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

A Reduction in Mitophagy Is Associated with Glaucomatous Neurodegeneration in Rodent Models of Glaucoma

Glaucoma is a heterogenous group of optic neuropathies characterized by the degeneration of optic nerve axons and the progressive loss of retinal ganglion cells (RGCs), which could ultimately lead to vision loss. Elevated intraocular pressure (IOP) is a major risk factor in the development of glaucoma, and reducing IOP remains the main therapeutic strategy. Endothelin-1 (ET-1), a potent vasoactive peptide, has been shown to produce neurodegenerative effects in animal models of glaucoma. However, the detailed mechanisms underlying ET-1-mediated neurodegeneration in glaucoma are not completely understood. In the current study, using a Seahorse Mitostress assay, we report that ET-1 treatment for 4 h and 24 h time points causes a significant decline in various parameters of mitochondrial function, including ATP production, maximal respiration, and spare respiratory capacity in cultured RGCs. This compromise in mitochondrial function could trigger activation of mitophagy as a quality control mechanism to restore RGC health. Contrary to our expectation, we observed a decrease in mitophagy following ET-1 treatment for 24 h in cultured RGCs. Using Morrison's model of ocular hypertension in rats, we investigated here, for the first time, changes in mitophagosome formation by analyzing the co-localization of LC-3B and TOM20 in RGCs. We also injected ET-1 (24 h) into transgenic GFP-LC3 mice to analyze the formation of mitophagosomes in vivo. In Morrison's model of ocular hypertension, as well as in ET-1 injected GFP-LC3 mice, we found a decrease in co-localization of LC3 and TOM20, indicating reduced mitophagy. Taken together, these results demonstrate that both ocular hypertension and ET-1 administration in rats and mice lead to reduced mitophagy, thus predisposing RGCs to neurodegeneration.

Longitudinal evaluation of morphological, functional and vascular alterations in a rat model of experimental glaucoma

Glaucoma, the leading cause of irreversible blindness worldwide, is a neurodegenerative disease characterized by chronic axonal damages and progressive loss of retinal ganglion cells, with increased intraocular pressure (IOP) as the primary risk factor. While current treatments focus solely on reducing IOP, understanding glaucoma through experimental models is essential for developing new therapeutic strategies and biomarkers for early diagnosis. Our research group developed an ocular hypertension rat model based on limbal plexus cautery, which provides significant glaucomatous neurodegeneration up to four weeks after injury. We evaluated long-term morphological, functional, and vascular alterations in this model. Our results showed that transient ocular hypertension, lasting approximately one week, can lead to progressive increase in optic nerve cupping and retinal ganglion cells loss. Remarkably, the pressure insult caused several vascular changes, such as arteriolar and venular thinning, and permanent choroidal vascular swelling. This study provides evidence of the longitudinal effects of a pressure insult on retinal structure and function using clinical modalities and techniques. The multifactorial changes reported in this model resemble the complex retinal ganglion cell degeneration found in glaucoma patients, and therefore may also provide a unique tool for the development of novel interventions to either halt or slow down disease progression.

Comparative proteomic analysis of retinal hypoxia-ischemia in an acute ocular hypertension model using tandem mass tag-based quantitative proteomics

The main symptom of acute glaucoma is acute ocular hypertension (AOH), which leads to the death of retinal ganglion cells (RGCs) and permanent loss of vision. However, effective treatments for these conditions are lacking. This study aimed to identify major regulators and overall protein changes involved in AOH-induced RGC death. Proteomic patterns of the retinal protein extracts from the AOH and sham groups were analyzed using mass spectrometry (MS), followed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. Proteomic analysis revealed 92 proteins in the AOH group compared to the control group; 58 proteins were upregulated and 34 were downregulated. Alterations in fatty acid-binding protein 7 (FABP7) and caveolin-1 (Cav-1), which are related to fatty acid metabolism and ocular inflammatory signaling, were detected using western blotting and biochemical assays. Variations in the expression of galectin-1 (Gal-1), S100 calcium-binding protein A6 (S100a6), and visinin-like protein-1 (VILIP) have been associated with neuronal ischemia. Our investigation demonstrates that neuroinflammation and fatty acid metabolism are involved in retinal impairment following AOH, suggesting a possible treatment approach for acute glaucoma.

From Gut to Eye: Exploring the Role of Microbiome Imbalance in Ocular Diseases

Background: The gut microbiome plays a crucial role in human health, and recent research has highlighted its potential impact on ocular health through the gut-eye axis. Dysbiosis, or an imbalance in the gut microbiota, has been implicated in various ocular diseases. Methods: A comprehensive literature search was conducted using relevant keywords in major electronic databases, prioritizing recent peer-reviewed articles published in English. Results: The gut microbiota influences ocular health through immune modulation, maintenance of the blood-retinal barrier, and production of beneficial metabolites. Dysbiosis can disrupt these mechanisms, contributing to ocular inflammation, tissue damage, and disease progression in conditions such as uveitis, age-related macular degeneration, diabetic retinopathy, dry eye disease, and glaucoma. Therapeutic modulation of the gut microbiome through probiotics, prebiotics, synbiotics, and fecal microbiota transplantation shows promise in preclinical and preliminary human studies. Conclusions: The gut-eye axis represents a dynamic and complex interplay between the gut microbiome and ocular health. Targeting the gut microbiome through innovative therapeutic strategies holds potential for improving the prevention and management of various ocular diseases.

The Prodrug DHED Delivers 17β-Estradiol into the Retina for Protection of Retinal Ganglion Cells and Preservation of Visual Function in an Animal Model of Glaucoma

We report a three-pronged phenotypic evaluation of the bioprecursor prodrug 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED) that selectively produces 17β-estradiol (E2) in the retina after topical administration and halts glaucomatous neurodegeneration in a male rat model of the disease. Ocular hypertension (OHT) was induced by hyperosmotic saline injection into an episcleral vein of the eye. Animals received daily DHED eye drops for 12 weeks. Deterioration of visual acuity and contrast sensitivity by OHT in these animals were markedly prevented by the DHED-derived E2 with concomitant preservation of retinal ganglion cells and their axons. In addition, we utilized targeted retina proteomics and a previously established panel of proteins as preclinical biomarkers in the context of OHT-induced neurodegeneration as a characteristic process of the disease. The prodrug treatment provided retina-targeted remediation against the glaucomatous dysregulations of these surrogate endpoints without increasing circulating E2 levels. Collectively, the demonstrated significant neuroprotective effect by the DHED-derived E2 in the selected animal model of glaucoma supports the translational potential of our presented ocular neuroprotective approach owing to its inherent therapeutic safety and efficacy.

Regulated Necrosis in Glaucoma: Focus on Ferroptosis and Pyroptosis

Glaucoma is one of the most common causes of irreversible blindness worldwide. This neurodegenerative disease is characterized by progressive and irreversible damage to retinal ganglion cells (RGCs) and optic nerves, which can lead to permanent loss of peripheral and central vision. To date, maintaining long-term survival of RGCs using traditional treatments, such as medication and surgery, remains challenging, as these do not promote optic nerve regeneration. Therefore, it is of great clinical and social significance to investigate the mechanisms of optic nerve degeneration in depth and find reliable targets to provide pioneering methods for the prevention and treatment of glaucoma. Regulated necrosis is a form of genetically programmed cell death associated with the maintenance of homeostasis and disease progression in vivo. An increasing body of innovative evidence has recognized that aberrant activation of regulated necrosis pathways is a common feature in neurodegenerative diseases, such as Alzheimer's, Parkinson's, and glaucoma, resulting in unwanted loss of neuronal cells and function. Among them, ferroptosis and pyroptosis are newly discovered forms of regulated cell death actively involved in the pathophysiological processes of RGCs loss and optic nerve injury. This was shown by a series of in vivo and in vitro studies, and these mechanisms have been emerging as a key new area of scientific research in ophthalmic diseases. In this review, we focus on the molecular mechanisms of ferroptosis and pyroptosis and their regulatory roles in the pathogenesis of glaucoma, with the aim of exploring their implications as potential therapeutic targets and providing new perspectives for better clinical decision-making in glaucoma treatment.

The Role of Complement Dysregulation in Glaucoma

Glaucoma is a progressive neurodegenerative disease characterized by damage to the optic nerve that results in irreversible vision loss. While the exact pathology of glaucoma is not well understood, emerging evidence suggests that dysregulation of the complement system, a key component of innate immunity, plays a crucial role. In glaucoma, dysregulation of the complement cascade and impaired regulation of complement factors contribute to chronic inflammation and neurodegeneration. Complement components such as C1Q, C3, and the membrane attack complex have been implicated in glaucomatous neuroinflammation and retinal ganglion cell death. This review will provide a summary of human and experimental studies that document the dysregulation of the complement system observed in glaucoma patients and animal models of glaucoma driving chronic inflammation and neurodegeneration. Understanding how complement-mediated damage contributes to glaucoma will provide opportunities for new therapies.

Sustained Vision Recovery by OSK Gene Therapy in a Mouse Model of Glaucoma

Glaucoma, a chronic neurodegenerative disease, is a leading cause of age-related blindness worldwide and characterized by the progressive loss of retinal ganglion cells (RGCs) and their axons. Previously, we developed a novel epigenetic rejuvenation therapy, based on the expression of the three transcription factors Oct4, Sox2, and Klf4 (OSK), which safely rejuvenates RGCs without altering cell identity in glaucomatous and old mice after 1 month of treatment. In the current year-long study, mice with continuous or cyclic OSK expression induced after glaucoma-induced vision damage had occurred were tracked for efficacy, duration, and safety. Surprisingly, only 2 months of OSK fully restored impaired vision, with a restoration of vision for 11 months with prolonged expression. In RGCs, transcription from the doxycycline (DOX)-inducible Tet-On AAV system, returned to baseline 4 weeks after DOX withdrawal. Significant vision improvements remained for 1 month post switching off OSK, after which the vision benefit gradually diminished but remained better than baseline. Notably, no adverse effects on retinal structure or body weight were observed in glaucomatous mice with OSK continuously expressed for 21 months providing compelling evidence of efficacy and safety. This work highlights the tremendous therapeutic potential of rejuvenating gene therapies using OSK, not only for glaucoma but also for other ocular and systemic injuries and age-related diseases.

Targeting the Gut-Eye Axis: An Emerging Strategy to Face Ocular Diseases

The human microbiota refers to a large variety of microorganisms (bacteria, viruses, and fungi) that live in different human body sites, including the gut, oral cavity, skin, and eyes. In particular, the presence of an ocular surface microbiota with a crucial role in maintaining ocular surface homeostasis by preventing colonization from pathogen species has been recently demonstrated. Moreover, recent studies underline a potential association between gut microbiota (GM) and ocular health. In this respect, some evidence supports the existence of a gut-eye axis involved in the pathogenesis of several ocular diseases, including age-related macular degeneration, uveitis, diabetic retinopathy, dry eye, and glaucoma. Therefore, understanding the link between the GM and these ocular disorders might be useful for the development of new therapeutic approaches, such as probiotics, prebiotics, symbiotics, or faecal microbiota transplantation through which the GM could be modulated, thus allowing better management of these diseases.

Pathologically high intraocular pressure disturbs normal iron homeostasis and leads to retinal ganglion cell ferroptosis in glaucoma

Glaucoma can result in retinal ganglion cell (RGC) death and permanently damaged vision. Pathologically high intraocular pressure (ph-IOP) is the leading cause of damaged vision during glaucoma; however, controlling ph-IOP alone does not entirely prevent the loss of glaucomatous RGCs, and the underlying mechanism remains elusive. In this study, we reported an increase in ferric iron in patients with acute primary angle-closure glaucoma (the most typical glaucoma with ph-IOP damage) compared with the average population by analyzing free iron levels in peripheral serum. Thus, iron metabolism might be involved in regulating the injury of RGCs under ph-IOP. In vitro and in vivo studies confirmed that ph-IOP led to abnormal accumulation of ferrous iron in cells and retinas at 1-8 h post-injury and elevation of ferric iron in serum at 8 h post-injury. Nuclear receptor coactivator 4 (NCOA4)-mediated degradation of ferritin heavy polypeptide 1(FTH1) is essential to disrupt iron metabolism in the retina after ph-IOP injury. Furthermore, knockdown of Ncoa4 in vivo inhibited FTH1 degradation and reduced the retinal ferrous iron level. Elevated ferrous iron induced by ph-IOP led to a marked accumulation of pro-ferroptotic factors (lipid peroxidation and acyl CoA synthetase long-chain family member 4) and a depletion of anti-ferroptotic factors (glutathione, glutathione peroxidase 4, and nicotinamide adenine dinucleotide phosphate). These biochemical changes resulted in RGC ferroptosis. Deferiprone can pass through the blood-retinal barrier after oral administration and chelated abnormally elevated ferrous iron in the retina after ph-IOP injury, thus inhibiting RGC ferroptosis and protecting visual function. In conclusion, this study revealed the role of NCOA4-FTH1-mediated disturbance of iron metabolism and ferroptosis in RGCs during glaucoma. We demonstrate the protective effect of Deferiprone on RGCs via inhibition of ferroptosis, providing a research direction to understand and treat glaucoma via the iron homeostasis and ferroptosis pathways.

Mimicking chronic glaucoma over 6 months with a single intracameral injection of dexamethasone/fibronectin-loaded PLGA microspheres

To create a chronic glaucoma animal model by a single intracameral injection of biodegradable poly lactic-co-glycolic acid (PLGA) microspheres (Ms) co-loaded with dexamethasone and fibronectin (MsDexaFibro). MsDexaFibro were prepared by a water-in-oil-in-water emulsion method including dexamethasone in the organic phase and fibronectin in the inner aqueous phase. To create the chronic glaucoma model, an interventionist and longitudinal animal study was performed using forty-five Long Evans rats (4-week-old). Rats received a single intracameral injection of MsDexafibro suspension (10%w/v) in the right eye. Ophthalmological parameters such as clinical signs, intraocular pressure (IOP), neuro-retinal functionality by electroretinography (ERG), retinal structural analysis by optical coherence tomography (OCT), and histology were evaluated up to six months. According to the results obtained, the model proposed was able to induce IOP increasing in both eyes over the study, higher in the injected eyes up to 6 weeks (p < 0.05), while preserving the ocular surface. OCT quantified progressive neuro-retinal degeneration (mainly in the retinal nerve fiber layer) in both eyes but higher in the injected eye. Ganglion cell functionality decreased in injected eyes, thus smaller amplitudes in PhNR were detected by ERG. In conclusion, a new chronic glaucoma animal model was created by a single injection of MsDexaFibro very similar to open-angle glaucoma occurring in humans. This model would impact in different fields such as ophthalmology, allowing long period of study of this pathology; pharmacology, evaluating the neuroprotective activity of active compounds; and pharmaceutical technology, allowing the correct evaluation of the efficacy of long-term sustained ocular drug delivery systems.

The Effect of Oral Citicoline and Docosahexaenoic Acid on the Visual Field of Patients with Glaucoma: A Randomized Trial

The role of nutraceuticals in the treatment of glaucoma remains controversial. The aim of this study was to evaluate the effect of citicoline, vitamin C, and docosahexaenoic acid (DHA) in patients with glaucoma. Methods: This was a prospective, randomized study. Patients with glaucoma were randomized to one of four groups and treated for 3 months with vitamin C, DHA, citicoline, or a combination of DHA and citicoline. We conducted a complete ophthalmic examination and visual fields each month and calculated the slopes of field indices. Changes in visual field indices (VFIs) and their slopes were assessed in each group and compared. Results: Seventy-three persons were included in the study. Mean defect (MD) significantly improved (p = 0.001) from −9.52 ± 4.36 to −7.85 ± 4.36 dB during the study period in persons taking DHA + citicoline. Similarly, the mean VFI significantly improved (p = 0.001) in this group. The only treatment group showing a statistically significant improvement (p = 0.006) in the MD (from −0.1041 ± 0.2471 to 0.1383 ± 0.2544 dB/month) and VFI slope was the group treated with DHA+citicoline. Conclusions: The combination of oral treatment with DHA + citicoline significantly improved VF indices and their slopes in patients with glaucoma after 3 months of treatment.

Age-related visual impairments and retinal ganglion cells axonal degeneration in a mouse model harboring OPTN (E50K) mutation

Retinal ganglion cells (RGCs) axons are the signal carriers of visual information between retina and brain. Therefore, they play one of the important roles affected in many optic neurodegenerative diseases like glaucoma. Among the genetic risks associated with glaucoma, the E50K mutation in the Optineurin (OPTN) gene are known to result in glaucoma in the absence of increased intraocular pressure (IOP), whereas the relevant pathological mechanism and neurological issues remain to be further investigated. In this study, the OPTN (E50K) mutant mouse model was established through CRISPR/Cas9-mediated genome editing, and aging-related RGCs loss and the visual dysfunction were identified. In E50K mice 16 months old, the axonal transport decreased comparing to wild-type (WT) mice at the same age. Furthermore, results of electron microscopy demonstrated significant morphological anomaly of mitochondria in RGCs axons of young E50K mice 3 months old, and these changes were aggravated with age. These indicated that the damaged mitochondria-associated dysfunction of RGCs axon should play an etiological role in glaucoma as an age-related outcome of OPTN (E50K) mutation. The findings of this study have potential implications for the targeted prevention and treatment of NTG.

Proteomic Analysis of Retinal Tissue in an S100B Autoimmune Glaucoma Model

Glaucoma is a neurodegenerative disease that leads to damage of retinal ganglion cells and the optic nerve. Patients display altered antibody profiles and increased antibody titer, e.g., against S100B. To identify the meaning of these antibodies, animals were immunized with S100B. Retinal ganglion cell loss, optic nerve degeneration, and increased glial cell activity were noted. Here, we aimed to gain more insights into the pathophysiology from a proteomic point of view. Hence, rats were immunized with S100B, while controls received sodium chloride. After 7 and 14 days, retinae were analyzed through mass spectrometry and immunohistology. Using data-independent acquisition-based mass spectrometry, we identified more than 1700 proteins on a high confidence level for both study groups, respectively. Of these 1700, 43 proteins were significantly altered in retinae after 7 days and 67 proteins revealed significant alterations at 14 days. For example, α2-macroglobulin was found significantly increased not only by mass spectrometry analysis, but also with immunohistological staining in S100B retinae at 7 and 14 days. All in all, the identified proteins are often associated with the immune system, such as heat shock protein 60. Once more, these data underline the important role of immunological factors in glaucoma pathogenesis.

Cell-Based Neuroprotection of Retinal Ganglion Cells in Animal Models of Optic Neuropathies

Retinal ganglion cells (RGCs) comprise a heterogenous group of projection neurons that transmit visual information from the retina to the brain. Progressive degeneration of these cells, as it occurs in inflammatory, ischemic, traumatic or glaucomatous optic neuropathies, results in visual deterioration and is among the leading causes of irreversible blindness. Treatment options for these diseases are limited. Neuroprotective approaches aim to slow down and eventually halt the loss of ganglion cells in these disorders. In this review, we have summarized preclinical studies that have evaluated the efficacy of cell-based neuroprotective treatment strategies to rescue retinal ganglion cells from cell death. Intraocular transplantations of diverse genetically nonmodified cell types or cells engineered to overexpress neurotrophic factors have been demonstrated to result in significant attenuation of ganglion cell loss in animal models of different optic neuropathies. Cell-based combinatorial neuroprotective approaches represent a potential strategy to further increase the survival rates of retinal ganglion cells. However, data about the long-term impact of the different cell-based treatment strategies on retinal ganglion cell survival and detailed analyses of potential adverse effects of a sustained intraocular delivery of neurotrophic factors on retina structure and function are limited, making it difficult to assess their therapeutic potential.

Non-drug interventions in glaucoma: Putative roles for lifestyle, diet and nutritional supplements

Glaucoma is a major ocular neurodegenerative disease characterized by progressive retinal ganglion cells degeneration and sight loss. Current treatment options have been limited to reducing intraocular pressure (IOP), known as the leading risk factor for this disease; however, glaucoma can develop even with low or normal IOP and progress despite controlling IOP values. Lifestyle, dietary habits, and supplementation may influence some of the risk factors and pathophysiological mechanisms underlying glaucoma development and progression; thus, the role of this complementary and alternative medicine in glaucoma has received great interest from both patients and ophthalmologists. We provide a summary of the current evidence concerning the relationship between lifestyle, dietary habits, and effects of supplements on the incidence and progression of glaucoma and their targets and associated mechanisms. The data suggest the existence of a therapeutic potential that needs to be further explored with both preclinical and rigorous clinical studies.

Impact of dorzolamide, benzalkonium-preserved dorzolamide and benzalkonium-preserved brinzolamide on selected biomarkers of oxidative stress in the tear film

Background

Long-term use of topical, especially benzalkonium chloride (BAC)-preserved, antiglaucoma medications can cause a negative impact on the ocular surface. The aim of the study was to assess the effect of topical carbonic anhydrase inhibitors (CAIs) on selected oxidative stress biomarkers in the tear film.

Methods

The patients were divided into four sex-matched groups: group C (n = 25) – control group – subjects who did not use topical antiglaucoma medications, group DL (n = 14) – patients using preservative-free dorzolamide, group DL + BAC (n = 16) – patients using topical BAC-preserved dorzolamide, group BL + BAC (n = 17) – patients using BAC-preserved brinzolamide. Subjects in all the study groups have been using the eye drops two times daily for 6–12 months. The oxidative stress biomarkers in the tear film samples were measured: total protein (TP) concentration, advanced oxidation protein products (AOPP) content, total sulfhydryl (-SH) groups content, the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), as well as Total Oxidant Status (TOS), Total Antioxidant Response (TAR), and Oxidative Stress Index (OSI).

Results

The advanced oxidation protein products content, Total Oxidant Status as well as superoxide dismutase and catalase activities in the group DL + BAC and BL + BAC were higher in comparison with the group C. The total sulfhydryl groups content was lower in the group DL + BAC and BL + BAC when compared to group C. Oxidative Stress Index was higher in the groups DL + BAC and BL + BAC in comparison with the groups DL and C.

Conclusions

Use of topical benzalkonium chloride-preserved carbonic anhydrase inhibitors increases oxidative stress in the tear film.

Protective effects of human umbilical cord mesenchymal stem cells on retinal ganglion cells in mice with acute ocular hypertension

AIM

To observe the protective effect of human umbilical cord mesenchymal stem cells (hucMSCs) on retinal ganglion cells (RGCs) injury in mice with acute ocular hypertension (AOH).

METHODS

Fifty-six adult male C57BL/6 mice were randomly divided into four groups: normal group, AOH group, hucMSCs group, normal saline (NS) group. Left eye of mice was induced by 90 mm Hg intraocular pressure for 1h to establish AOH model. hucMSCs 1×10⁵/µL, 1 µL or NS 1 µL was injected into the vitreous body the next day. CM-Dil fluorescent dye was used to label the 3^rd generation of hucMSCs, for tracing the cells in the vitreous cavity of mice. Seven days after the model established, hematoxylin-eosin (HE) staining was used to observe the thickness of the inner retina layer in four groups. Numbers and loss rate of RGCs were evaluated by counting Brn-3a positive cells stained by immunofluorescencein.

RESULTS

On the 7^th day after AOH established, labeled hucMSCs were found in the vitreous cavity. HE staining showed that the thickness of retinal inner layer in AOH group was significantly lower than that in normal group and hucMSCs group (P<0.05), same as that in NS group (P>0.05). Compared with AOH group, the RGCs in normal group was significantly higher; RGCs number increased in hucMSCs group and the loss rate was lower (P<0.05). Injection of NS had no protective effect on RGCs.

CONCLUSION

In AOH mouse model, vitreous injection of hucMSCs have shown a protection for RGCs.

Microglia Activation in Retinal Ischemia Triggers Cytokine and Toll-Like Receptor Response

Mechanisms and progression of ischemic injuries in the retina are still incompletely clarified. Therefore, the time course of microglia activation as well as resulting cytokine expression and downstream signaling were investigated. Ischemia was induced in one eye by transiently elevated intraocular pressure (60 min) followed by reperfusion; the other eye served as a control. Eyes were processed for RT-qPCR and immunohistochemistry analyses at 2, 6, 12, and 24 h as well as at 3 and 7 days. Already 2 h after ischemia, more microglia/macrophages were in an active state in the ischemia group. This was accompanied by an upregulation of pro-inflammatory cytokines, like IL-1β, IL-6, TNFα, and TGFβ. Activation of TLR3, TLR2, and the adaptor molecule Myd88 was also observed after 2 h. NFκB revealed a wave-like activation pattern. In addition, an extrinsic caspase pathway activation was noted at early time points, while enhanced numbers of cleaved caspase 3⁺ cells could be observed in ischemic retinae throughout the study. Retinal ischemia induced an early and strong microglia/macrophage response as well as cytokine and apoptotic activation processes. Moreover, in early and late ischemic damaging processes, TLR expression and downstream signaling were involved, suggesting an involvement in neuronal death in ischemic retinae. Graphical Abstract.

Rho-kinase (ROCK) Inhibitors - A Neuroprotective Therapeutic Paradigm with a Focus on Ocular Utility

BACKGROUND

Glaucoma is a progressive optic neuropathy causing visual impairment and Retinal Ganglionic Cells (RGCs) death gradually posing a need for neuroprotective strategies to minimize the loss of RGCs and visual field. It is recognized as a multifactorial disease, Intraocular Pressure (IOP) being the foremost risk factor. ROCK inhibitors have been probed for various possible indications, such as myocardial ischemia, hypertension, kidney diseases. Their role in neuroprotection and neuronal regeneration has been suggested to be of value in the treatment of neurological diseases, like spinal-cord injury, Alzheimer's disease and multiple sclerosis but recently Rho-associated Kinase inhibitors have been recognized as potential antiglaucoma agents.

EVIDENCE SYNTHESIS

Rho-Kinase is a serine/threonine kinase with a kinase domain which is constitutively active and is involved in the regulation of smooth muscle contraction and stress fibre formation. Two isoforms of Rho-Kinase, ROCK-I (ROCK β) and ROCK-II (ROCK α) have been identified. ROCK II plays a pathophysiological role in glaucoma and hence the inhibitors of ROCK may be beneficial to ameliorate the vision loss. These inhibitors decrease the intraocular pressure in the glaucomatous eye by increasing the aqueous humour outflow through the trabecular meshwork pathway. They also act as anti-scarring agents and hence prevent post-operative scarring after the glaucoma filtration surgery. Their major role involves axon regeneration by increasing the optic nerve blood flow which may be useful in treating the damaged optic neurons. These drugs act directly on the neurons in the central visual pathway, interrupting the RGC apoptosis and therefore serve as a novel pharmacological approach for glaucoma neuroprotection.

CONCLUSION

Based on the results of high-throughput screening, several Rho kinase inhibitors have been designed and developed comprising of diverse scaffolds exhibiting Rho kinase inhibitory activity from micromolar to subnanomolar ranges. This diversity in the scaffolds with inhibitory potential against the kinase and their SAR development will be intricated in the present review. Ripasudil is the only Rho kinase inhibitor marketed to date for the treatment of glaucoma. Another ROCK inhibitor AR-13324 has recently passed the clinical trials whereas AMA0076, K115, PG324, Y39983 and RKI-983 are still under trials. In view of this, a detailed and updated account of ROCK II inhibitors as the next generation therapeutic agents for glaucoma will be discussed in this review.

A subacute model of glaucoma based on limbal plexus cautery in pigmented rats

Glaucoma is a neurodegenerative disorder characterized by the progressive functional impairment and degeneration of the retinal ganglion cells (RGCs) and their axons, and is the leading cause of irreversible blindness worldwide. Current management of glaucoma is based on reduction of high intraocular pressure (IOP), one of its most consistent risk factors, but the disease proceeds in almost half of the patients despite such treatments. Several experimental models of glaucoma have been developed in rodents, most of which present shortcomings such as high surgical invasiveness, slow learning curves, damage to the transparency of the optic media which prevents adequate functional assessment, and variable results. Here we describe a novel and simple method to induce ocular hypertension in pigmented rats, based on low-temperature cauterization of the whole circumference of the limbal vascular plexus, a major component of aqueous humor drainage and easily accessible for surgical procedures. This simple, low-cost and efficient method produced a reproducible subacute ocular hypertension with full clinical recovery, followed by a steady loss of retinal ganglion cells and optic axons, accompanied by functional changes detected both by electrophysiological and behavioral methods.

Brimonidine enhances inhibitory postsynaptic activity of OFF- and ON-type retinal ganglion cells in a Wistar rat chronic glaucoma model

Glaucoma is a multifactorial disease in which retinal ganglion cells (RGCs) undergo excitotoxic damage, leading to their degeneration. The α2-adrenoceptor (α2-AR) agonist brimonidine exerts a neuroprotective effect by regulating postsynaptic excitatory N-methyl-D-aspartate (NMDA) receptor activity in RGCs. However, researchers have not clearly determined whether or how brimonidine regulates inhibitory synaptic transmission in rat models of chronic glaucoma. Whole-cell voltage-clamp and current-clamp recordings were performed in ON- and OFF-type RGCs in retinal slices. Brimonidine directly and acutely enhanced γ-aminobutyric acidergic (GABAergic) transmission mediated by ionotropic GABA_A receptors in ON- and OFF-type RGCs in rat retinal slices; this effect occurred at the synaptic terminals and was independent of action potentials and multi-synaptic connections. The highly selective α2-AR antagonist yohimbine blocked the effects of brimonidine. Regarding the postsynaptic GABA receptor sensitivity, brimonidine also increased the amplitude of the GABA-induced current. Additionally, compared to RGCs from the control group, the frequencies and amplitudes of spontaneous excitatory postsynaptic currents (sEPSCs) and miniature excitatory postsynaptic currents (mEPSCs) did not change after brimonidine gravity perfusion. Brimonidine significantly decreased the spontaneous firing frequency of rat RGCs with intact synaptic inputs and decreased the resting membrane potential of RGCs, changes that were blocked by the highly selective GABA_A receptor antagonist SR95531. SR95531 alone increased spontaneous action potentials and the resting membrane potential. Based on these findings, an α2-AR agonist facilitated the frequency of the GABAergic inhibitory postsynaptic currents (IPSCs), directly increased the amplitude of the postsynaptic GABA-induced current (GABA receptor reactivity/sensitivity), suppressed the firing frequency of spontaneous action in RGCs with intact synaptic inputs and decreased the resting membrane potential of RGCs, thus deactivating RGCs from the neural network level and reducing the excitotoxic damage occurring during the pathological process of chronic glaucoma.

Immune Mediated Degeneration and Possible Protection in Glaucoma

The underlying pathomechanisms for glaucoma, one of the most common causes of blindness worldwide, are still not identified. In addition to increased intraocular pressure (IOP), oxidative stress, excitotoxicity, and immunological processes seem to play a role. Several pharmacological or molecular/genetic methods are currently investigated as treatment options for this disease. Altered autoantibody levels were detected in serum, aqueous humor, and tissue sections of glaucoma patients. To further analyze the role of the immune system, an IOP-independent, experimental autoimmune glaucoma (EAG) animal model was developed. In this model, immunization with ocular antigens leads to antibody depositions, misdirected T-cells, retinal ganglion cell death and degeneration of the optic nerve, similar to glaucomatous degeneration in patients. Moreover, an activation of the complement system and microglia alterations were identified in the EAG as well as in ocular hypertension models. The inhibition of these factors can alleviate degeneration in glaucoma models with and without high IOP. Currently, several neuroprotective approaches are tested in distinct models. It is necessary to have systems that cover underlying pathomechanisms, but also allow for the screening of new drugs. In vitro models are commonly used, including single cell lines, mixed-cultures, and even organoids. In ex vivo organ cultures, pathomechanisms as well as therapeutics can be investigated in the whole retina. Furthermore, animal models reveal insights in the in vivo situation. With all these models, several possible new drugs and therapy strategies were tested in the last years. For example, hypothermia treatment, neurotrophic factors or the blockage of excitotoxity. However, further studies are required to reveal the pressure independent pathomechanisms behind glaucoma. There is still an open issue whether immune mechanisms directly or indirectly trigger cell death pathways. Hence, it might be an imbalance between protective and destructive immune mechanisms. Moreover, identified therapy options have to be evaluated in more detail, since deeper insights could lead to better treatment options for glaucoma patients.

Inhibition of retinal ganglion cell apoptosis: regulation of mitochondrial function by PACAP

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an endogenous peptide with neuroprotective effects on retinal neurons, but the precise mechanism underlying these effects remains unknown. Considering the abundance of mitochondria in retinal ganglion cells (RGCs), we postulate that the protective effect of PACAP is associated with the regulation of mitochondrial function. RGC-5 cells were subjected to serum deprivation for 48 hours to induce apoptosis in the presence or absence of 100 nM PACAP. As revealed with the Cell Counting Kit-8 assay, PACAP at different concentrations significantly increased the viability of RGC-5 cells. PACAP also inhibited the excessive generation of reactive oxygen species in RGC-5 cells subjected to serum deprivation. We also showed by flow cytometry that PACAP inhibited serum deprivation-induced apoptosis in RGC-5 cells. The proportions of apoptotic cells and cells with mitochondria depolarization were significantly decreased with PACAP treatment. Western blot assays demonstrated that PACAP increased the levels of Bcl-2 and inhibited the compensatory increase of PAC1. Together, these data indicate protective effects of PACAP against serum deprivation-induced apoptosis in RGCs, and that the mechanism of this action is associated with maintaining mitochondrial function.

Primary Open-Angle Glaucoma Genetics in African Americans

PURPOSE OF REVIEW

Individuals of African descent are at highest risk for developing primary open-angle glaucoma (POAG), a devastating disease and major contributor of blindness worldwide. Currently, there is a large dearth of knowledge in this area despite a critical need for better understanding the underlying genetic and environmental factors afflicting this population. Here we highlight the current literature exploring the genetics of POAG in African Americans.

RECENT FINDINGS

Current studies have yet to replicate European POAG index variants (i.e. CDKN2B-AS1 and SIX1/SIX6) in African Americans or to definitely exclude that these loci contribute to risk in African descent populations. Recent studies have evaluated clinical features that may account for some differences in POAG risk between African Americans and European Americans.

SUMMARY

In summary, little headway has been made in elucidating the genetics of primary open-angle glaucoma in African Americans and other individuals of African descent.

Neurodegeneration beyond the primary visual pathways in a population with a high incidence of normal-pressure glaucoma

PURPOSE

Glaucoma is the most common age-related neurodegenerative eye disease in western society. It is an insidious disease that, when untreated or detected too late, leads inevitably to blindness. An outstanding issue is whether glaucoma should be considered exclusively an eye disease or also a brain disease. To further examine it, we used Diffusion Tensor Imaging (DTI) to study white matter integrity in a Japanese glaucoma population. This population has a very high incidence of normal-pressure glaucoma, in which optic nerve damage occurs in the absence of the elevated eye pressure that characterises the more common form of glaucoma.

METHODS

We performed DTI in 30 participants with normal-pressure glaucoma and 21 age-matched healthy controls. We used voxel-wise tract-based spatial statistics to compare fractional anisotropy and mean diffusivity of the white matter of the brain between patients and control group. Whole-brain and region of interest-based analyses served to find associations between diffusion indices and clinical measures of glaucomatous damage.

RESULTS

Fractional Anisotropy was significantly lower in glaucoma patients in a cluster in the right occipital lobe (p < 0.05; family-wise error-corrected) comprising fibres of both the optic radiation and the forceps major. Additional analysis confirmed bilateral involvement of the optic radiations and forceps major and additionally revealed damage to the corpus callosum and parietal lobe (p < 0.09; family-wise error-corrected). The region of interest-based analysis revealed a positive association between Fractional Anisotropy of the optic radiation and optic nerve damage.

CONCLUSIONS

In this specific population, glaucoma is associated with lower Fractional Anisotropy in the optic radiations, forceps major and corpus callosum. We interpret these reductions as evidence for white matter degeneration in these loci. In particular, the degeneration of the corpus callosum suggests the presence of neurodegeneration of the brain beyond what can be explained on the basis of propagated retinal and pre-geniculate damage. We discuss how this finding links to the emerging view that a brain component that is independent from the eye damage plays a role in the aetiology of glaucoma.

Effect of benzalkonium chloride on trabecular meshwork cells in a new in vitro 3D trabecular meshwork model for glaucoma

PURPOSE

To validate a new culture model of primary human trabecular meshwork cells (p-hTMCs) using Matrigel®, in order to mimic in vitro 3D-TM organization, and to investigate the proinflammatory effect of benzalkonium chloride (BAK) in 3D p-hTMC cultures.

METHODS

p-hTMCs, seeded onto Matrigel®-coated inserts were stimulated with BAK (10^-4%), dexamethasone (DEX) (10^-6M) or transforming growth factor-beta 2 (TGF-β2) (5ng/ml) for 48h and observed with confocal microscopy. The BAK effect at 10^-4% or 5.10^-3% on the gene expressions of interleukin-6 (IL-6), interleukin-8 (IL-8) and matrix metalloproteinase (MMP-9) was investigated using qRT-PCR in 2D and 3D p-hTMC cultures.

RESULTS

p-hTMCs seeded in Matrigel® were able to organize themselves in a 3D-spatial conformation in the different conditions tested with cross-linked actin network (CLAN) formation in presence of DEX or TGF-β2 and intercellular space contraction with TGF-β2. IL-6 and IL-8 gene expressions increased in presence of BAK in 2D and in 3D p-hTMC cultures. BAK 10^-4% only showed a tendency to stimulate MMP-9 expression in p-hTMCs after 24h-recovery.

CONCLUSIONS

We investigated this new 3D-TM in vitro model in Matrigel^® matrix for pathophysiological and toxicological purposes. It appears as a new promising tool for a better understanding of TM behavior in physiological and stress conditions, as well as toxicological evaluations of antiglaucoma eyedrops and preservatives.

Ischemic optic neuropathy as a model of neurodegenerative disorder: A review of pathogenic mechanism of axonal degeneration and the role of neuroprotection

Optic neuropathy is a neurodegenerative disease which involves optic nerve injury. It is caused by acute or intermittent insults leading to visual dysfunction. There are number of factors, responsible for optic neuropathy, and the optic nerve axon is affected in all type which causes the loss of retinal ganglion cells. In this review we will highlight various mechanisms involved in the cell loss cascades during axonal degeneration as well as ischemic optic neuropathy. These mechanisms include oxidative stress, excitotoxicity, angiogenesis, neuroinflammation and apoptosis following retinal ischemia. We will also discuss the effect of neuroprotective agents in attenuation of the negative effect of factors involve in the disease occurrence and progression.

Elevated urine formaldehyde in elderly patients with primary open angle glaucoma

AIM

To investigate the risk factor of primary open angle glaucoma (POAG), which is the leading cause of irreversible blindness worldwide. An abnormally high level of endogenous formaldehyde (FA) has recently been found correlated with cell death and neurodegenerative disease, raising the possibility of a putative correlation of abnormal endogenous FA with POAG.

METHODS

Thirty-four elderly patients with POAG and sixteen healthy controls were enrolled. Glaucomatous visual defects were present at both the functional (visual field) and structural [retinal nerve fiber layer (RNFL) thickness] levels. Morning urine samples were obtained and were analyzed by high-performance liquid chromatography (HPLC) to detect the endogenous FA level in a double blind manner.

RESULTS

Patients with POAG (P<0.05) had significantly higher urine FA levels. The urine FA level of patients with severe visual field defects [mean deviation (MD)≥12 dB] was significantly (P<0.001) greater than that of patients with mild to moderate defects (MD<12 dB). By optical coherence tomography (OCT), the superior and inferior RNFL thickness of POAG group was significantly (P<0.001) thinner than in controls. Furthermore, the superior and inferior thinning of the RNFL was correlated with the elevation of urine FA concentration.

CONCLUSION

Endogenous FA level is positively correlated with the neuronal defects of POAG.

Structural brain MRI studies in eye diseases: are they clinically relevant? A review of current findings

Many eye diseases reduce visual acuity or are associated with visual field defects. Because of the well-defined retinotopic organization of the connections of the visual pathways, this may affect specific parts of the visual pathways and cortex, as a result of either deprivation or transsynaptic degeneration. For this reason, over the past several years, numerous structural magnetic resonance imaging (MRI) studies have examined the association of eye diseases with pathway and brain changes. Here, we review structural MRI studies performed in human patients with the eye diseases albinism, amblyopia, hereditary retinal dystrophies, age-related macular degeneration (AMD) and glaucoma. We focus on two main questions. First, what have these studies revealed? Second, what is the potential clinical relevance of their findings? We find that all the aforementioned eye diseases are indeed associated with structural changes in the visual pathways and brain. As such changes have been described in very different eye diseases, in our view the most parsimonious explanation is that these are caused by the loss of visual input and the subsequent deprivation of the visual pathways and brain regions, rather than by transsynaptic degeneration. Moreover, and of clinical relevance, for some of the diseases - in particular glaucoma and AMD - present results are compatible with the view that the eye disease is part of a more general neurological or neurodegenerative disorder that also affects the brain. Finally, establishing structural changes of the visual pathways has been relevant in the context of new therapeutic strategies to restore retinal function: it implies that restoring retinal function may not suffice to also effectively restore vision. Future structural MRI studies can contribute to (i) further establish relationships between ocular and neurological neurodegenerative disorders, (ii) investigate whether brain degeneration in eye diseases is reversible, (iii) evaluate the use of neuroprotective medication in ocular disease, (iv) determine optimal timing for retinal implant insertion and (v) establish structural MRI examination as a diagnostic tool in ophthalmology.

Neuroprotective therapies in glaucoma: I. Neurotrophic factor delivery

Glaucoma is a neurodegenerative eye disease that causes permanent blindness at the progressive stage and the number of people affected worldwide is expected to reach over 79 million by 2020. Currently, glaucoma management relies on pharmacological and invasive surgical treatments mainly by reducing the intraocular pressure (IOP), which is the most important risk factor for the progression of the visual field loss. Recent research suggests that neuroprotective or neuroregenerative approaches are necessary to prevent retinal ganglion cells (RGCs) loss and visual impairment over time. Neuroprotection is a new therapeutic strategy that attempts to keep RGCs alive and functional. New gene and cell therapeutics encoding neurotrophic factors (NTFs) are emerging for both neuroprotection and regenerative treatments for retinal diseases. This article briefly reviews the role of NTFs in glaucoma and the potential delivery systems.

Contribution of microglia-mediated neuroinflammation to retinal degenerative diseases

Retinal degenerative diseases are major causes of vision loss and blindness worldwide and are characterized by chronic and progressive neuronal loss. One common feature of retinal degenerative diseases and brain neurodegenerative diseases is chronic neuroinflammation. There is growing evidence that retinal microglia, as in the brain, become activated in the course of retinal degenerative diseases, having a pivotal role in the initiation and propagation of the neurodegenerative process. A better understanding of the events elicited and mediated by retinal microglia will contribute to the clarification of disease etiology and might open new avenues for potential therapeutic interventions. This review aims at giving an overview of the roles of microglia-mediated neuroinflammation in major retinal degenerative diseases like glaucoma, age-related macular degeneration, and diabetic retinopathy.

Cumulative mtDNA damage and mutations contribute to the progressive loss of RGCs in a rat model of glaucoma

Glaucoma is a chronic neurodegenerative disease characterized by the progressive loss of retinal ganglion cells (RGCs). Mitochondrial DNA (mtDNA) alterations have been documented as a key component of many neurodegenerative disorders. However, whether mtDNA alterations contribute to the progressive loss of RGCs and the mechanism whereby this phenomenon could occur are poorly understood. We investigated mtDNA alterations in RGCs using a rat model of chronic intraocular hypertension and explored the mechanisms underlying progressive RGC loss. We demonstrate that the mtDNA damage and mutations triggered by intraocular pressure (IOP) elevation are initiating, crucial events in a cascade leading to progressive RGC loss. Damage to and mutation of mtDNA, mitochondrial dysfunction, reduced levels of mtDNA repair/replication enzymes, and elevated reactive oxygen species form a positive feedback loop that produces irreversible mtDNA damage and mutation and contributes to progressive RGC loss, which occurs even after a return to normal IOP. Furthermore, we demonstrate that mtDNA damage and mutations increase the vulnerability of RGCs to elevated IOP and glutamate levels, which are among the most common glaucoma insults. This study suggests that therapeutic approaches that target mtDNA maintenance and repair and that promote energy production may prevent the progressive death of RGCs.

Differential up-regulation of Vesl-1/Homer 1 protein isoforms associated with decline in visual performance in a preclinical glaucoma model

Glaucoma is a multifactorial progressive ocular pathology, clinically presenting with damage to the retina and optic nerve, ultimately leading to blindness. Retinal ganglion cell loss in glaucoma ultimately results in vision loss. Vesl/Homer proteins are scaffolding proteins that are critical for maintaining synaptic integrity by clustering, organizing and functionally regulating synaptic proteins. Current anti-glaucoma therapies target IOP as the sole modifiable clinical parameters. Long-term pharmacotherapy and surgical treatment do not prevent gradual visual field loss as the disease progresses, highlighting the need for new complementary, alternative and comprehensive treatment approaches. Vesl/Homer expression was measured in the retinae of DBA/2J mice, a preclinical genetic glaucoma model with spontaneous mutations resulting in a phenotype reminiscent of chronic human pigmentary glaucoma. Vesl/Homer proteins were differentially expressed in the aged, glaucomatous DBA/2J retina, both at the transcriptional and translational level. Immunoreactivity for the long Vesl-1L/Homer 1c isoform, but not of the immediate early gene product Vesl-1S/Homer 1a was increased in the synaptic layers of the retina. This increased protein level of Vesl-1L/Homer 1c was correlated with phenotypes of increased disease severity and a decrease in visual performance. The increased expression of Vesl-1L/Homer 1c in the glaucomatous retina likely results in increased intracellular Ca(2+) release through enhancement of synaptic coupling. The ensuing Ca(2+) toxicity may thus activate neurodegenerative pathways and lead to the progressive loss of synaptic function in glaucoma. Our data suggest that higher levels of Vesl-1L/Homer 1c generate a more severe disease phenotype and may represent a viable target for therapy development.

Rho-kinase inhibitors offer a new approach in the treatment of glaucoma

INTRODUCTION

Primary open-angle glaucoma (POAG) is a leading cause for worldwide blindness and is characterized by progressive optic nerve damage. The etiology of POAG is unknown, but elevated intraocular pressure (IOP) and advanced age have been identified as risk factors. IOP reduction is the only known treatment for glaucoma. Recently, drugs that inhibit rho-associated protein kinase (ROCK) have been studied in animals and people for their ability to lower IOP and potentially treat POAG. ROCK inhibitors lower IOP through a trabecular mechanism and may represent a new therapeutic paradigm for the treatment of POAG.

AREAS COVERED

Exploring the place that ROCK inhibitors may occupy in our treatment of POAG requires a thorough understanding of pathophysiology and treatment. This article summarizes current research on the incidence, proposed etiologies and mechanisms of action for this drug class. ROCK inhibitor research is presented and considered in light of the current standard of pharmacologic care.

EXPERT OPINION

ROCK inhibitors alter the cell shape and extracellular matrix (ECM) of the trabecular meshwork. Preclinical studies demonstrate that these drugs have the potential to become a new therapy for glaucoma. However, ROCK inhibitors can affect multiple cell types, and their utility can be proven only after clinical studies in patients.

Western blot patterns of serum autoantibodies against optic nerve antigens in dogs with goniodysgenesis-related glaucoma

OBJECTIVE

To investigate whether differences existed between clinically normal dogs and dogs with goniodysgenesis-related glaucoma (GDRG) in serum autoantibodies against optic nerve antigens.

ANIMALS

16 dogs with GDRG, 17 healthy dogs with unremarkable pectinate ligament and iridocorneal angle morphology, and 13 euthanized dogs with no major ocular abnormalities or underlying diseases.

PROCEDURES

Western blotting was performed with optic nerve extracts from the euthanized dogs as an antigen source and serum from clinically normal dogs and dogs with GDRG as a primary antibody (autoantibody) source. Blots were evaluated for presence and density of bands.

RESULTS

Multiple bands were identified on western blots from all dogs with GDRG and all clinically normal dogs, with a high degree of variability among individual dogs. Dogs with GDRG were significantly more likely than healthy dogs to have bands present at 38, 40, and 68 kDa. Dogs with GDRG had significant increases in autoreactivity at 40 and 53 kDa and a significant decrease in autoreactivity at 48 kDa.

CONCLUSIONS AND CLINICAL RELEVANCE

Significant differences in serum autoantibodies against optic nerve antigens were found in dogs with versus without GDRG. Although it remains unclear whether these differences were part of the pathogenesis of disease or were sequelae to glaucomatous changes, these findings provide support for the hypothesis that immune-mediated mechanisms play a role in the development or progression of GDRG. However, the high degree of variability among individual dogs and the considerable overlap between groups suggest that the clinical usefulness of this technique for distinguishing dogs with GDRG from clinically normal dogs is likely limited.

Sensory stimulation for lowering intraocular pressure, improving blood flow to the optic nerve and neuroprotection in primary open-angle glaucoma

Primary open-angle glaucoma is a group of optic neuropathies that can lead to irreversible blindness. Sensory stimulation in the form of acupuncture or ear acupressure may contribute to protecting patients from blindness when used as a complementary method to orthodox treatment in the form of drops, laser or surgery. The objective of this article is to provide a narrative overview of the available literature up to July 2012. It summarises reported evidence on the potential beneficial effects of sensory stimulation for glaucoma. Sensory stimulation appears to significantly enhance the pressure-lowering effect of orthodox treatments. Studies suggest that it may also improve blood flow to the eye and optic nerve head. Furthermore, it may play a role in neuroprotection through regulating nerve growth factor and brain-derived neurotrophic factor and their receptors, thereby encouraging the survival pathway in contrast to the pathway to apoptosis. Blood flow and neuroprotection are areas that are not directly influenced by orthodox treatment modalities. Numerous different treatment protocols were used to investigate the effect of sensory stimulation on intraocular pressure, blood flow or neuroprotection of the retina and optic nerve in the animal model and human pilot studies. Objective outcomes were reported to have been evaluated with Goldmann tonometry, Doppler ultrasound techniques and electrophysiology (pattern electroretinography, visually evoked potentials), and supported with histological studies in the animal model. Taken together, reported evidence from these studies strongly suggests that sensory stimulation is worthy of further research.

Melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine potentiate adrenergic receptor-mediated ocular hypotensive effects in rabbits: significance for combination therapy in glaucoma

Melatonin is currently considered a promising drug for glaucoma treatment because of its ocular hypotensive and neuroprotective effects. We have investigated the effect of melatonin and its analog 5-methoxycarbonylamino-N-acetyltryptamine, 5-MCA-NAT, on β₂/α(2A)-adrenergic receptor mRNA as well as protein expression in cultured rabbit nonpigmented ciliary epithelial cells. Quantitative polymerase chain reaction and immunocytochemical assays revealed a significant β₂-adrenergic receptor downregulation as well as α(2A)-adrenergic receptor up-regulation of treated cells (P < 0.001, maximal significant effect). In addition, we have studied the effect of these drugs upon the ocular hypotensive action of a nonselective β-adrenergic receptor (timolol) and a selective α₂-adrenergic receptor agonist (brimonidine) in normotensive rabbits. Intraocular pressure (IOP) experiments showed that the administration of timolol in rabbits pretreated with melatonin or 5-MCA-NAT evoked an additional IOP reduction of 14.02% ± 5.8% or 16.75% ± 5.48% (P < 0.01) in comparison with rabbits treated with timolol alone for 24 hours. Concerning brimonidine hypotensive action, an additional IOP reduction of 29.26% ± 5.21% or 39.07% ± 5.81% (P < 0.001) was observed in rabbits pretreated with melatonin or 5-MCA-NAT when compared with animals treated with brimonidine alone for 24 hours. Additionally, a sustained potentiating effect of a single dose of 5-MCA-NAT was seen in rabbits treated with brimonidine once daily for up 4 days (extra IOP decrease of 15.57% ± 5.15%, P < 0.05, compared with brimonidine alone). These data confirm the indirect action of melatoninergic compounds on adrenergic receptors and their remarkable effect upon the ocular hypotensive action mainly of α₂-adrenergic receptor agonists but also of β-adrenergic antagonists.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.