability to reduce peroxides. In addition to its peroxidase function, Prdx6 also demonstrates phospholipase A₂ and lysophosphatidylcholine acyl transferase (LPCAT) activities. These enzymatic activities play an important role in regenerating oxidized membrane phospholipids and maintaining an appropriate balance of intracellular reactive oxygen species. Development of clinical pathologies, including those within the eye, have been linked to dysregulation of Prdx6 function. Interplay between external stressors like exposure to UV light, transforming growth factor β (TGF-β), and hyperglycemia in conjunction with diminished Prdx6 levels and loss of redox balance is associated with cellular changes in a variety of ophthalmic pathologies including cataracts, glaucoma, and retinal degeneration. Many of these cellular abnormalities can be rescued through supplementation with exogenous Prdx6. Additionally, corneal endothelial cells have been found to express high levels of Prdx6 in the plasma membrane. These findings highlight the importance of Prdx6 as an essential regulator of oxidative stress in the eye.

Evaluation of corpus geniculatum laterale and vitreous fluid by magnetic resonance spectroscopy in patients with glaucoma; a preliminary study

PURPOSE

To determine whether metabolite changes at cellular level occur in vitreous and lateral geniculate body (LGB) in patients with glaucoma by magnetic resonance spectroscopy (MRS).

METHODS

A total of 29 eyes of 29 patients with glaucoma, no existing ocular pathology and systemic disease (group 1), and 13 eyes of 13 healthy individuals whose routine ophthalmological examinations were normal (group 2) were included in the study. Single-voxel MRS examination was performed by placing region of interest in vitreous and LGB. Glutamate-glutamine (Glx)/creatin (Cr) ratios and lactate peaks in the vitreous, and the Glx/Cr, N-acetyl aspartate (NAA)/Cr, choline (Cho)/Cr ratios in the ipsilateral area of LGB were evaluated.

RESULTS

A statistically significant difference was found between the two groups for the Glx/Cr ratio in both the vitreous and LGB (P=0.0001). There was no statistically significant difference between the two groups for the NAA/Cr, and Cho/Cr ratios in the LGB (P=0.108). A lactate peak was established in the vitreous of 11 glaucoma patients.

CONCLUSION

Determining increased Glx/Cr ratios in both the vitreous and LGB of glaucoma patients, supports the theory of apoptosis in the etiopathogenesis of glaucoma. The MRS method, which can measure biochemical structures and metabolites of tissues, and also shows on a single spectrum, may be a new and non-invasive method for confirming the diagnosis of glaucoma and the role of apoptosis in the etiopathogenesis of glaucoma.

Trabecular meshwork gene expression after selective laser trabeculoplasty

BACKGROUND

Trabecular meshwork and Schlemm's canal are the tissues appointed to modulate the aqueous humour outflow from the anterior chamber. The impairment of their functions drives to an intraocular pressure increase. The selective laser trabeculoplasty is a laser therapy of the trabecular meshwork able to decrease intraocular pressure. The exact response mechanism to this treatment has not been clearly delineated yet. The herein presented study is aimed at studying the gene expression changes induced in trabecular meshwork cells by selective laser trabeculoplasty (SLT) in order to better understand the mechanisms subtending its efficacy.

METHODOLOGY/PRINCIPAL FINDINGS

Primary human trabecular meshwork cells cultured in fibroblast medium underwent selective laser trabeculoplasty treatment. RNA was extracted from a pool of cells 30 minutes after treatment while the remaining cells were further cultured and RNA was extracted respectively 2 and 6 hours after treatment. Control cells stored in incubator in absence of SLT treatment were used as reference samples. Gene expression was evaluated by hybridization on miRNA-microarray and laser scanner analysis. Scanning electron microscopic examination was performed on 2 Trabecular meshwork samples after SLT at 4(th) and 6(th) hour from treatment. On the whole, selective laser trabeculoplasty modulates in trabecular meshwork the expression of genes involved in cell motility, intercellular connections, extracellular matrix production, protein repair, DNA repair, membrane repair, reactive oxygen species production, glutamate toxicity, antioxidant activities, and inflammation.

CONCLUSIONS/SIGNIFICANCE

SLT did not induce any phenotypic alteration in TM samples. TM is a complex tissue possessing a great variety of function pivotal for the active regulation of aqueous humour outflow from the anterior chamber. SLT is able to modulate these functions at the postgenomic molecular level without inducing damage either at molecular or phenotypic levels.

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.

Neuroprotective effect of peroxiredoxin 6 against hypoxia-induced retinal ganglion cell damage

Background

The ability to respond to changes in the extra-intracellular environment is prerequisite for cell survival. Cellular responses to the environment include elevating defense systems, such as the antioxidant defense system. Hypoxia-evoked reactive oxygen species (ROS)-driven oxidative stress is an underlying mechanism of retinal ganglion cell (RGC) death that leads to blinding disorders. The protein peroxiredoxin 6 (PRDX6) plays a pleiotropic role in negatively regulating death signaling in response to stressors, and thereby stabilizes cellular homeostasis.

Results

We have shown that RGCs exposed to hypoxia (1%) or hypoxia mimetic cobalt chloride display reduced expression of PRDX6 with higher ROS expression and activation of NF-κB. These cells undergo apoptosis, while cells with over-expression of PRDX6 demonstrate resistance against hypoxia-driven RGC death. The RGCs exposed to hypoxia either with 1% oxygen or cobalt chloride (0-400 μM), revealed ~30%-70% apoptotic cell death after 48 and 72 h of exposure. Western analysis and real-time PCR showed elevated expression of PRDX6 during hypoxia at 24 h, while PRDX6 protein and mRNA expression declined from 48 h onwards following hypoxia exposure. Concomitant with this, RGCs showed increased ROS expression and activation of NF-κB with IkB phosphorylation/degradation, as examined with H2DCF-DA and transactivation assays. These hypoxia-induced adverse reactions could be reversed by over-expression of PRDX6.

Conclusion

Because an abundance of PRDX6 in cells was able to attenuate hypoxia-induced RGC death, the protein could possibly be developed as a novel therapeutic agent acting to postpone RGC injury and delay the progression of glaucoma and other disorders caused by the increased-ROS-generated death signaling related to hypoxia.

Glutamate-induced NFkappaB activation in the retina

PURPOSE

To determine the distribution and glutamate-mediated activation of nuclear factor (NF) kappaB members in the retina and pan-purified retinal ganglion cells (RGCs) and to characterize steps in the signal transduction events that lead to NFkappaB activation.

METHODS

Retinal expression patterns and RGCs were evaluated for five NFkappaB proteins with the aid of immunohistochemistry. Retinal explants or RGCs were treated with glutamate with or without the presence of the NDMA receptor antagonist memantine, the calcium chelator EGTA, or a specific inhibitor for calcium/calmodulin-dependent protein kinase-II (CaMKII). Characterizations of NFkappaB activation were performed with the aid of electrophoretic mobility shift assays and supershift assays.

RESULTS

All five NFkappaB proteins were present in the retina and in the pan-purified RGCs. In response to a glutamate stimulus, all NFkappaB proteins except c-Rel were activated. P65 was unique in that it was not constitutively active but showed a glutamate-inducible activation in the retina and in the cultured RGCs. Memantine, EGTA, or autocamtide-2-related inhibitory peptide (AIP) inhibited NFkappaB activation in the retina. Furthermore, AIP significantly reduced the level of glutamate-induced degradation of IkappaBs.

CONCLUSIONS

These data indicate that glutamate activates distinct NFkappaB proteins in the retina. P65 activation may be especially important with regard to RGC responses to glutamate given that its activity is induced by conditions known to lead to the death of these cells. The NMDA receptor-Ca(2+)-CaMKII signaling pathway is involved in glutamate-induced NFkappaB activation. Because AIP blocks the degradation of IkappaB, its regulation is clearly downstream of CaMKII.

Peroxiredoxin 6 delivery attenuates TNF-alpha-and glutamate-induced retinal ganglion cell death by limiting ROS levels and maintaining Ca2+ homeostasis

Higher expression of reactive oxygen species (ROS) is implicated in neurological disorders. A major event in glaucoma, the death of retinal ganglion cells (RGCs), has been associated with elevated levels of glutamate and TNF-alpha in the RGCs' local microenvironment. Herein we show that the transduction of Peroxiredoxin 6 (PRDX6) attenuates TNF-alpha- and glutamate-induced RGC death, by limiting ROS and maintaining Ca2+ homeostasis. Immunohistochemical staining of rat retina disclosed the presence of PRDX6 in RGCs, and Western and real-time PCR analysis revealed an abundance of PRDX6 protein and mRNA. RGCs treated with glutamate and/or TNF-alpha displayed elevated levels of ROS and reduced expression of PRDX6, and underwent apoptosis. A supply of PRDX6 protected RGCs from glutamate and TNF-alpha induced cytotoxicity by reducing ROS level and NF-kappaB activation, and limiting increased intracellular Ca2+ influx. Results provide a rationale for use of PRDX6 for blocking ROS-mediated pathophysiology in glaucoma and other neuronal disorders.

Development of spontaneous optic neuropathy in NF-kappaBetap50-deficient mice: requirement for NF-kappaBetap50 in ganglion cell survival

Although the transcription factor NF-kappaBeta is known to regulate cell death and survival, its precise role in cell death within the central nervous system remains unknown. The purpose of this study was to investigate the role of NF-kappaBetap50 in the age-related survival of retinal ganglion cells (RGCs). Eyes of mice with a deleted NF-kappaBetap50 gene and its wild-type mice at each of age were studied by histopathological studies. The number of RGCs was counted using retrograde labelling methods. Mice were subjected to intravitreous injection of N-methyl-D aspartate (NMDA) to induce RGC death. In p50-deficient mice, the number of RGCs significantly decreased with age in total independence of intraocular pressure measurement. Optic nerves of p50-deficient mice showed hypertrophy astrocytes and enlargement of the axons, together with a decreased number of axons. Immunohistochemistry showed a strong expression of glial fibrillary acidic protein. The histological results show obvious excavation of the optic nerve head in p50-deficient mice at 10 months of age. Intravitreal injection of NMDA in young p50-deficient mice damaged RGCs more intensively than in control animals. We further noticed that autoantibodies against RGCs were produced in p50-deficient mice. Our results show that p50 deficiency induced age-related RGC death, indicating a new insight into the role of p50 in the pathophysiology of neuropathy, and further experiments with p50-deficient mice may provide new targets for therapeutic intervention for human glaucoma.

High infusion pressure in conjunction with vitreous surgery alters the morphology and function of the retina of rabbits

PURPOSE

To investigate the effects of high infusion pressure in conjunction with pars plana vitrectomy (PPV) on retinal morphology and function in rabbits.

METHODS

Pars plana vitrectomy was performed under urethane (0.8 mg/kg) anaesthesia in the right eye of albino rabbits following phacoemulsification and aspiration (PEA). The left eyes were not touched. After PEA, the animals were divided into two groups. In six eyes, intraocular pressure (IOP) was increased to 80 mmHg for 30 mins (high-pressure group) and in five eyes IOP was maintained at 40 mmHg for 30 mins (low-pressure group). The IOPs were regulated by the height of the bottle of balanced salt solution (BSS) and monitored with a pressure transducer. After the pressure elevation, vitreous fluid was collected to measure the glutamate concentration. Then, PPV was performed for 15 mins in both groups under an infusion pressure of 40 mmHg. In five additional rabbits, PEA alone was performed in the right eye, and vitreous fluid was collected (PEA group). Functional alterations were assessed by recording visual evoked potentials (VEPs) and electroretinograms (ERGs). Ten days after the IOP changes, the animals were killed with intravenous pentobarbital sodium and the eyes were prepared for histological analysis. Damage to retinal ganglion cells (RGCs) was quantified by counting the number of cells in the ganglion cell layer (GCL). The contralateral eyes in the high-pressure group served as controls (n = 6).

RESULTS

The mean implicit time (IT) of the VEPs in the high-pressure group was significantly longer than that before the IOP elevation, by 114-124% (p < 0.05, paired t-test), and also than that of control eyes (p < 0.05, anova followed by t-test). No significant changes in the VEPs were detected in either the low-pressure group or the PEA group. There were significantly fewer cells in the GCL in the high-pressure group (24.7/mm) than in the control animals (41.4/mm; p < 0.05, Dunnett's test). The number of cells in the GCL in the low-pressure and PEA groups did not significantly differ to that in the controls. The amplitudes of the ERG a- and b-waves were not significantly changed (p > 0.05, paired t-test).

CONCLUSIONS

These results suggest that high infusion pressure in conjunction with PPV leads to morphological and functional changes in the retina. The absence of ERG changes and presence of VEP changes suggest that these changes were due to damage to RGCs, which supports the morphological observations.