Reaction of Müller cells in an experimental rat model of increased intraocular pressure following timolol, latanoprost and brimonidine

Pressure-Related Effects on Homeostatic Müller Cell Proteins in the Adult Porcine in Vitro Retina

PURPOSE

To explore early pressure-related effects on Müller cell homeostatic proteins in the in vitro adult porcine retina.

METHODS

Retinal explants were subjected to 0-, 10-, 30-, or 60-mmHg of pressure for 24 or 48 h in culture. Retinal explants fixed immediately after enucleation were used as controls. Müller cell proteins were evaluated by GFAP, GS, CRALBP, and bFGF immunohistochemistry.

RESULTS

GFAP-labeling revealed no differences in fluorescence intensity after 24 or 48 h in any of the pressure groups compared with control retinas. However, a higher intensity was found in the 30- and 60-mmHg groups compared with 0-mmHg counterparts after 24 and 48 h. A higher intensity in GS-labeled sections was found in the 10-and 60-mmHg groups compared with controls and remaining pressure groups after 48 h. Compared with control retinas, CRALBP labeling revealed a higher intensity in the 60-mmHg group after 24 h and in the 10-, 30-, and 60-mmHg groups after 48 h. After 24 and 48 h, a lower intensity was found in bFGF-labeled cells in the 0-, 10-, and 30-mmHg groups compared with controls, while no difference was seen for the 60-mmHg group.

CONCLUSIONS

Müller cells in the cultured porcine adult retina respond early to pressure by altering the expression of GFAP as well as the homeostatic proteins GS, CRALBP, and bFGF.

Oxidative stress in the brain and retina after traumatic injury

The brain and the retina share many physiological similarities, which allows the retina to serve as a model of CNS disease and disorder. In instances of trauma, the eye can even indicate damage to the brain via abnormalities observed such as irregularities in pupillary reflexes in suspected traumatic brain injury (TBI) patients. Elevation of reactive oxygen species (ROS) has been observed in neurodegenerative disorders and in both traumatic optic neuropathy (TON) and in TBI. In a healthy system, ROS play a pivotal role in cellular communication, but in neurodegenerative diseases and post-trauma instances, ROS elevation can exacerbate neurodegeneration in both the brain and the retina. Increased ROS can overwhelm the inherent antioxidant systems which are regulated via mitochondrial processes. The overabundance of ROS can lead to protein, DNA, and other forms of cellular damage which ultimately result in apoptosis. Even though elevated ROS have been observed to be a major cause in the neurodegeneration observed after TON and TBI, many antioxidants therapeutic strategies fail. In order to understand why these therapeutic approaches fail further research into the direct injury cascades must be conducted. Additional therapeutic approaches such as therapeutics capable of anti-inflammatory properties and suppression of other neurodegenerative processes may be needed for the treatment of TON, TBI, and neurodegenerative diseases.

Fermented maize slurry (Ogi) and its supernatant (Omidun) mitigate elevated intraocular pressure by modulating BDNF expression and glial plasticity in the retina-gut axis of glaucomatous rats

OBJECTIVES

Growing interest has been reported on the health benefits of fermented foods, which includes cognition enhancement and inflammation attenuation. BDNF is a known protectant against retinal degeneration, however, therapies that target this neurotrophic factor has been limited. Therefore, we assessed the reaction of BDNF and glial cells in glaucomatous rats and their response to treatment with fermented maize products.

METHODS

Thirty male adult rats were either injected via the episcleral vein with hypertonic saline to elevate intraocular pressure (IOP) or treated with fermented maize slurry (Ogi) or its supernatant (Omidun). Following sacrifice, the retina and duodenum were studied by immunohistochemical analysis using antibodies directed against GFAP, AIF-1 and BDNF.

RESULTS

Hypertonic saline injection produced hypertrophy of the Müller cells and increased GFAP and AIF-1 expression in the retina and gut when compared to the control. Treatment with Ogi and Omidun produced varying degrees of reduction of gliosis, protection against hypertonic saline-induced retinal ganglion cell loss, and reduced intraocular pressure. BDNF expression was downregulated following the hypertonic saline assault, while Omidun and Ogi treatment abrogated its reduction following the hypertonic saline assault.

CONCLUSIONS

Collectively, our findings suggest that acute elevation of IOP alters crosstalk between gut and retina with consequent aberrant activation of glial cells; and that probiotic bacteria like the lactic acid bacteria rich in fermented foods including Ogi and Omidun may offer neuroprotection to the ganglionic cells by attenuating the retinal glial reaction and improving BDNF activity.

Mettl14-mediated m6A modification is essential for visual function and retinal photoreceptor survival

Background

As the most abundant epigenetic modification of eukaryotic mRNA, N6-methyladenosine (m6A) modification has been shown to play a role in mammalian nervous system development and function by regulating mRNA synthesis and degeneration. However, the role of m6A modification in retinal photoreceptors remains unknown.

Results

We generated the first retina-specific Mettl14-knockout mouse models using the Rho-Cre and HRGP-Cre lines and investigated the functions of Mettl14 in retinal rod and cone photoreceptors. Our data showed that loss of Mettl14 in rod cells causes a weakened scotopic photoresponse and rod degeneration. Further study revealed the ectopic accumulation of multiple outer segment (OS) proteins in the inner segment (IS). Deficiency of Mettl14 in cone cells led to the mislocalization of cone opsin proteins and the progressive death of cone cells. Moreover, Mettl14 depletion resulted in drastic decreases in METTL3/WTAP levels and reduced m6A methylation levels. Mechanistically, transcriptomic analyses in combination with MeRIP-seq illustrated that m6A depletion via inactivation of Mettl14 resulted in reduced expression levels of multiple phototransduction- and cilium-associated genes, which subsequently led to compromised ciliogenesis and impaired synthesis and transport of OS-residing proteins in rod cells.

Conclusions

Our data demonstrate that Mettl14 plays an important role in regulating phototransduction and ciliogenesis events and is essential for photoreceptor function and survival, highlighting the importance of m6A modification in visual function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01335-x.

[Neuroprotective properties of latanoprost]

Glaucoma is the main cause of irreversible blindness in the world. Latanoprost - an ester prodrug of prostaglandin F_2α (PGF_2α) - was the first prostaglandin analogue used to treat glaucoma. The review shows that latanoprost possesses direct neuroprotective properties such as blocking the entry of calcium ions into neurons and inhibiting the action of caspase-3, inhibiting the activity of cyclooxygenase and activation of polypeptide 2B1 (OATP2B1) and Klotho protein. It is emphasized that when the drug is instilled into the eye, the concentration of the drug inside the vitreous body is twice as high as what is required to ensure the survival of retinal ganglion cells.

Neuroprotective Effect of Statins in a Rat Model of Chronic Ocular Hypertension

Glaucoma is an optic neuropathy in which the degeneration of retinal ganglion cells (RGCs) results in irreversible vison loss. Therefore, neuroprotection of RGCs from glaucomatous afflictions is crucial for glaucoma treatment. In this study, we aimed to investigate the beneficial effects of statins in the protection of RGCs using a rat model. Glaucomatous injury was induced in rats by chronic ocular hypertension (OHT) achieved after performing a circumlimbal suture. The rats were given either statins such as simvastatin and atorvastatin or a solvent weekly for 6 weeks. Retina sections underwent hematoxylin and eosin, Brn3a, or cleaved casepase-3 staining to evaluate RGC survival. In addition, modulation of glial activation was assessed. While the retinas without statin treatment exhibited increased RGC death due to chronic OHT, statins promoted the survival of RGCs and reduced apoptosis. Statins also suppressed chronic OHT-mediated glial activation in the retina. Our results demonstrate that statins exert neuroprotective effects in rat retinas exposed to chronic OHT, which may support the prospect of statins being a glaucoma treatment.

Tmem30a deficiency leads to retinal rod bipolar cell degeneration

Phospholipids are asymmetrically distributed across the mammalian plasma membrane, with phosphatidylserine (PS) and phosphatidylethanolamine concentrated in the cytoplasmic leaflet of the membrane bilayer and phosphatidylcholine in the exoplasmic leaflet. This asymmetric distribution is dependent on a group of P4 ATPases called PS flippases. The proper transport and function of PS flippases require a β-subunit transmembrane protein 30A (TMEM30A). Disruption of PS flippases leads to several human diseases. Tmem30a is essential for photoreceptor survival. However, the roles of Tmem30a in the retinal rod bipolar cells (RBC) remain elusive. To investigate the role of Tmem30a in the RBCs, we generated a RBC-specific Tmem30a knockout (cKO) mouse model using PCP2-Cre line. The Tmem30a cKO mice exhibited defect in RBC function and progressive RBC death. PKCα staining of retinal cryosections from cKO mice revealed a remarkable dendritic sprouting of rod bipolar cells during the early degenerative process. Immunostaining analysis of PSD95 and mGluT6 expression demonstrated that rod bipolar cells in Tmem30a cKO retinas exhibited aberrant dendritic sprouting as a result of impaired synaptic efficacy, which implied a crucial role for Tmem30a in synaptic transmission in the retina. In addition, loss of Tmem30a led to reactive gliosis with increased expression of glial fibrillary acidic protein and CD68. TUNEL staining suggested that apoptotic cell death occurred in the retinal inner nuclear layer (INL). Our data show that loss of Tmem30a in RBCs results in dendritic sprouting of rod bipolar cells, increased astrogliosis and RBC death. Taken together, our studies demonstrate an essential role for Tmem30a in the retinal bipolar cells. Cover Image for this issue: doi: 10.1111/jnc.14492.

All roads lead to glaucoma: Induced retinal injury cascades contribute to a common neurodegenerative outcome

Glaucoma describes a distinct optic neuropathy with complex etiology and a variety of associated risk factors, but with similar pathological endpoints. Risk factors such as age, increased intraocular pressure (IOP), low mean arterial pressure, and autoimmune disease, can all be associated with death of retinal ganglion cells (RGCs) and optic nerve head remodeling. Today, IOP management remains the standard of care, even though IOP elevation is not pathognomonic of glaucoma, and patients can continue to lose vision despite effective IOP control. A contemporary view of glaucoma as a complex, neurodegenerative disease has developed, along with the recognition of a need for new disease modifying retinal treatment strategies and improved outcomes. However, the distinction between risk factors triggering the disease process and retinal injury responses is not always clear. In this review, we attempt to distinguish between the various triggers, and their association with subsequent key RGC injury mechanisms. We propose that distinct glaucomatous risk factors result in similar retinal and optic nerve injury cascades, including oxidative and metabolic stress, glial reactivity, and altered inflammatory responses, which induce common molecular signals to induce RGC apoptosis. This organization forms a coherent disease framework and presents conserved targets for therapeutic intervention that are not limited to specific risk factors.

Novel Therapeutics in Glaucoma Management

BACKGROUND

Glaucoma is a progressive optic neuropathy characterized by retinal ganglion cell death and alterations of visual field. Elevated intraocular pressure (IOP) is considered the main risk factor of glaucoma, even though other factors cannot be ruled out, such as epigenetic mechanisms.

OBJECTIVE

An overview of the ultimate promising experimental drugs to manage glaucoma has been provided.

RESULTS

In particular, we have focused on purinergic ligands, KATP channel activators, gases (nitric oxide, carbon monoxide and hydrogen sulfide), non-glucocorticoid steroidal compounds, neurotrophic factors, PI3K/Akt activators, citicoline, histone deacetylase inhibitors, cannabinoids, dopamine and serotonin receptors ligands, small interference RNA, and Rho kinase inhibitors.

CONCLUSIONS

The review has been also endowed of a brief chapter on last reports about potential neuroprotective benefits of anti-glaucoma drugs already present in the market.

Optic neuropathy and increased retinal glial fibrillary acidic protein due to microbead-induced ocular hypertension in the rabbit

AIM

To characterize whether a glaucoma model with chronic elevation of the intraocular pressure (IOP) was able to be induced by anterior chamber injection of microbeads in rabbits.

METHODS

In order to screen the optimal dose of microbead injection, IOP was measured every 3d for 4wk using handheld applanation tonometer after a single intracameral injection of 10 µL, 25 µL, 50 µL or 100 µL microbeads (5×10⁶ beads/mL; n=6/group) in New Zealand White rabbits. To prolong IOP elevation, two intracameral injections of 50 µL microbeads or phosphate buffer saline (PBS) were made respectively at days 0 and 21 (n=24/group). The fellow eye was not treated. At 5wk after the second injection of microbeads or PBS, bright-field microscopy and transmission electron microscopy (TEM) were used to assess the changes in the retina. The expression of glial fibrillary acidic protein (GFAP) in the retina was evaluated by immunofluorescence, quantitative real-time polymerase chain reaction and Western blot at 5wk after the second injection of microbeads.

RESULTS

Following a single intracameral injection of 10 µL, 25 µL, 50 µL or 100 µL microbead, IOP levels showed a gradual increase and a later decrease over a 4wk period after a single injection of microbead into the anterior chamber of rabbits. A peak IOP was observed at day 15 after injection. No significant difference in peak value of IOP was found between 10 µL and 25 µL groups (17.13±1.25 mm Hg vs 17.63±0.74 mm Hg; P=0.346). The peak value of IOP from 50 µL group (23.25±1.16 mm Hg) was significantly higher than 10 µL and 25 µL groups (all P<0.05). Administration of 100 µL microbead solution (23.00±0.93 mm Hg) did not lead to a significant increase in IOP compared to the 50 µL group (P=0.64). A prolonged elevated IOP duration up to 8wk was achieved by administering two injections of 50 µL microbeads (20.48±1.21 mm Hg vs 13.60±0.90 mm Hg in PBS-injected group; P<0.05). The bright-field and TEM were used to assess the changes of retinal ganglion cells (RGCs). Compared with PBS-injected group, the extended IOP elevation was associated with the degeneration of optic nerve, the reduction of RGC axons (47.16%, P<0.05) and the increased GFAP expression in the retina (4.74±1.10 vs 1.00±0.46, P<0.05).

CONCLUSION

Two injections of microbeads into the ocular anterior chamber of rabbits lead to a prolonged IOP elevation which results in structural abnormality as well as loss in RGCs and their axons without observable ocular structural damage or inflammatory response. We have therefore established a novel and practical model of experimental glaucoma in rabbits.

Amacrine cells coupled to ganglion cells via gap junctions are highly vulnerable in glaucomatous mouse retinas

We determined whether the structural and functional integrity of amacrine cells (ACs), the largest cohort of neurons in the mammalian retina, are affected in glaucoma. Intraocular injection of microbeads was made in mouse eyes to elevate intraocular pressure as a model of experimental glaucoma. Specific immunocytochemical markers were used to identify AC and displaced (d)ACs subpopulations in both the inner nuclear and ganglion cell layers, respectively, and to distinguish them from retinal ganglion cells (RGCs). Calretinin- and γ-aminobutyric acid (GABA)-immunoreactive (IR) cells were highly vulnerable to glaucomatous damage, whereas choline acetyltransferase (ChAT)-positive and glycinergic AC subtypes were unaffected. The AC loss began 4 weeks after initial microbead injection, corresponding to the time course of RGC loss. Recordings of electroretinogram (ERG) oscillatory potentials and scotopic threshold responses, which reflect AC and RGC activity, were significantly attenuated in glaucomatous eyes following a time course that matched that of the AC and RGC loss. Moreover, we found that it was the ACs coupled to RGCs via gap junctions that were lost in glaucoma, whereas uncoupled ACs were largely unaffected. Our results suggest that AC loss in glaucoma occurs secondary to RGC death through the gap junction-mediated bystander effect. J. Comp. Neurol. 527:159-173, 2019. © 2016 Wiley Periodicals, Inc.

Brimonidine Can Prevent In Vitro Hydroquinone Damage on Retinal Pigment Epithelium Cells and Retinal Müller Cells

PURPOSE

Brimonidine is a selective alpha-2 adrenergic agonist used to reduce intraocular pressure and it has been shown to have some neuroprotective effects. Hydroquinone (HQ) is a toxicant present in cigarette smoke, and other sources. In this study, we investigated the cyto-protective effects in vitro of Brimonidine on human retinal pigment epithelium cells (ARPE-19) and human retinal Müller cells (MIO-M1) that had been treated with HQ.

METHODS

Cells were pretreated for 6 h with different doses of Brimonidine tartrate 0.1% (1/2×, 1×, 5×, 10×), followed by a 24-h exposure to 100 μM of HQ, while the Brimonidine was still present. Assays were used to measure cell viability, mitochondrial membrane potential (ΔΨm), reactive oxygen species (ROS) production, and lactate dehydrogenase (LDH) release.

RESULTS

Brimonidine increased the cell viability at all concentrations studied in both cell lines studied. ΔΨm also improved at all Brimonidine doses in ARPE-19 cells and in the 5× and 10× dosages MIO-M1 cells. The ROS levels decreased at 1×, 5×, and 10× doses of Brimonidine in ARPE-19 but only at 10× on MIO-M1 cells. The 10×-Brimonidine ARPE-19 cells had decreased LDH release, but no LDH changes were observed on MIO-M1 cells.

CONCLUSION

HQ-induced toxicity is mediated through mitochondrial damaging, oxidative stress-related and necrosis-related pathways; Brimonidine significantly prevented the mitochondrial damaging and oxidative stress-related effects but had little effect on blocking the necrosis component of HQ-toxicity. Brimonidine protective effects differ between the different retinal cell types and high concentrations of Brimonidine (10×) have minimal damaging effects on human retinal cells.

α2-Adrenergic modulation of the glutamate receptor and transporter function in a chronic ocular hypertension model

Excitotoxicity, glutamate-induced toxic effects to retinal ganglion cells (RGCs), is one of several mechanisms of RGC loss suggested in glaucoma. In this study, we focused on the role of glutamate transporter of glial cells as well as N-methyl-d-aspartate (NMDA) receptor with regard to glutamate toxicity in glaucoma. We also investigated whether α2-adrenoceptor activation could modulate glutamate transporters and NMDA receptors in a chronic ocular hypertension model. Brimonidine 0.15% was administered topically to the eyes of experimental glaucoma and control animals twice daily. After 8 weeks of intraocular pressure (IOP) elevation, staining with terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) revealed an increase in the ganglion cell layer, and the number of TUNEL-positive cells was reduced by brimonidine treatment (P<0.05). Animals with experimentally induced glaucoma exhibited an increase in retinal stress marker glial fibrillary acidic protein (GFAP) immunoreactivity; brimonidine treatment reduced GFAP. Excitatory amino acid transporter 1(EAAT1) expression remained stable throughout the period of chronic ocular hypertension. α2-Adrenergic treatment upregulated EAAT1 protein levels (P<0.05). NMDA receptor (GluN1) expression was stimulated by chronic elevation of IOP, and GluN1-positive cells in ganglion cell layer were co-localized with TUNEL staining. Brimonidine administration suppressed GluN1 levels (P<0.05). These results indicate that brimonidine decreased RGC apoptosis, upregulating EAAT1 and downregulating NMDA receptors. We suggest that topical brimonidine treatment may decrease the glutamate excitotoxicity through modulation of glutamate transporter and NMDA receptor in glaucoma.

Natural compounds and retinal ganglion cell neuroprotection

Glaucoma, the second leading cause of blindness in the world, is a chronic optic neuropathy often associated with increased intraocular pressure and characterized by progressive retinal ganglion cell (RGC) axons degeneration and death leading to typical optic nerve head damage and distinctive visual field defects. Although the pathogenesis of glaucoma is still largely unknown, it is hypothesized that RCGs become damaged through various insults/mechanisms, including ischemia, oxidative stress, excitotoxicity, defective axonal transport, trophic factor withdrawal, and neuroinflammation. In this review, we summarize the potential benefits of several natural compounds for RGCs neuroprotection.

The interactions of genes, age, and environment in glaucoma pathogenesis

Glaucoma, a progressive degenerative condition that results in the death of retinal ganglion cells, is one of the leading causes of blindness, affecting millions worldwide. The mechanisms underlying glaucoma are not well understood, although years of studies have shown that the largest risk factors are elevated intraocular pressure, age, and genetics. Eleven genes and multiple loci have been identified as contributing factors. These genes act by a number of mechanisms, including mechanical stress, ischemic/oxidative stress, and neurodegeneration. We summarize the recent advances in the understanding of glaucoma and propose a unified hypothesis for glaucoma pathogenesis. Glaucoma does not result from a single pathological mechanism, but rather a combination of pathways that are influenced by genes, age, and environment. In particular, we hypothesize that, in the presence of genetic risk factors, exposure to environment stresses results in an earlier age of onset for glaucoma. This hypothesis is based upon the overlap of the molecular pathways in which glaucoma genes are involved. Because of the interactions between these processes, it is likely that there are common therapies that may be effective for different subtypes of glaucoma.

Pathophysiology of blast-induced ocular trauma in rats after repeated exposure to low-level blast overpressure

BACKGROUND

The incidence of blast-induced ocular injury has dramatically increased due to advances in weaponry and military tactics. A single exposure to blast overpressure (BOP) has been shown to cause damage to the eye in animal models; however, on the battlefield, military personnel are exposed to BOP multiple times. The effects of repeated exposures to BOP on ocular tissues have not been investigated. The purpose of this study is to characterize the effects of single or repeated exposure on ocular tissues.

METHODS

A compressed air shock tube was used to deliver 70 ± 7 KPa BOP to rats, once (single blast overpressure [SBOP]) or once daily for 5 days (repeated blast overpressure [RBOP]). Immunohistochemistry was performed to characterize the pathophysiology of ocular injuries induced by SBOP and RBOP. Apoptosis was determined by quantification activated caspase 3. Gliosis was examined by detection of glial fibrillary acidic protein (GFAP). Inflammation was examined by detection of CD68.

RESULTS

Activated caspase 3 was detected in ocular tissues from all animals subjected to BOP, while those exposed to RBOP had more activated caspase 3 in the optic nerve than those exposed to SBOP. GFAP was detected in the retinas from all animals subjected to BOP. CD68 was detected in optic nerves from all animals exposed to BOP.

CONCLUSION

SBOP and RBOP induced retinal damage. RBOP caused more apoptosis in the optic nerve than SBOP, suggesting that RBOP causes more severe optic neuropathy than SBOP. SBOP and RBOP caused gliosis in the retina and increased inflammation in the optic nerve.

Brimonidine prevents neurodegeneration in a mouse model of normal tension glaucoma

Glaucoma is one of the leading causes of irreversible blindness that is characterized by progressive degeneration of optic nerves and retinal ganglion cells (RGCs). In the mammalian retina, excitatory amino-acid carrier 1 (EAAC1) is expressed in neural cells, including RGCs, and the loss of EAAC1 leads to RGC degeneration without elevated intraocular pressure (IOP). Brimonidine (BMD) is an α2-adrenergic receptor agonist and it is commonly used in a form of eye drops to lower IOP in glaucoma patients. Recent studies have suggested that BMD has direct protective effects on RGCs involving IOP-independent mechanisms, but it is still controversial. In the present study, we examined the effects of BMD in EAAC1-deficient (KO) mice, an animal model of normal tension glaucoma. BMD caused a small decrease in IOP, but sequential in vivo retinal imaging and electrophysiological analysis revealed that treatment with BMD was highly effective for RGC protection in EAAC1 KO mice. BMD suppressed the phosphorylation of the N-methyl-D-aspartate receptor 2B (NR2B) subunit in RGCs in EAAC1 KO mice. Furthermore, in cultured Müller glia, BMD stimulated the production of several neurotrophic factors that enhance RGC survival. These results suggest that, in addition to lowering IOP, BMD prevents glaucomatous retinal degeneration by stimulating multiple pathways including glia–neuron interactions.

Inflammatory demyelination induces glia alterations and ganglion cell loss in the retina of an experimental autoimmune encephalomyelitis model

Background

Multiple sclerosis (MS) is often accompanied by optic nerve inflammation. And some patients experience permanent vision loss. We examined if the grade of optic nerve infiltration and demyelination affects the severity of clinical signs in an experimental autoimmune encephalomyelitis (EAE) model. The loss of retinal ganglion cells (RGC) and alterations in glia activity were also investigated.

Methods

C57BL/6 mice were immunized with peptide MOG_35-55 in complete Freund’s adjuvant (CFA) and controls received PBS in CFA. Then 23 days post immunization eyes were prepared for flatmounts and stained with Nissl to evaluated neuronal density. Clinical EAE symptoms as well as cell infiltration and demyelination in the optic nerve were examined. Retinal sections were stained with hematoxylin and eosin and silver stain. Immunohistochemistry was used to label RGCs (Brn-3a), apoptotic cells (caspase 3), macroglia (glial fibrillary acidic protein (GFAP)), microglia (Iba1), macrophages (F 4/80) and interleukin-6 (IL-6) secretion.

Results

EAE symptoms started at day 8 and peaked at day 15. Cell infiltrations (P = 0.0047) and demyelination (P = 0.0018) of EAE nerves correlated with the clinical score (r > 0.8). EAE led to a significant loss of RGCs (P< 0.0001). Significantly more caspase 3⁺ cells were noted in these animals (P = 0.0222). They showed an increased expression of GFAP (P< 0.0002) and a higher number of microglial cells (P< 0.0001). Also more macrophages and IL-6 secretion were observed in EAE mice.

Conclusions

MOG immunization leads to optic neuritis and RGC loss. EAE severity is related to the severity of optic nerve inflammation and demyelination. EAE not only affects activation of apoptotic signals, but also causes a glial response in the retina.

Apoptotic retinal ganglion cell death in an autoimmune glaucoma model is accompanied by antibody depositions

Glaucoma is characterized by death of retinal ganglion cells (RGC), but its cause is still unknown. We used an autoimmune glaucoma model to study (1) apoptosis, (2) antibody occurrence, and (3) gliosis by immunohistochemistry. Rats were immunized with optic nerve homogenate (ONA). At 8 days no significant apoptosis or difference in RGCs was noted, but ONA retinas had a significantly higher GFAP(+) area (p = 0.02). At 14 days, significantly more TUNEL(+) (p = 0.0002) and caspase 3(+) (p = 0.004) were detected in ONA animals, but no difference in RGC density. Distinct IgM and IgG deposits (p = 0.04) were observed in ONA retinas. At 22 days, a significantly higher number of TUNEL(+) cells (p = 0.0002), caspase 3(+) cells (p = 0.0007), and concurrent a lower RGC density (p = 0.04) was noted in ONA animals. IgM and IgG deposits were observed in the ganglion cell layer of ONA retinas. The largest percentage of GFAP(+) area in the ONA group was observed at 22 days (p = 0.02). This data suggest that immunization with ocular antigens leads to apoptotic retinal ganglion cell death. Based on the co-localization of antibody deposits and apoptotic cells, we conclude that antibodies are engaged in eliciting RGC apoptosis in this animal model.

Hypoxic-ischemic retinal injury in rat pups

BACKGROUND

Visual loss associated with brain damage, especially hypoxic-ischemic (HI) encephalopathy, is the most common cause of visual impairment in children in developed countries. We hypothesized that HI insults can cause long-term damage in immature eyes.

METHODS

In postnatal day 7 rat pups, HI was induced by unilateral common carotid artery ligation followed by hypoxia. Retina damage was assessed by electroretinography (ERG) and cell counting. Neuronal injury and astrogliosis were evaluated by terminal deoxynucleotidyl transferase nick-end labeling, cleaved caspase 3, ED1, and glial fibrillary acidic protein immunostaining.

RESULTS

We observed rapid and persistently extensive injuries in the ganglia cell layer (GCL), inner plexiform layer, and inner nuclear layer (INL) in ipsilateral retinas after HI injury, corresponding to the marked alteration in ERG. HI insult caused prominent microglial and Műller cell activation in ipsilateral inner retinas. Neuronal death in the GCL and INL after HI injury was mainly apoptotic, involving caspase-dependent pathways.

CONCLUSION

Our study demonstrated the first evidence of HI retinal damage at both the pathological and functional level using the Vannucci model in neonatal rats. Because retinal damage is often associated with HI injury, it is important to demonstrate that a particular neuroprotective strategy effectively preserves the retina in addition to the brain.

Retinal ganglion cell loss is accompanied by antibody depositions and increased levels of microglia after immunization with retinal antigens

Background

Antibodies against retinal and optic nerve antigens are detectable in glaucoma patients. Recent studies using a model of experimental autoimmune glaucoma demonstrated that immunization with certain ocular antigens causes an immun-mediated retinal ganglion cell loss in rats.

Methodology/Principal Findings

Rats immunized with a retinal ganglion cell layer homogenate (RGA) had a reduced retinal ganglion cell density on retinal flatmounts (p = 0.007) and a lower number of Brn3⁺retinal ganglion cells (p = 0.0001) after six weeks. The autoreactive antibody development against retina and optic nerve was examined throughout the study. The levels of autoreactive antibodies continuously increased up to 6 weeks (retina: p = 0.004; optic nerve: p = 0.000003). Additionally, antibody deposits were detected in the retina (p = 0.02). After 6 weeks a reactive gliosis (GFAP density: RGA: 174.7±41.9; CO: 137.6±36.8, p = 0.0006; %GFAP⁺ area: RGA: 8.5±3.4; CO: 5.9±3.6, p = 0.006) as well as elevated level of Iba1⁺ microglia cells (p = 0.003) was observed in retinas of RGA animals.

Conclusions/Significance

Our findings suggest that these antibodies play a substantial role in mechanisms leading to retinal ganglion cell death. This seems to lead to glia cell activation as well as the invasion of microglia, which might be associated with debris clearance.

Protecting the retinal neurons from glaucoma: lowering ocular pressure is not enough

The retina is theater of a number of biochemical reactions allowing, within its layers, the conversion of light impulses into electrical signals. The axons of the last neuronal elements, the ganglion cells, form the optic nerve and transfer the signals to the brain. Therefore, an appropriate cellular communication, not only within the different retinal cells, but also between the retina itself and the other brain structures, is fundamental. One of the most diffuse pathologies affecting retinal function and communication, which thus reverberates in the whole visual system, is glaucoma. This insidious disease is characterized by a progressive optic nerve degeneration and sight loss which may finally lead to irreversible blindness. Nevertheless, the progressive nature of this pathology offers an opportunity for therapeutic intervention. To better understand the cellular processes implicated in the development of glaucoma useful to envision a targeted pharmacological strategy, this manuscript first examines the complex cellular and functional organization of the retina and subsequently identifies the targets sensitive to neurodegeneration. Within this context, high ocular pressure represents a key risk factor. However, recent literature findings highlight the concept that lowering ocular pressure is not enough to prevent/slow down glaucomatous damage, suggesting the importance of combining the hypotensive treatment with other pharmacological approaches, such as the use of neuroprotectants. Therefore, this important and more novel aspect is extensively considered in this review, also emphasizing the idea that the neuroprotective strategy should be extended to the entire visual system and not restricted to the retina.

In vitro effect of adenosine on the mRNA expression of Kir 2.1 and Kir 4.1 channels in rat retinal Müller cells at elevated hydrostatic pressure

The aim of this study was to investigate the expression of Kir 2.1 and Kir 4.1 channels at an elevated hydrostatic pressure in vitro, and to determine whether adenosine may modulate the mRNA expression of Kir 2.1 and Kir 4.1 channels in retinal Müller cells at an elevated hydrostatic pressure in vitro. Müller cells treated with 1 μM adenosine at 40 mmHg/24 h, and mRNA expression of Kir 2.1 and Kir 4.1 channels were examined using real-time PCR. Müller cells significantly increased the mRNA expression of Kir 2.1 and Kir 4.1 channels at 40 mmHg/24 h. When further treated with 1 μM adenosine at 40 mmHg/24 h, the mRNA expression of the Kir 2.1 channels decreased, while the mRNA expression of the Kir 4.1 channels continued to increase. When the pressure was elevated, Müller cells were still able to take up K(+) and mediate the potassium concentration of the retina. Adenosine upregulated the expression of the Kir 4.1 channels, but weakly affected the expression of the Kir 2.1 channels.

Endothelin antagonism as an active principle for glaucoma therapy

Endothelin, the most potent vasoactive peptide known to date, has been suggested to play a potential role in the pathogenesis of open-angle glaucoma. Open-angle glaucoma is the most common optic nerve head neuropathy and is associated with a loss of retinal ganglion cells and visual field damage. Although an increased intraocular pressure is a major risk factor for glaucomatous optic neuropathy, other factors such as a reduced ocular blood flow play an important role for appearance of the disease. Thus, treatment of glaucoma is focused on lowering of intraocular pressure and preventing the occurrence or progression of glaucomatous optic neuropathy. Endothelin participates in the regulation of intraocular pressure by an effect on trabecular outflow, the main route for aqueous humour outflow from the eye. Trabecular outflow is modulated by trabecular meshwork contractility which is affected by endothelin. In addition to the effects of endothelin in the anterior part of the eye, the vasoconstrictor causes a decrease in ocular blood flow followed by pathological changes in the retina and the optic nerve head which is assumed to contribute to the degeneration of retinal ganglion cells. In sum, inhibition of endothelin signalling leads to lowering of intraocular pressure and exerts neuroprotective effects. Thus, endothelin antagonism in the eye represents a promising approach for pharmacological treatment of glaucoma.

Update on the role of alpha-agonists in glaucoma management

Glaucoma is the second most common cause of world blindness (following cataract) with estimated cases reaching 79.6 million by 2020. Although the etiology of glaucoma is multi-factorial, intraocular pressure (IOP) is the only modifiable factor in glaucoma management proven to alter the natural course of the disease. Among various classes of IOP-lowering medications currently available, alpha-adrenergic receptor agonists are used either as monotherapy, as second-line therapy, or in fixed combination with beta-blockers. Non-selective adrenergic agonists such as epinephrine and dipivefrin are infrequently used today for the treatment of glaucoma or ocular hypertension, and have been replaced by the alpha-2-selective agonists. The use of apraclonidine for IOP reduction in glaucoma or OHT is limited due to a high rate of follicular conjunctivitis. The alpha-2-selective agonist in use today is brimonidine. The brimonidine-purite formulations are preferred to brimonidine-benzalkonium chloride (BAC) formulations due better tolerability while maintaining similar efficacy. Brimonidine is also effective when used in combination with a beta-blocker. Using brimonidine-timolol fixed combination (BTFC) as first-line therapy has an added potential for neuroprotection. This would be a valuable strategy for glaucoma treatment, for patients who are intolerant of prostaglandin analogs, or for patients where prostaglandin analogues are contraindicated as first-line therapy, such as in patients with inflammatory glaucoma.

Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons

BACKGROUND

Brimonidine is a common drug for lowering ocular pressure and may directly protect retinal ganglion cells in glaucoma. The disease involves early loss of retinal ganglion cell transport to brain targets followed by axonal and somatic degeneration. We examined whether brimonidine preserves ganglion cell axonal transport and abates degeneration in rats with elevated ocular pressure induced by laser cauterization of the episcleral veins.

RESULTS

Ocular pressure was elevated unilaterally by 90% for a period of 8 weeks post- cauterization. During this time, brimonidine (1mg/kg/day) or vehicle (phosphate-buffered saline) was delivered systemically and continuously via subcutaneous pump. Animals received bilateral intravitreal injections of fluorescent cholera toxin subunit β (CTB) two days before sacrifice to assess anterograde transport. In retinas from the vehicle group, elevated pressure induced a 44% decrease in the fraction of ganglion cells with intact uptake of CTB and a 14-42% reduction in the number of immuno-labelled ganglion cell bodies, with the worst loss occurring nasally. Elevated pressure also caused a 33% loss of ganglion cell axons in vehicle optic nerves and a 70% decrease in CTB transport to the superior colliculus. Each of these components of ganglion cell degeneration was either prevented or significantly reduced in the brimonidine treatment group.

CONCLUSIONS

Continuous and systemic treatment with brimonidine by subcutaneous injection significantly improved retinal ganglion cell survival with exposure to elevated ocular pressure. This effect was most striking in the nasal region of the retina. Brimonidine treatment also preserved ganglion cell axon morphology, sampling density and total number in the optic nerve with elevated pressure. Consistent with improved outcome in the optic projection, brimonidine also significantly reduced the deficits in axonal transport to the superior colliculus associated with elevated ocular pressure. As transport deficits to and from retinal ganglion cell projection targets in the brain are relevant to the progression of glaucoma, the ability of brimonidine to preserve optic nerve axons and active transport suggests its neuroprotective effects are relevant not only at the cell body, but throughout the entire optic projection.

Tyrosinase is the modifier of retinoschisis in mice

X-linked retinoschisis (XLRS) is a form of macular degeneration with a juvenile onset. This disease is caused by mutations in the retinoschisin (RS1) gene. The major clinical pathologies of this disease include splitting of the retina (schisis) and a loss in synaptic transmission. Human XLRS patients display a broad range in phenotypic severity, even among family members with the same mutation. This variation suggests the existence of genetic modifiers that may contribute to disease severity. Previously, we reported the identification of a modifier locus, named Mor1, which affects severity of schisis in a mouse model of XLRS (the Rs1tmgc1 mouse). Homozygosity for the protective AKR allele of Mor1 restores cell adhesion in Rs1tmgc1 mice. Here, we report our study to identify the Mor1 gene. Through collecting recombinant mice followed by progeny testing, we have localized Mor1 to a 4.4-Mb region on chromosome 7. In this genetic region, the AKR strain is known to carry a mutation in the tyrosinase (Tyr) gene. We observed that the schisis phenotype caused by the Rs1 mutation is rescued by a Tyr mutation in the C57BL/6J genetic background, strongly suggesting that Tyr is the Mor1 gene.