A role for aquaporin-4 in fluid regulation in the inner retina

Retinal glia in myopia: current understanding and future directions

Myopia, a major public health problem, involves axial elongation and thinning of all layers of the eye, including sclera, choroid and retina, which defocuses incoming light and thereby blurs vision. How the various populations of glia in the retina are involved in the disorder is unclear. Astrocytes and Müller cells provide structural support to the retina. Astrogliosis in myopia may influence blood oxygen supply, neuronal function, and axon diameter, which in turn may affect signal conduction. Müller cells act as a sensor of mechanical stretching in myopia and trigger downstream molecular responses. Microglia, for their part, may exhibit a reactive morphology and elevated response to inflammation in myopia. This review assesses current knowledge about how myopia may involve retinal glia, and it explores directions for future research into that question.

Osteopontin Regulates AQP4 Expression by TRPV4 Activation in Müller Cells: Implications for Retinal Homeostasis

During the intense neuronal activity in the retina, Müller cells are exposed to a hypotonic environment and activate a regulatory volume decrease (RVD) response, which depends on Aquaporin-4 (AQP4) and the calcium channel Transient Receptor Potential Vanilloid 4 (TRPV4). It was reported that Osteopontin (OPN), a cytokine and component of the extracellular matrix (ECM), may modulate the RVD of Müller cells. In other cell types, OPN participates in cell survival and migration, which Müller cells undergo to maintain retinal homeostasis. Therefore, the aim of this work was to study the putative crosstalk of OPN with AQP4 and/or TRPV4 in the main functions of Müller cells: RVD, morphology maintenance and migration. We used a human Müller cell line (MIO-M1) exposed to OPN and evaluated cell volume and osmotic permeability (Pf) during an osmotic swelling, AQP4 expression, cell morphology and migration. We observed that OPN induced a reduced Pf and RVD by downregulating AQP4 expression, which was prevented by TRPV4 inhibition. OPN also induced significant changes in cell morphology with an increased number of cytoplasmic projections. Finally, OPN reduced the migration of Müller cells, being this effect dependent on TRPV4. We propose that OPN affects water permeability and cell volume regulation of Müller cells by activating TRPV4 to reduce AQP4 expression. This represents a novel mechanism of regulation of water permeability by the ECM in Müller cells. Additionally, OPN-induced changes in morphology and migration of Müller cells may have an impact on retinal physiology.

Aquaporin 4 beyond a water channel; participation in motor, sensory, cognitive and psychological performances, a comprehensive review

Aquaporin 4 (AQP4) is a protein highly expressed in the central nervous system (CNS) and peripheral nervous system (PNS) as well as various other organs, whose different sites of action indicate its importance in various functions. AQP4 has a variety of essential roles beyond water homeostasis. In this article, we have for the first time summarized different roles of AQP4 in motor and sensory functions, besides cognitive and psychological performances, and most importantly, possible physiological mechanisms by which AQP4 can exert its effects. Furthermore, we demonstrated that AQP4 participates in pathology of different neurological disorders, various effects depending on the disease type. Since neurological diseases involve a spectrum of dysfunctions and due to the difficulty of obtaining a treatment that can simultaneously affect these deficits, it is therefore suggested that future studies consider the role of this protein in different functional impairments related to neurological disorders simultaneously or separately by targeting AQP4 expression and/or polarity modulation.

Müller cell vulnerability in aging human retina: Implications on photoreceptor cell survival

Müller glial cells (MC) support various metabolic functions of the retinal neurons, and maintain the homeostasis. Oxidative stress is intensified with aging, and in human retina, MC and photoreceptors undergo lipid peroxidation and protein nitration. Information on how MC respond to oxidative stress is vital to understand the fate of aging retinal neurons. This study examined age-related changes in MC of donor human retina (age: 35-98 years; N = 18 donors). Ultrastructural and immunohistochemical observations indicate that MC undergo gliosis and increased lipid peroxidation, and show osmotic changes with advanced aging (>80 years). Photoreceptor cells also undergo oxidative-nitrosative stress with aging, and their synapses also show clear osmotic swelling. MC respond to oxidative stress via proliferation of smooth endoplasmic reticulum in their processes, and increased expression of aquaporin-4 in endfeet and outer retina. In advanced aged retinas (81-98 years), they showed mitochondrial disorganisation, accumulation of lipids and autophagosomes, lipofuscin granules and axonal remnants in phagolysosomes in their inner processes, suggesting a reduced phagocytotic potential in them with aging. Glutamine synthetase expression does not alter until advanced aging, when the retinas show its increased expression in endfeet and Henle fiber layer. It is evident that MC are vulnerable with normal aging and this could be a reason for photoreceptor cell abnormalities reported with aging of the human retina.

Aldehyde Dehydrogenase and Aldo-Keto Reductase Enzymes: Basic Concepts and Emerging Roles in Diabetic Retinopathy

Diabetic retinopathy (DR) is a complication of diabetes mellitus that can lead to vision loss and blindness. It is driven by various biochemical processes and molecular mechanisms, including lipid peroxidation and disrupted aldehyde metabolism, which contributes to retinal tissue damage and the progression of the disease. The elimination and processing of aldehydes in the retina rely on the crucial role played by aldehyde dehydrogenase (ALDH) and aldo-keto reductase (AKR) enzymes. This review article investigates the impact of oxidative stress, lipid-derived aldehydes, and advanced lipoxidation end products (ALEs) on the advancement of DR. It also provides an overview of the ALDH and AKR enzymes expressed in the retina, emphasizing their growing importance in DR. Understanding the relationship between aldehyde metabolism and DR could guide innovative therapeutic strategies to protect the retina and preserve vision in diabetic patients. This review, therefore, also explores various approaches, such as gene therapy and pharmacological compounds that have the potential to augment the expression and activity of ALDH and AKR enzymes, underscoring their potential as effective treatment options for DR.

Aquaporins in Eye

The major part of the eye consists of water. Continuous movement of water and ions between the ocular compartments and to the systemic circulation is pivotal for many physiological functions in the eye. The movement of water facilitates removal of the many metabolic products of corneal-, ciliary body-, lens-, and retinal metabolism, while maintaining transparency in the optical compartments. Transport across the corneal epithelium and endothelium maintains the corneal transparency. Also, aqueous humor is continuously secreted by the epithelia of the ciliary body and maintains the intraocular pressure. In the retina, water is transported into the vitreous body and across the retinal pigment epithelium to regulate the extracellular environment and the hydration of the retina. Aquaporins are a major contributor in the water transport throughout the eye.

Functions of retinal astrocytes and Müller cells in mammalian myopia

Background

Changes in the retina and choroid blood vessels are regularly observed in myopia. However, if the retinal glial cells, which directly contact blood vessels, play a role in mammalian myopia is unknown. We aimed to explore the potential role and mechanism of retinal glial cells in form deprived myopia.

Methods

We adapted the mice form-deprivation myopia model by covering the right eye and left the left eye open for control, measured the ocular structure with anterior segment optical coherence tomography, evaluated changes in the morphology and distribution of retinal glial cells by fluorescence staining and western blotting; we also searched the online GEO databases to obtain relative gene lists and confirmed them in the form-deprivation myopia mouse retina at mRNA and protein level.

Results

Compared with the open eye, the ocular axial length (3.54 ± 0.006 mm v.s. 3.48 ± 0.004 mm, p = 0.027) and vitreous chamber depth (3.07 ± 0.005 mm v.s. 2.98 ± 0.006 mm, p = 0.007) in the covered eye became longer. Both glial fibrillary acidic protein and excitatory amino acid transporters 4 elevated. There were 12 common pathways in human myopia and anoxic astrocytes. The key proteins were also highly relevant to atropine target proteins. In mice, two common pathways were found in myopia and anoxic Müller cells. Seven main genes and four key proteins were significantly changed in the mice form-deprivation myopia retinas.

Conclusion

Retinal astrocytes and Müller cells were activated in myopia. They may response to stimuli and secretory acting factors, and might be a valid target for atropine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-022-02643-0.

Ocular Lymphatic and Glymphatic Systems: Implications for Retinal Health and Disease

Clearance of ocular fluid and metabolic waste is a critical function of the eye in health and disease. The eye has distinct fluid outflow pathways in both the anterior and posterior segments. Although the anterior outflow pathway is well characterized, little is known about posterior outflow routes. Recent studies suggest that lymphatic and glymphatic systems play an important role in the clearance of fluid and waste products from the posterior segment of the eye. The lymphatic system is a vascular network that runs parallel to the blood circulatory system. It plays an essential role in maintenance of fluid homeostasis and immune surveillance in the body. Recent studies have reported lymphatics in the cornea (under pathological conditions), ciliary body, choroid, and optic nerve meninges. The evidence of lymphatics in optic nerve meninges is, however, limited. An alternative lymphatic system termed the glymphatic system was recently discovered in the rodent eye and brain. This system is a glial cell-based perivascular network responsible for the clearance of interstitial fluid and metabolic waste. In this review, we will discuss our current knowledge of ocular lymphatic and glymphatic systems and their role in retinal degenerative diseases.

The Correlation between the Increased Expression of Aquaporins on the Inner Limiting Membrane and the Occurrence of Diabetic Macular Edema

Purpose

Diabetic macular edema (DME) is a major cause of vision loss in patients with diabetic retinopathy; this study is aimed at comparing the expression of aquaporins (AQPs) on the inner limiting membranes (ILMs) of various vitreoretinal diseases and investigating the role of aquaporins expressed on the ILMs in mediating the occurrence of DME.

Methods

The whole-mounted ILM specimens surgically excised from patients with various vitreoretinal diseases (idiopathic macular hole, myopic traction maculopathy, and diabetic retinopathy) were analyzed by immunohistochemistry (IHC). The distribution and morphology of AQP4, AQP7, and AQP11 on the ILMs were correlated with immunohistochemical staining characteristics. Moreover, immunofluorescence of AQP4 was performed on the ILM specimens of the patient in four groups: the control group, negative control group, no DME group, and DME group. The immunofluorescence intensity value of AQP4 was measured using ImageJ. The difference between the four groups and the correction between the immunofluorescence value and central foveal thickness (CFT) were analyzed.

Results

In IHC sections, the expression of AQP4, AQP7, and AQP11 on ILMs of diabetic retinopathy (DR) with macular edema, respectively, seemed to be more abundant than in the idiopathic macular hole (iMH) and myopic traction maculopathy (MTM). Moreover, markedly higher fluorescence intensity of AQP4 of ILMs was determined in the DME group (51.05 ± 5.67) versus the other three groups (P < 0.001). A marked positive association was identified between the fluorescence intensity of AQP4 and CFT (r = 0.758; P = 0.011).

Conclusions

AQP4, AQP7, and AQP11 can be expressed on human ILM in vivo. The increased expression of AQPs on the ILMs of DR may be associated with the occurrence of DME. Moreover, the degree of DME may be positively correlated with the expression of AQP4 on the ILMs.

Astrocytic outer retinal layer thinning is not a feature in AQP4-IgG seropositive neuromyelitis optica spectrum disorders

BACKGROUND

Patients with anti-aquaporin-4 antibody seropositive (AQP4-IgG+) neuromyelitis optica spectrum disorders (NMOSDs) frequently suffer from optic neuritis (ON) leading to severe retinal neuroaxonal damage. Further, the relationship of this retinal damage to a primary astrocytopathy in NMOSD is uncertain. Primary astrocytopathy has been suggested to cause ON-independent retinal damage and contribute to changes particularly in the outer plexiform layer (OPL) and outer nuclear layer (ONL), as reported in some earlier studies. However, these were limited in their sample size and contradictory as to the localisation. This study assesses outer retinal layer changes using optical coherence tomography (OCT) in a multicentre cross-sectional cohort.

METHOD

197 patients who were AQP4-IgG+ and 32 myelin-oligodendrocyte-glycoprotein antibody seropositive (MOG-IgG+) patients were enrolled in this study along with 75 healthy controls. Participants underwent neurological examination and OCT with central postprocessing conducted at a single site.

RESULTS

No significant thinning of OPL (25.02±2.03 µm) or ONL (61.63±7.04 µm) were observed in patients who were AQP4-IgG+ compared with patients who were MOG-IgG+ with comparable neuroaxonal damage (OPL: 25.10±2.00 µm; ONL: 64.71±7.87 µm) or healthy controls (OPL: 24.58±1.64 µm; ONL: 63.59±5.78 µm). Eyes of patients who were AQP4-IgG+ (19.84±5.09 µm, p=0.027) and MOG-IgG+ (19.82±4.78 µm, p=0.004) with a history of ON showed parafoveal OPL thinning compared with healthy controls (20.99±5.14 µm); this was not observed elsewhere.

CONCLUSION

The results suggest that outer retinal layer loss is not a consistent component of retinal astrocytic damage in AQP4-IgG+ NMOSD. Longitudinal studies are necessary to determine if OPL and ONL are damaged in late disease due to retrograde trans-synaptic axonal degeneration and whether outer retinal dysfunction occurs despite any measurable structural correlates.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

Intravitreal Ranibizumab Had Limited Effect on Cystoid Macular Edema Due to Albumin-Bound Paclitaxel: A Case Report and Literature Review

Angiographically silent cystoid macular edema (CME) is a rare complication from nab-paclitaxel. Here we report a 45-year-old woman with breast cancer who developed CME after several months of treatment with albumin-bound paclitaxel (nab-paclitaxel). Her visual acuity did not improve significantly with the cessation of nab-paclitaxel and intravitreal ranibizumab treatment. Then, brinzolamide eye drops were prescribed. One month later, her vision improved, with the macular edema significantly subsided. Finally, we reviewed other cases of CME induced by nab-paclitaxel that have been reported in the literature and discussed the underlying pathogenesis of nab-paclitaxel-induced CME.

Retinal optical coherence tomography and magnetic resonance imaging in neuromyelitis optica spectrum disorders and MOG-antibody associated disorders: an updated review

INTRODUCTION

Neuromyelitis optica spectrum disorders (NMOSD) and myelin oligodendrocyte glycoprotein IgG antibody-associated disorders (MOGAD) comprise two groups of rare neuroinflammatory diseases that cause attack-related damage to the central nervous system (CNS). Clinical attacks are often characterized by optic neuritis, transverse myelitis, and to a lesser extent, brainstem encephalitis/area postrema syndrome. Retinal optical coherence tomography (OCT) is a non-invasive technique that allows for in vivo thickness quantification of the retinal layers. Apart from OCT, magnetic resonance imaging (MRI) plays an increasingly important role in NMOSD and MOGAD diagnosis based on the current international diagnostic criteria. Retinal OCT and brain/spinal cord/optic nerve MRI can help to distinguish NMOSD and MOGAD from other neuroinflammatory diseases, particularly from multiple sclerosis, and to monitor disease-associated CNS-damage.

AREAS COVERED

This article summarizes the current status of imaging research in NMOSD and MOGAD, and reviews the clinical relevance of OCT, MRI and other relevant imaging techniques for differential diagnosis, screening and monitoring of the disease course.

EXPERT OPINION

Retinal OCT and MRI can visualize and quantify CNS damage in vivo, improving our understanding of NMOSD and MOGAD pathology. Further efforts on the standardization of these imaging techniques are essential for implementation into clinical practice and as outcome parameters in clinical trials.

Dose Response of Bumetanide on Aquaporins and Angiogenesis Biomarkers in Human Retinal Endothelial Cells Exposed to Intermittent Hypoxia

Aquaporins (AQPs) are important for regulating cellular water, solute transport, and balance. Recently, AQPs have also been recognized as playing a key role in cell migration and angiogenesis. In the retina, hypoxia induces vascular endothelial growth factor (VEGF), a potent angiogenic and vascular permeability factor, resulting in retinal edema, which is facilitated by AQPs. Bumetanide is a diuretic agent and AQP 1–4 blocker. We tested the hypothesis that bumetanide suppression of AQPs ameliorates intermittent hypoxia (IH)-induced angiogenesis and oxidative stress in human microvascular retinal endothelial cells (HMRECs). HMRECs were treated with a low-dose (0.05 µg/mL) or high-dose (0.2 µg/mL) of bumetanide and were exposed to normoxia (Nx), hyperoxia (50% O₂), or IH (50% O₂ with brief hypoxia 5% O₂) for 24, 48, and 72 h. Angiogenesis and oxidative stress biomarkers were determined in the culture media, and the cells were assessed for tube formation capacity and AQP-1 and -4 expression. Both doses of bumetanide significantly decreased oxidative stress and angiogenesis biomarkers. This response was reflected by reductions in tube formation capacity and AQP expression. These findings confirm the role of AQPs in retinal angiogenesis. Therapeutic targeting of AQPs with bumetanide may be advantageous for IH-induced aberrant retinal development.

Microcystic macular degeneration in autosomal hereditary optic neuropathies: A cross-sectional retrospective study

OBJECTIVE

Patients with autosomal optic neuropathies (AON) may develop microcystic macular degeneration (MMD), observed on retinal optical coherence tomography (OCT) examination. This study aimed to report the prevalence of MMD in AON patients and to assess the consequences of MMD on retinal architecture.

METHODS

Retrospective single-center study conducted between 2001 and 2018. Patients affected by AON secondary to OPA1 or WFS1 gene mutations were included. The following data were collected: visual acuity, macular volume, vitreomacular interface and presence or absence of MMD.

RESULTS

Forty-two subjects (34 OPA1, 8 WFS1) were included. MMD was found in 12 (29%) patients, i.e. 6 of the 8 WFS1 patients (75%) and 6 of the 34 OPA1 patients (17%). In cases with MMD, total retinal volume was greater (P=0.02) in accordance with thickening of the inner nuclear layer (P<0.001). WFS1 subjects had the highest total retinal volume (P=0.01), in relation to a thickening of the inner plexiform layer (P=0.02), inner nuclear layer (P<0.001) and outer plexiform layer (P=0.002). MMD was significantly associated with the WFS1 mutation (P<0.001). No significant association was found between the presence of vitreomacular adhesion and MMD.

CONCLUSION

MMD was found in 29% of patients affected by AON and was more frequent in cases with a WFS1 gene mutation. MMD appears to be related to primary ganglion cell degeneration and Müller cell dysfunction. The vitreomacular interface does not appear to play a role in the occurrence of MMD.

Foveal changes in aquaporin-4 antibody seropositive neuromyelitis optica spectrum disorder are independent of optic neuritis and not overtly progressive

BACKGROUND AND PURPOSE

Foveal changes were reported in aquaporin-4 antibody (AQP4-Ab) seropositive neuromyelitis optica spectrum disorder (NMOSD) patients; however, it is unclear whether they are independent of optic neuritis (ON), stem from subclinical ON or crossover from ON in fellow eyes. Fovea morphometry and a statistical classification approach were used to investigate if foveal changes in NMOSD are independent of ON and progressive.

METHODS

This was a retrospective longitudinal study of 27 AQP4-IgG + NMOSD patients (49 eyes; 15 ON eyes and 34 eyes without a history of ON [NON eyes]), follow-up median (first and third quartile) 2.32 (1.33-3.28), and 38 healthy controls (HCs) (76 eyes), follow-up median (first and third quartile) 1.95 (1.83-2.54). The peripapillary retinal nerve fibre layer thickness and the volume of combined ganglion cell and inner plexiform layer as measures of neuroaxonal damage from ON were determined by optical coherence tomography. Nineteen foveal morphometry parameters were extracted from macular optical coherence tomography volume scans. Data were analysed using orthogonal partial least squares discriminant analysis and linear mixed effects models.

RESULTS

At baseline, foveal shape was significantly altered in ON eyes and NON eyes compared to HCs. Discriminatory analysis showed 81% accuracy distinguishing ON vs. HCs and 68% accuracy in NON vs. HCs. NON eyes were distinguished from HCs by foveal shape parameters indicating widening. Orthogonal partial least squares discriminant analysis discriminated ON vs. NON with 76% accuracy. In a follow-up of 2.4 (20.85) years, no significant time-dependent foveal changes were found.

CONCLUSION

The parafoveal area is altered in AQP4-Ab seropositive NMOSD patients suggesting independent neuroaxonal damage from subclinical ON. Longer follow-ups are needed to confirm the stability of the parafoveal structure over time.

Retinal Aquaporin-4 and Regulation of Water Inflow Into the Vitreous Body

Purpose

Details of the posterior eye water dynamics are unclear. Aquaporin-4 (AQP4), a water channel, plays an important role in water dynamics in the central nervous system and is also present in the ocular tissue. The purpose of this study was to reveal the role of AQP4 in the water dynamics of the posterior eye using in vivo JJ vicinal coupling proton exchange (JJVCPE) magnetic resonance imaging (MRI) of AQP4 knockout (KO) mice and their wild-type littermates (controls).

Methods

JJVCPE MRI of the eye was performed on five AQP4 KO mice and seven control mice. We assessed the normalized signal intensities of a region of interest (ROI) set in the vitreous body after H₂¹⁷O administration. The results of the two groups were compared using a two-tailed Mann-Whitney U test.

Results

A statistical analysis revealed that the normalized ROI signal intensities at the steady state were significantly lower (P = 0.010, <0.05) in the AQP4 KO mice (mean ± SD, 84.5% ± 2.7%) than the controls (mean ± SD, 88.8% ± 1.9%).

Conclusions

The present study using JJVCPE MRI of the eye demonstrated that retinal AQP4 has a potential role in the regulation of water inflow into the vitreous body. Absence of AQP4 in the KO mice probably induces lower water outflow from the vitreous body. Our results could help clarify the pathogenesis of various ocular diseases.

Altered fovea in AQP4-IgG-seropositive neuromyelitis optica spectrum disorders

Objective

To investigate disease-specific foveal shape changes in patients with neuromyelitis optica spectrum disorders (NMOSDs) using foveal morphometry.

Methods

This cross-sectional study included macular spectral domain optical coherence tomography scans of 52 eyes from 28 patients with aquaporin-4 immunoglobulin G (AQP4-IgG)-seropositive NMOSD, 116 eyes from 60 patients with MS, and 123 eyes from 62 healthy controls (HCs), retrospectively, and an independent confirmatory cohort comprised 33/33 patients with NMOSD/MS. The fovea was characterized using 3D foveal morphometry. We included peripapillary retinal nerve fiber layer (pRNFL) thickness and combined macular ganglion cell and inner plexiform layer (GCIPL) volume to account for optic neuritis (ON)-related neuroaxonal damage.

Results

Group comparison showed significant differences compared with HC in the majority of foveal shape parameters in NMOSD, but not MS. Pit flat disk area, average pit flat disk diameter, inner rim volume, and major slope disk length, as selected parameters, showed differences between NMOSD and MS (p value = 0.017, 0.002, 0.005, and 0.033, respectively). This effect was independent of ON. Area under the curve was between 0.7 and 0.8 (receiver operating characteristic curve) for discriminating between NMOSD and MS. Pit flat disk area and average pit flat disk diameter changes independent of ON were confirmed in an independent cohort.

Conclusions

Foveal morphometry reveals a wider and flatter fovea in NMOSD in comparison to MS and HC. Comparison to MS and accounting for ON suggest this effect to be at least in part independent of ON. This supports a primary retinopathy in AQP4-IgG–seropositive NMOSD.

Neuroretinal rim response to transient changes in intraocular pressure in healthy non-human primate eyes

Optic nerve head (ONH) neuroretinal rim thickness, quantified as minimum rim width (BMO-MRW), is a sensitive measure for assessing early glaucomatous disease. The BMO-MRW is sensitive to transient fluctuations in intraocular pressure (IOP), but the time course over which BMO-MRW decreases and recovers with changes in IOP remains unknown. The goal of this study was to investigate the dynamics of BMO-MRW changes over 2-h periods of mild or moderate IOP elevation, and subsequent recovery, in healthy non-human primate eyes. Eight non-human primates were included in the study. For each animal, in two different sessions separated by at least 2 weeks, the anterior chamber IOP of one eye was maintained at either 25 mmHg or 40 mmHg for 2 h and, subsequently, at 10 mmHg for 2 h. For the duration of anterior chamber cannulation, optical coherence tomography (OCT) radial scans centered on the ONH were acquired every 5 min and used to quantify BMO-MRW. An exponential decay or rise to maximum function was used to determine the extent and rate of structural change. Additionally, Bruch's membrane opening (BMO) area, BMO height/displacement, and BMO-referenced anterior lamina cribrosa surface depth (BMO-ALCSD) were computed from radial scans. A circular scan was used to quantify retinal nerve fiber layer thickness (RNFLT) and circumpapillary choroid thickness. The primary results demonstrated that the BMO-MRW changed over an extended duration, while BMO displacement was rapid and remained stable with sustained IOP. The mean maximum predicted BMO-MRW thinning following 2 h of IOP elevation was significantly related to pressure (34.2 ± 13.8 μm for an IOP of 25 mmHg vs 40.5 ± 12.6 μm for 40 mmHg, p = 0.03). The half-life for BMO-MRW thinning was 21.9 ± 9.2 min for 25 mmHg and 20.9 ± 4.2 min for 40 mmHg, not significantly different between IOP levels (p = 0.76). Subsequently, after 2 h of IOP at 10 mmHg, all animals exhibited partial recovery of BMO-MRW with similar degrees of persistent residual thinning for the two IOP levels (21.5 ± 13.7 vs 21.0 ± 12.3 μm, p = 0.88). Similar to BMO-MRW, choroid thickness exhibited gradual thinning with IOP elevation and residual thinning following IOP reduction. However, there was no significant change in BMO area or BMO-ALCSD in either experimental session. The RNFLT gradually decreased over the duration of IOP elevation, with continued decreases following IOP reduction for the 40 mmHg session, resulting in total changes from baseline of -2.24 ± 0.81 and -2.45 ± 1.21 μm for 25 and 40 mmHg, respectively (p < 0.001). The sum of the results demonstrate that the ONH neural tissue is sensitive to changes in IOP, the effects of which are gradual over an extended time course and different for increased vs. decreased pressure. Understanding the duration over which IOP influences BMO-MRW has important implications for studies investigating the effects of IOP on the ONH. Additionally, individual variability in ONH response to IOP may improve our understanding of the risk and progression of disease.

Glial Cells in the Fish Retinal Nerve Fiber Layer Form Tight Junctions, Separating and Surrounding Axons

In the retina of teleost fish, cell addition continues throughout life involving proliferation and axonal growth. To study how this is achieved in a fully functioning retina, we investigated the nerve fiber layer (NFL) of the cichlid fish Astatotilapia burtoni for components that might regulate the extracellular environment. We hypothesized that growing axons are surrounded by different cell structures than signal conducting axons. Using immunohistochemistry and freeze fracture electron microscopy we found that the endfeet of Müller cells (MCs) expressed aquaporin-4 but not in high densities as in mammals. The presence of this water channel indicates the involvement of MCs in water homeostasis. Remarkably, we discovered conspicuous tight junctions in the retinal NFL. These tight junctions formed branching strands between myelin-like wrappings of ganglion cell axons that differed morphologically from any known myelin, and also an elaborate meshwork on large membrane faces between axons. We speculated that these tight junctions have additional functions than solely facilitating nerve conductance. Immunostainings against the adaptor protein ZO-1 labeled the NFL as did antibodies against the mammalian claudin-1, 3, and 19. Performing PCR analysis, we showed expression of claudin-1, 3, 5a, 5b, 9, 11, and 19 in the fish retina, claudins that typically occur at brain barriers or myelin. We could show by immunostains for doublecortin, a marker for differentiating neurons, that new axons are not surrounded by the myelin-like wrappings but only by the endfeet of MCs. We hypothesize that the tight junctions in the NFL of fish might contribute to the separation of an extracellular space around axons facilitating conductance, from a growth-promoting environment. For a functional test we applied Evans Blue dye to eye cup preparations which showed a retention of the dye in the NFL. This indicates that these remarkable tight junctions can indeed act as a diffusion barrier.

Microcystic macular oedema in optic neuropathy: case series and literature review

Cavitations in the inner nuclear layer associated with severe optic atrophy and loss of retinal ganglion cells have clinically been termed microcystic macular oedema (MME). We describe a case series of MME in patients of all ages but predominantly younger patients with a wide range of optic atrophies ranging from acute onset optic disc drusen associated ischemic optic neuropathy to slowly progressive disease as glaucoma. There were no physical distinctions between MME in different causes of optic atrophy suggesting a common causative mechanism. We reviewed the literature on MME and it appears that MME is associated with more severe visual loss, and is more common in hereditary optic neuropathies and neuromyelitis optica spectrum disease rather than in patients with optic atrophy secondary to multiple sclerosis and glaucoma. Three main causative mechanisms have been proposed, including increased vitreal traction on the macular as the ganglion cells are lost. Others have suggested that trans-synaptic loss of cells in the inner nuclear layer causes formation of empty spaces or cavities. Finally, some have hypothesized a disturbance in the fluid homeostasis of the inner retina as Müller cells are lost or their function is impaired. There are no known treatments of MME. In conclusion, MME seems to be a marker of severe optic nerve atrophy irrespective of the underlying cause.

Potential role of the methylation of VEGF gene promoter in response to hypoxia in oxygen-induced retinopathy: beneficial effect of the absence of AQP4

Hypoxia‐dependent accumulation of vascular endothelial growth factor (VEGF) plays a major role in retinal diseases characterized by neovessel formation. In this study, we investigated whether the glial water channel Aquaporin‐4 (AQP4) is involved in the hypoxia‐dependent VEGF upregulation in the retina of a mouse model of oxygen‐induced retinopathy (OIR). The expression levels of VEGF, the hypoxia‐inducible factor‐1α (HIF‐1α) and the inducible form of nitric oxide synthase (iNOS), the production of nitric oxide (NO), the methylation status of the HIF‐1 binding site (HBS) in the VEGF gene promoter, the binding of HIF‐1α to the HBS, the retinal vascularization and function have been determined in the retina of wild‐type (WT) and AQP4 knock out (KO) mice under hypoxic (OIR) or normoxic conditions. In response to 5 days of hypoxia, WT mice were characterized by (i) AQP4 upregulation, (ii) increased levels of VEGF, HIF‐1α, iNOS and NO, (iii) pathological angiogenesis as determined by engorged retinal tufts and (iv) dysfunctional electroretinogram (ERG). AQP4 deletion prevents VEGF, iNOS and NO upregulation in response to hypoxia thus leading to reduced retinal damage although in the presence of high levels of HIF‐1α. In AQP4 KO mice, HBS demethylation in response to the beginning of hypoxia is lower than in WT mice reducing the binding of HIF‐1α to the VEGF gene promoter. We conclude that in the absence of AQP4, an impaired HBS demethylation prevents HIF‐1 binding to the VEGF gene promoter and the relative VEGF transactivation, reducing the VEGF‐induced retinal damage in response to hypoxia.

Common and Rare Manifestations of Neuromyelitis Optica Spectrum Disorder

The discovery of a highly specific biomarker of neuromyelitis optica (NMO)-the anti-aquaporin-4 (AQP4) antibody-has opened new paths to understanding disease pathogenesis and afforded a way to confirm the diagnosis in clinical practice. An important consequence of the discovery is the broadening of the spectrum of syndromes seen in the context of AQP4 autoimmunity. These syndromes have been subsumed under the rubric of NMO spectrum disorder (NMOSD). The current classification recognizes not only optic neuritis and myelitis as core syndromes of NMOSD but also cerebral, diencephalic, brainstem, and area postrema syndromes. These neurologic syndromes are the focus of our review. AQP4 is also expressed in many organs outside of the central nervous system, and this may explain some of the unusual, non-neurologic features that have been occasionally reported in NMOSD. Our review catalogues non-neurologic manifestations seen in NMOSD and concludes with a discussion of frequently associated autoimmune and neoplastic comorbidities of NMOSD.

Implication of VEGF and aquaporin 4 mediating Müller cell swelling to diabetic retinal edema

PURPOSE

Aquaporin 4 (AQP4), a water channel protein, is known to be expressed in retinal Müller cells. The purpose of this study was to determine the effects of VEGF and AQP4 channels on the volumetric changes in Müller cells.

METHODS

Retinas from diabetic rats and a cultured Müller cell line, TR-MUL5, were used in this study. Intravitreal injections of VEGF or PBS were performed on either streptozotocin (STZ)-induced diabetic or normoglycemic rats. Retinal sections were immunostained for anti-glial fibrillary acidic protein (GFAP), anti-AQP4, and anti-VEGF. VEGF protein levels from collected retinas were determined by western blot analysis. Volumetric changes and nitric oxide (NO) levels in cultured Müller cells were determined using flow cytometry (FACS), in the presence or absence of VEGF and TGN-020, a selective AQP4 inhibitor.

RESULTS

In the diabetic rat retina, VEGF immunoreactivity was concentrated in the internal retinal layers, and AQP4 immunoreactivity was higher than controls. The expressions of AQP4 were colocalized with GFAP. Protein levels of VEGF in the hyperglycemic rat retina were significantly higher than controls. FACS analyses showed that exposure to VEGF enlarged Müller cells, while exposure to TGN-020 suppressed the enlargement. Intracellular levels of NO were increased after exposure to VEGF, which was suppressed following the addition of TGN-020.

CONCLUSION

The observed Müller cell swelling is mediated by VEGF and AQP4.

A pharmacological approach in newly established retinal vein occlusion model

The mechanism underlying the effects of anti-vascular endothelial growth factor (VEGF) antibody in retinal vein occlusion (RVO) treatment is poorly understood, partly due to the lack of RVO animal models that mimic clinical pathology. The aims of this study were to establish a suitable RVO model, clarify the pathogenic mechanisms, and evaluate the effects of anti-VEGF antibody in the model. Mouse retinal veins were occluded by laser photocoagulation after rose bengal injection. Reduction of the b/a wave amplitude ratio, retinal nonperfusion, cystoid edema, and hard exudates were observed after occlusion, and expression of RVO-related genes was altered. Administration of anti-VEGF antibody immediately, or 7 days, after occlusion resulted in reduction and increase of the nonperfused area, respectively. We conclude that the present model will be useful for clarification of the pathogenic mechanisms, and that the timing of anti-VEGF antibody administration is important for the successful amelioration of retinal nonperfusion.

Aquaporins in the Eye

The major part of the eye consists of water . Continuous movement of water and ions between the ocular compartments and to the systemic circulation is pivotal for many physiological functions in the eye. The movement of water facilitates removal of the many metabolic products of corneal-, ciliary body-, lens- and retinal metabolism, while maintaining transparency in the optical compartments. Transport across the corneal epithelium and endothelium maintains the corneal transparency. Also, aqueous humour is continuously secreted by the epithelia of the ciliary body and maintains the intraocular pressure. In the retina, water is transported into the vitreous body and across the retinal pigment epithelium to regulate the extracellular environment and the hydration of the retina. Aquaporins (AQPs ) take part in the water transport throughout the eye.

Aquaporin-4 in Astroglial Cells in the CNS and Supporting Cells of Sensory Organs-A Comparative Perspective

The main water channel of the brain, aquaporin-4 (AQP4), is one of the classical water-specific aquaporins. It is expressed in many epithelial tissues in the basolateral membrane domain. It is present in the membranes of supporting cells in most sensory organs in a specifically adapted pattern: in the supporting cells of the olfactory mucosa, AQP4 occurs along the basolateral aspects, in mammalian retinal Müller cells it is highly polarized. In the cochlear epithelium of the inner ear, it is expressed basolaterally in some cells but strictly basally in others. Within the central nervous system, aquaporin-4 (AQP4) is expressed by cells of the astroglial family, more specifically, by astrocytes and ependymal cells. In the mammalian brain, AQP4 is located in high density in the membranes of astrocytic endfeet facing the pial surface and surrounding blood vessels. At these locations, AQP4 plays a role in the maintenance of ionic homeostasis and volume regulation. This highly polarized expression has not been observed in the brain of fish where astroglial cells have long processes and occur mostly as radial glial cells. In the brain of the zebrafish, AQP4 immunoreactivity is found along the radial extent of astroglial cells. This suggests that the polarized expression of AQP4 was not present at all stages of evolution. Thus, a polarized expression of AQP4 as part of a control mechanism for a stable ionic environment and water balanced occurred at several locations in supporting and glial cells during evolution. This initially basolateral membrane localization of AQP4 is shifted to highly polarized expression in astrocytic endfeet in the mammalian brain and serves as a part of the neurovascular unit to efficiently maintain homeostasis.

Expression and Distribution Pattern of Aquaporin 4, 5 and 11 in Retinas of 15 Different Species

Aquaporins (AQPs) are small integral membrane proteins with 13 members in mammals and are essential for water transport across membranes. They are found in many different tissues and cells. Currently, there are conflicting results regarding retinal aquaporin expression and subcellular localization between genome and protein analyses and among various species. AQP4, 7, 9 and 11 were described in the retina of men; whereas AQP6, 8 and 10 were earlier identified in rat retinas and AQP4, 5 and 11 in horses. Since there is a lack of knowledge regarding AQP expression on protein level in retinas of different animal models, we decided to analyze retinal cellular expression of AQP4, 5 and 11 in situ with immunohistochemistry. AQP4 was detected in all 15 explored species, AQP5 and AQP11 in 14 out of 15. Interestingly, AQP4 was unambiguously expressed in Muller glial cells, whereas AQP5 was differentially allocated among the species analyzed. AQP11 expression was Muller glial cell-specific in 50% of the animals, whereas in the others, AQP11 was detected in ganglion cell layer and at photoreceptor outer segments. Our data indicate a disparity in aquaporin distribution in retinas of various animals, especially for AQP5 and 11.

Opposing effects of atropine and timolol on the color and luminance emmetropization mechanisms in chicks

This study analyzed the luminance and color emmetropization response in chicks treated with the nonselective parasympathetic antagonist atropine and the sympathetic β-receptor blocker timolol. Chicks were binocularly exposed (8h/day) for 4days to one of three illumination conditions: 2Hz sinusoidal luminance flicker, 2Hz sinusoidal blue/yellow color flicker, or steady light (mean 680lux). Atropine experiments involved monocular daily injections of either 20μl of atropine (18nmol) or 20μl of phosphate-buffered saline. Timolol experiments involved monocular daily applications of 2 drops of 0.5% timolol or 2 drops of distilled H2O. Changes in the experimental eye were compared with those in the fellow eye after correction for the effects of saline/water treatments. Atropine caused a reduction in axial length with both luminance flicker (-0.078±0.021mm) and color flicker (-0.054±0.017mm), and a reduction in vitreous chamber depth with luminance flicker (-0.095±0.023mm), evoking a hyperopic shift in refraction (3.40±1.77D). Timolol produced an increase in axial length with luminance flicker (0.045±0.030mm) and a myopic shift in refraction (-4.07±0.92D), while color flicker caused a significant decrease in axial length (-0.046±0.017mm) that was associated with choroidal thinning (-0.046±0.015mm). The opposing effects on growth and refraction seen with atropine and timolol suggest a balancing mechanism between the parasympathetic and β-receptor mediated sympathetic system through stimulation of the retina with luminance and color contrast.

Vasopressin Hypersecretion-Associated Brain Edema Formation in Ischemic Stroke: Underlying Mechanisms

BACKGROUND

Brain edema formation is a major cause of brain damages and the high mortality of ischemic stroke. The aim of this review is to explore the relationship between ischemic brain edema formation and vasopressin (VP) hypersecretion in addition to the oxygen and glucose deprivation and the ensuing reperfusion injury.

METHODS

Pertinent studies involving ischemic stroke, brain edema formation, astrocytes, and VP were identified by a search of the PubMed and the Web of Science databases in January 2016. Based on clinical findings and reports of animal experiments using ischemic stroke models, this systematic review reanalyzes the implication of individual reports in the edema formation and then establishes the inherent links among them.

RESULTS

This systematic review reveals that cytotoxic edema and vasogenic brain edema in classical view are mainly under the influence of a continuous malfunction of astrocytic plasticity. Adaptive VP secretion can modulate membrane ion transport, water permeability, and blood-brain barrier integrity, which are largely via changing astrocytic plasticity. Maladaptive VP hypersecretion leads to disruptions of ion and water balance across cell membranes as well as the integrity of the blood-brain barrier. This review highlights our current understandings of the cellular mechanisms underlying ischemic brain edema formation and its association with VP hypersecretion.

CONCLUSIONS

VP hypersecretion promotes brain edema formation in ischemic stroke by disrupting hydromineral balance in the neurovascular unit; suppressing VP hypersecretion has the potential to alleviate ischemic brain edema.

Müller glia activation by VEGF-antagonizing drugs: An in vitro study on rat primary retinal cultures

The effects of the anti-Vascular Endothelial Growth Factor (VEGF) drugs ranibizumab and aflibercept were studied in Müller glia in primary mixed cultures from rat neonatal retina. Treatment with both agents induced activation of Müller glia, demonstrated by increased levels of Glial Fibrillary Acidic Protein. In addition, phosphorylated Extracellular-Regulated Kinase 1/2 (ERK 1/2) showed enhanced immunoreactivity in activated Müller glia. Treatment with aflibercept induced an increase in K(+) channel (Kir) 4.1 levels and both drugs upregulated Aquaporin 4 (AQP4) in activated Müller glia. The results show that VEGF-antagonizing drugs influence the homeostasis of Müller cells in primary retinal cultures, inducing an activated phenotype. Upregulation of Kir4.1 and AQP4 suggests that Müller glia activation following anti-VEGF drugs may not depict a detrimental gliotic reaction. Indeed, it could represent one of the mechanisms able to contribute to the therapeutic effects of these drugs, particularly in the presence of macular edema.

TRPV4 and AQP4 Channels Synergistically Regulate Cell Volume and Calcium Homeostasis in Retinal Müller Glia

Brain edema formation occurs after dysfunctional control of extracellular volume partly through impaired astrocytic ion and water transport. Here, we show that such processes might involve synergistic cooperation between the glial water channel aquaporin 4 (AQP4) and the transient receptor potential isoform 4 (TRPV4), a polymodal swelling-sensitive cation channel. In mouse retinas, TRPV4 colocalized with AQP4 in the end feet and radial processes of Müller astroglia. Genetic ablation of TRPV4 did not affect the distribution of AQP4 and vice versa. However, retinas from Trpv4(-/-) and Aqp4(-/-) mice exhibited suppressed transcription of genes encoding Trpv4, Aqp4, and the Kir4.1 subunit of inwardly rectifying potassium channels. Swelling and [Ca(2+)]i elevations evoked in Müller cells by hypotonic stimulation were antagonized by the selective TRPV4 antagonist HC-067047 (2-methyl-1-[3-(4-morpholinyl)propyl]-5-phenyl-N-[3-(trifluoromethyl)phenyl]-1H-pyrrole-3-carboxamide) or Trpv4 ablation. Elimination of Aqp4 suppressed swelling-induced [Ca(2+)]i elevations but only modestly attenuated the amplitude of Ca(2+) signals evoked by the TRPV4 agonist GSK1016790A [(N-((1S)-1-{[4-((2S)-2-{[(2,4-dichlorophenyl)sulfonyl]amino}-3-hydroxypropanoyl)-1-piperazinyl]carbonyl}-3-methylbutyl)-1-benzothiophene-2-carboxamide]. Glial cells lacking TRPV4 but not AQP4 showed deficits in hypotonic swelling and regulatory volume decrease. Functional synergy between TRPV4 and AQP4 during cell swelling was confirmed in the heterologously expressing Xenopus oocyte model. Importantly, when the swelling rate was osmotically matched for AQP4-positive and AQP4-negative oocytes, TRPV4 activation became independent of AQP4. We conclude that AQP4-mediated water fluxes promote the activation of the swelling sensor, whereas Ca(2+) entry through TRPV4 channels reciprocally modulates volume regulation, swelling, and Aqp4 gene expression. Therefore, TRPV4-AQP4 interactions constitute a molecular system that fine-tunes astroglial volume regulation by integrating osmosensing, calcium signaling, and water transport and, when overactivated, triggers pathological swelling. Significance statement: We characterize the physiological features of interactions between the astroglial swelling sensor transient receptor potential isoform 4 (TRPV4) and the aquaporin 4 (AQP4) water channel in retinal Müller cells. Our data reveal an elegant and complex set of mechanisms involving reciprocal interactions at the level of glial gene expression, calcium homeostasis, swelling, and volume regulation. Specifically, water influx through AQP4 drives calcium influx via TRPV4 in the glial end foot, which regulates expression of Aqp4 and Kir4.1 genes and facilitates the time course and amplitude of hypotonicity-induced swelling and regulatory volume decrease. We confirm the crucial facets of the signaling mechanism in heterologously expressing oocytes. These results identify the molecular mechanism that contributes to dynamic regulation of glial volume but also provide new insights into the pathophysiology of glial reactivity and edema formation.

Differential Expression of AQP1 and AQP4 in Avascular Chick Retina Exposed to Moderate Light of Variable Photoperiods

Aquaporins (AQPs) are integral membrane proteins which maintain cellular water and ion homeostasis. Alterations in AQP expression have been reported in rod-dominated rodent retinas exposed to light. In rodents and also in birds, light of moderate intensities (700-2000 lux) damages the retina, though detailed changes were not examined in birds. The aim of our study was to see if light affects cone dominated retinas, which would be reflected in expression levels of AQPs. We examined AQP1 and AQP4 expressions in chick retina exposed to 2000 lux under 12 h light:12 h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod) and 24L:0D (constant light). Additionally, morphological changes, apoptosis (by TUNEL) and levels of glutamate and GFAP (a marker of injury) in the retina were examined to correlate these with AQP expressions. Constant light caused damage in outer and inner nuclear layer (ONL, INL) and ganglion cell layer (GCL). Also, there were associated increases in GFAP and glutamate levels in retinal extracts. In normal photoperiod, AQP1 was expressed in GCL, outer part of INL and photoreceptor inner segments of. AQP4 was additionally expressed in nerve fiber layer. Immunohistochemistry and Western blotting revealed over all decreased AQP1 and AQP4 expression in constant light condition compared to those in other two groups. The elevated GFAP and glutamate levels might be involved in the reduction of AQPs in constant light group. Such decreases in AQP expressions are perhaps linked with retinal cell damage seen in constant light condition, while their relatively enhanced expression in two other conditions may help in maintaining a normal retinal architecture, indicating their neuroprotective potential.

Dystrophin Dp71 gene deletion induces retinal vascular inflammation and capillary degeneration

We have previously shown that the deletion of the dystrophin Dp71 gene induces a highly permeable blood-retinal barrier (BRB). Given that BRB breakdown is involved in retinal inflammation and the pathophysiology of many blinding eye diseases, here we investigated whether the absence of Dp71 brings out retinal vascular inflammation and vessel loss by using specific Dp71-null mice. The expression of vascular endothelial growth factor (VEGF), quantified by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay methods, was higher in the retina of Dp71-null mice than in wild-type mice. In contrast, no differences were observed in VEGFR-2 and tumor necrosis factor-α expression. Moreover, mRNA expression of water channel, aquaporin 4 (AQP4) was increased after Dp71 deletion. The Dp71 deletion was also associated with the overexpression of intercellular adhesion molecule 1, which is expressed on endothelial cells surface to recruit leukocytes. Consistent with these findings, the total number of adherent leukocytes per retina, assessed after perfusion with fluorescein isothiocyanate-conjugated concanavalin A, was increased in the absence of Dp71. Finally, a significant increase in capillary degeneration quantified after retinal trypsin digestion was observed in mice lacking Dp71. These data illustrate for the first time that the deletion of Dp71 was associated with retinal vascular inflammation, vascular lesions with increased leukocyte adhesion and capillary degeneration. Thus, dystrophin Dp71 could play a critical role in retinal vascular inflammation disease, and therefore represent a potential therapeutic target.

Retinal vasculopathy is reduced by dietary salt restriction: involvement of Glia, ENaCα, and the renin-angiotensin-aldosterone system

OBJECTIVE

Neovascularization and vaso-obliteration are vision-threatening events that develop by interactions between retinal vascular and glial cells. A high-salt diet is causal in cardiovascular and renal disease, which is linked to modulation of the renin-angiotensin-aldosterone system. However, it is not known whether dietary salt influences retinal vasculopathy and if the renin-angiotensin-aldosterone system is involved. We examined whether a low-salt (LS) diet influenced vascular and glial cell injury and the renin-angiotensin-aldosterone system in ischemic retinopathy.

APPROACH AND RESULTS

Pregnant Sprague Dawley rats were fed LS (0.03% NaCl) or normal salt (0.3% NaCl) diets, and ischemic retinopathy was induced in the offspring. An LS diet reduced retinal neovascularization and vaso-obliteration, the mRNA and protein levels of the angiogenic factors, vascular endothelial growth factor, and erythropoietin. Microglia, which influence vascular remodeling in ischemic retinopathy, were reduced by LS as was tumor necrosis factor-α. Macroglial Müller cells maintain the integrity of the blood-retinal barrier, and in ischemic retinopathy, LS reduced their gliosis and also vascular leakage. In retina, LS reduced mineralocorticoid receptor, angiotensin type 1 receptor, and renin mRNA levels, whereas, as expected, plasma levels of aldosterone and renin were increased. The aldosterone/mineralocorticoid receptor-sensitive epithelial sodium channel alpha (ENaCα), which is expressed in Müller cells, was increased in ischemic retinopathy and reduced by LS. In cultured Müller cells, high salt increased ENaCα, which was prevented by mineralocorticoid receptor and angiotensin type 1 receptor blockade. Conversely, LS reduced ENaCα, angiotensin type 1 receptor, and mineralocorticoid receptor expression.

CONCLUSIONS

An LS diet reduced retinal vasculopathy, by modulating glial cell function and the retinal renin-angiotensin-aldosterone system.

Diabetic retinopathy: loss of neuroretinal adaptation to the diabetic metabolic environment

Diabetic retinopathy (DR) impairs vision of patients with type 1 and type 2 diabetes, associated with vascular dysfunction and occlusion, retinal edema, hemorrhage, and inappropriate growth of new blood vessels. The recent success of biologic treatments targeting vascular endothelial growth factor (VEGF) demonstrates that treating the vascular aspects in the later stages of the disease can preserve vision in many patients. It would also be highly desirable to prevent the onset of the disease or arrest its progression at a stage preceding the appearance of overt microvascular pathologies. The progression of DR is not necessarily linear but may follow a series of steps that evolve over the course of multiple years. Abundant data suggest that diabetes affects the entire neurovascular unit of the retina, with an early loss of neurovascular coupling, gradual neurodegeneration, gliosis, and neuroinflammation occurring before observable vascular pathologies. In this article, we consider the pathology of DR from the point of view that diabetes causes measurable dysfunctions in the complex integral network of cell types that produce and maintain human vision.

Aquaporins in the eye: expression, function, and roles in ocular disease

BACKGROUND

All thirteen known mammalian aquaporins have been detected in the eye. Moreover, aquaporins have been identified as playing essential roles in ocular functions ranging from maintenance of lens and corneal transparency to production of aqueous humor to maintenance of cellular homeostasis and regulation of signal transduction in the retina.

SCOPE OF REVIEW

This review summarizes the expression and known functions of ocular aquaporins and discusses their known and potential roles in ocular diseases.

MAJOR CONCLUSIONS

Aquaporins play essential roles in all ocular tissues. Remarkably, not all aquaporin function as a water permeable channel and the functions of many aquaporins in ocular tissues remain unknown. Given their vital roles in maintaining ocular function and their roles in disease, aquaporins represent potential targets for future therapeutic development.

GENERAL SIGNIFICANCE

Since aquaporins play key roles in ocular physiology, an understanding of these functions is important to improving ocular health and treating diseases of the eye. It is likely that future therapies for ocular diseases will rely on modulation of aquaporin expression and/or function. This article is part of a Special Issue entitled Aquaporins.

Aquaporins 6-12 in the human eye

PURPOSE

Aquaporins (AQPs) are widely expressed and have diverse distribution patterns in the eye. AQPs 0-5 have been localized at the cellular level in human eyes. We investigated the presence of the more recently discovered AQPs 6-12 in the human eye.

METHODS

RT-PCR was performed on fresh tissue from two human eyes divided into the cornea, corneal limbus, ciliary body and iris, lens, choroid, optic nerve, retina and sclera. Each structure was examined to detect the mRNA of AQPs 6-12. Twenty-one human eyes were examined using immunohistochemical and immunofluorescence techniques to determine the topographical localization of AQPs 6-12.

RESULTS

mRNA transcripts of AQP7, AQP9 and AQP11 were found in the ciliary body, corneo-limbal tissue, optic nerve, retina and sclera. AQP9 and AQP11 mRNA was also detected in the choroid. No mRNA of AQP6, AQP8, AQP10 or AQP12 was detected. Anti-AQP7 immunolabelling was detected in the corneal epithelium, corneal endothelium, trabecular meshwork endothelium, ciliary epithelia, lens epithelium, the inner and outer limiting membrane of the retina, the retinal pigment epithelium and the capillary endothelium of all parts of the eye. AQP9 immunolabelling was detected in the nonpigmented ciliary epithelium and retinal ganglion cells. AQP11 immunolabelling was detected in the corneo-limbal epithelium, nonpigmented ciliary epithelium and inner limiting membrane of the retina.

CONCLUSION

Selective expression of AQP7, AQP9 and AQP11 was found within various structures of the human eye. The detection of these aquaporins in the eye implies a role that may be related not only to water transport but also to the transport of glycerol, lactate and ammonia, with importance for metabolism, especially in the retina.

Age-related changes of aquaporin expression patterns in the postnatal rat retina

Previous studies revealed that the rat retina contains numerous membrane-located water channels, the aquaporins (AQPs). Protein expression patterns of AQP1-4, 6 and 9 were examined by immunohistochemistry. In the present study, we investigated the immunolocalization of AQP1-4, 6 and 9 during postnatal development in the rat retina and examined the effect of age on the tissue distribution of these channels. AQP1, 3, 4, 6 and 9 showed gradually increased expression in rat retinas from postnatal week 1 to week 12, and decreased in the 40-week-old rat retinas. AQP2 expression was barely seen in the first week in rat retinas and displayed a significant increase from week 1 to week 4, however no significant alteration of AQP2 was observed after 4weeks of development. AQP1 and 4 immunoreactivities were present in the inner limiting membrane (ILM), the ganglion cell layer (GCL), inner nuclear layer (INL) and retinal pigment epithelium (RPE) in the 4-, 12- and 40-week-old rat retinas. The RPE, OLM and ILM showed a remarkable expression of AQP1-4, 6 and 9 in the 4, 12 and 40-week-old rat retinas. The reduced expression of AQPs in aged rat retinas may indicate the involvement of AQPs in the pathogenesis of age-related retinal diseases.

Molecular basis of the inner blood-retinal barrier and its breakdown in diabetic macular edema and other pathological conditions

Breakdown of the inner endothelial blood-retinal barrier (BRB), as occurs in diabetic retinopathy, age-related macular degeneration, retinal vein occlusions, uveitis and other chronic retinal diseases, results in vasogenic edema and neural tissue damage, causing loss of vision. The central mechanism of altered BRB function is a change in the permeability characteristics of retinal endothelial cells caused by elevated levels of growth factors, cytokines, advanced glycation end products, inflammation, hyperglycemia and loss of pericytes. Subsequently, paracellular but also transcellular transport across the retinal vascular wall increases via opening of endothelial intercellular junctions and qualitative and quantitative changes in endothelial caveolar transcellular transport, respectively. Functional changes in pericytes and astrocytes, as well as structural changes in the composition of the endothelial glycocalyx and the basal lamina around BRB endothelium further facilitate BRB leakage. As Starling's rules apply, active transcellular transport of plasma proteins by the BRB endothelial cells causing increased interstitial osmotic pressure is probably the main factor in the formation of macular edema. The understanding of the complex cellular and molecular processes involved in BRB leakage has grown rapidly in recent years. Although appropriate animal models for human conditions like diabetic macular edema are lacking, these insights have provided tools for rational design of drugs aimed at restoring the BRB as well as for design of effective transport of drugs across the BRB, to treat the chronic retinal diseases such as diabetic macular edema that affect the quality-of-life of millions of patients.

Environmental light and endogenous antioxidants as the main determinants of non-cancer ocular diseases

The human eye is constantly exposed to sunlight and artificial lighting. Exogenous sources of reactive oxygen species (ROS) such as UV light, visible light, ionizing radiation, chemotherapeutics, and environmental toxins contribute to oxidative damage in ocular tissues. Long-term exposure to these insults places the aging eye at considerable risk for pathological consequences of oxidative stress. Furthermore, in eye tissues, mitochondria are an important endogenous source of ROS. Over time, all ocular structures, from the tear film to the retina, undergo oxidative stress, and therefore, the antioxidant defenses of each tissue assume the role of a safeguard against degenerative ocular pathologies. The ocular surface and cornea protect the other ocular tissues and are significantly exposed to oxidative stress of environmental origin. Overwhelming of antioxidant defenses in these tissues clinically manifests as pathologies including pterygium, corneal dystrophies, and endothelial Fuch's dystrophy. The crystalline lens is highly susceptible to oxidative damage in aging because its cells and their intracellular proteins are not turned over or replaced, thus providing the basis for cataractogenesis. The trabecular meshwork, which is the anterior chamber tissue devoted to aqueous humor drainage, has a particular susceptibility to mitochondrial oxidative injury that affects its endothelium and leads to an intraocular pressure increase that marks the beginning of glaucoma. Photo-oxidative stress can cause acute or chronic retinal damage. The pathogenesis of age-related macular degeneration involves oxidative stress and death of the retinal pigment epithelium followed by death of the overlying photoreceptors. Accordingly, converging evidence indicates that mutagenic mechanisms of environmental and endogenous sources play a fundamental pathogenic role in degenerative eye diseases.

Effects of avastin on expression of AQP4 in Müller cells under hypoxia

The aim of this study was to investigate the effects of Avastin on aquaporin4 (AQP4) expression in human retinal Müller cells in vitro under hypoxia, so as to explore the mechanism of Avastin treating retinal edema. The human Müller cells were cultured using the enzymatic digestion method. Müller cells were identified under the transmission electron microscopy and by using immunofluorescence staining. By using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), the expression of AQP4 mRNA and VEGF mRNA in Müller cells cultured with 500 μmol/L CoCl(2) for 0, 3, 6, 12 and 24 h, and with 0, 100, 300, 500 and 700 μmol/L CoCl(2) for 24 h was detected. The expression of AQP4 mRNA in Müller cells cultured with 50 ng/mL exogenous vascular endothelial growth factor (VEGF) for 0, 0.5, 1, 2 and 4 h, and with 0, 25, 50 and 75 ng/mL VEGF for 24 h was detected. Amplified cDNA products of AQP4 mRNA in Müller cells cultured with 500 μmol/L CoCl(2) and 200 μg/mL Avastin for 24 h were detected. The results showed that more than 95% cells displayed positive immunofluorescence reaction. Characteristic 8-10 nm intracellular filaments could be seen in the cytoplasm under the transmission electron microscopy. In the CoCl(2) experimental groups, the expression of AQP4 mRNA and VEGF mRNA in Müller cells was increased as compared with the control group. Alteration of AQP4 mRNA and VEGF mRNA levels showed a significantly positive correlation (r (2)=0.822, P<0.05). The expression of AQP4 mRNA in Müller cells was increased by VEGF. The expression of AQP4 mRNA was significantly decreased by Avastin as compared with the control group. It is suggested that Avastin can decrease the expression of AQP4 mRNA in human Müller cells under chemical hypoxic conditions partially via VEGF path, which may be one of the mechanisms of Avastin treating retinal edema.

Aquaporin 4 is a Ubiquitously Expressed Isoform in the Dogfish (Squalus acanthias) Shark

The dogfish ortholog of aquaporin 4 (AQP4) was amplified from cDNA using degenerate PCR followed by cloning and sequencing. The complete coding region was then obtained using 5' and 3' RACE techniques. Alignment of the sequence with AQP4 amino acid sequences from other species showed that dogfish AQP4 has high levels (up to 65.3%) of homology with higher vertebrate sequences but lower levels of homology to Agnathan (38.2%) or teleost (57.5%) fish sequences. Northern blotting indicated that the dogfish mRNA was approximately 3.2 kb and was highly expressed in the rectal gland (a shark fluid secretory organ). Semi-quantitative PCR further indicates that AQP4 is ubiquitous, being expressed in all tissues measured but at low levels in certain tissues, where the level in liver > gill >  intestine. Manipulation of the external environmental salinity of groups of dogfish showed that when fish were acclimated in stages to 120% seawater (SW) or 75% SW, there was no change in AQP4 mRNA expression in either rectal gland, kidney, or esophagus/cardiac stomach. Whereas quantitative PCR experiments using the RNA samples from the same experiment, showed a significant 63.1% lower abundance of gill AQP4 mRNA expression in 120% SW-acclimated dogfish. The function of dogfish AQP4 was also determined by measuring the effect of the AQP4 expression in Xenopus laevis oocytes. Dogfish AQP4 expressing-oocytes, exhibited significantly increased osmotic water permeability (P(f)) compared to controls, and this was invariant with pH. Permeability was not significantly reduced by treatment of oocytes with mercury chloride, as is also the case with AQP4 in other species. Similarly AQP4 expressing-oocytes did not exhibit enhanced urea or glycerol permeability, which is also consistent with the water-selective property of AQP4 in other species.

Light modulation, not choroidal vasomotor action, is a regulator of refractive compensation to signed optical blur

BACKGROUND AND PURPOSE

The nitric oxide system has two proposed sites and mechanisms of action within the ocular growth/refractive compensation platform-neuromodulatory effects on retinal physiology, and vascular/smooth muscle effects in the choroid. The relative contribution of these mechanisms are tested here with drugs that perturb the nitric oxide system and with slow flicker modulation of the ON and OFF pathways of the retina.

EXPERIMENTAL APPROACH

Intravitreal injection of saline or 900 nmol N(G) -nitro-L-arginine methyl ester or L-arginine in saline was followed by monocular defocus with ±10 D lens (or no lens), from days 5-9 under standard diurnal (SD) or daytime 1 Hz ramped flicker conditions. Biometric, electrophysiological and histological analyses were conducted.

KEY RESULTS

After 4 days of SD conditions, both drugs enhanced electroretinogram (ERG) b-wave cf. d-wave amplitudes compared with saline and reduced refractive compensation to -10 D lenses. Under flicker conditions compensation to +10 D lenses was suppressed. Choroidal thinning was observed in the drug, no lens groups under SD conditions, whereas choroidal thickening was seen in most groups under flicker conditions, irrespective of refractive outcomes.

CONCLUSIONS AND IMPLICATIONS

As choroidal thickness was not predictive of final refractive compensation across any of the variables of drug, defocus sign or light condition, it is unlikely that choroidal thickness is a primary mechanism underlying refractive compensation across the range of parameters of this study. Rather, the changes in refractive compensation observed under these particular drug and light conditions are more likely due to a neuromodulatory action on retinal ON and OFF pathways.

Aquaporin 4 in the sensory organs of adult zebrafish (Danio rerio)

The aquaporins (AQPs) are a family (AQP-AQP10) of transmembrane channel proteins that mediate the transport of water, ions, gases, and small molecules across the cell membrane, thus regulating cell homeostasis. AQP4 has the highest water permeability and it is involved in hearing and vision in mammals. Here, we used immunohistochemistry to map the presence of AQP4 in the sensory organs of adult zebrafish. The antibody used detected by Western blot proteins of 34 kDa (equivalent to that of mammalian AQP4) and maps in the sensory cells of taste buds, the hair sensory cells of the neuromast and of the maculae, and cristae ampullaris of the inner ear. Moreover, the retinal photoreceptors display AQP4 immunoreactivity. The non-sensory cells in these organs were AQP4 negative. These results suggest that the AQP4 could play a role in the regulation of water balance and ion transport in the sensory cells of zebrafish, bringing new data for the utilizing of this experimental model in the biology of sensory system.