Expression of glial fibrillary acidic protein in primary cultures of human Müller cells

Müller cell degeneration and microglial dysfunction in the Alzheimer's retina

Amyloid beta (Aβ) deposits in the retina of the Alzheimer's disease (AD) eye may provide a useful diagnostic biomarker for AD. This study focused on the relationship of Aβ with macroglia and microglia, as these glial cells are hypothesized to play important roles in homeostasis and clearance of Aβ in the AD retina. Significantly higher Aβ load was found in AD compared to controls, and specifically in the mid-peripheral region. AD retina showed significantly less immunoreactivity against glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) compared to control eyes. Immunoreactivity against ionized calcium binding adapter molecule-1 (IBA-1), a microglial marker, demonstrated a higher level of microgliosis in AD compared to control retina. Within AD retina, more IBA-1 immunoreactivity was present in the mid-peripheral retina, which contained more Aβ than the central AD retina. GFAP co-localized rarely with Aβ, while IBA-1 co-localized with Aβ in more layers of control than AD donor retina. These results suggest that dysfunction of the Müller and microglial cells may be key features of the AD retina.

Downregulation of Retinal Connexin 43 in GFAP-Expressing Cells Modifies Vasoreactivity Induced by Perfusion Ocular Pressure Changes

Purpose

Glia and their communication via connexin 43 (Cx43) gap junctions are known to mediate neurovascular coupling, a process driven by metabolic demand. However, it is unclear whether Cx43 mediated glial communication intermediates classical autoregulation. Here we used viral transfection and a glial fibrillary acidic protein (GFAP) promoter to downregulate glial Cx43 to evaluate its role in retinal vascular autoregulation to ocular perfusion pressure (OPP) reduction.

Methods

Adult rats were intravitreally injected with the viral active construct or a control. Three weeks after the injection, eyes were imaged using confocal scanning laser ophthalmoscopy before and during a period of OPP decrease induced by blood draw to lower blood pressure or by manometric IOP elevation. Vessel diameter responses to the OPP decrease were compared between Cx43-downregulated and control-injected eyes. The extent of Cx43 downregulation was evaluated by Western blot and immunohistochemistry.

Results

In control eyes, the OPP decrease induced dilatation of arterioles, but not venules. In Cx43-downregulated eyes, Cx43 expression in whole retina was decreased by approximately 40%. In these eyes, the resting diameter of the venules increased significantly, but there was no effect on arterioles. In Cx43-downregulated eyes, vasoreactivity evoked by blood pressure lowering was significantly compromised in both arterioles (P = 0.005) and venules (P = 0.001). Cx43 downregulation did not affect the arteriole responses to IOP elevation, whereas the responses of the venules showed a significantly greater decrease in diameter (P < 0.001).

Conclusions

The downregulation of retinal Cx43 in GFAP–expressing cells compromises vasoreactivity of both arterioles and venules in response to an OPP decrease achieved via blood pressure lowering or IOP elevation. The results also suggest that Cx43-mediated glial communication actively regulates resting venular diameter.

NRL-/- gene edited human embryonic stem cells generate rod-deficient retinal organoids enriched in S-cone-like photoreceptors

Organoid cultures represent a unique tool to investigate the developmental complexity of tissues like the human retina. NRL is a transcription factor required for the specification and homeostasis of mammalian rod photoreceptors. In Nrl-deficient mice, photoreceptor precursor cells do not differentiate into rods, and instead follow a default photoreceptor specification pathway to generate S-cone-like cells. To investigate whether this genetic switch mechanism is conserved in humans, we used CRISPR/Cas9 gene editing to engineer an NRL-deficient embryonic stem cell (ESC) line (NRL-/- ), and differentiated it into retinal organoids. Retinal organoids self-organize and resemble embryonic optic vesicles (OVs) that recapitulate the natural histogenesis of rods and cone photoreceptors. NRL-/- OVs develop comparably to controls, and exhibit a laminated, organized retinal structure with markers of photoreceptor synaptogenesis. Using immunohistochemistry and quantitative polymerase chain reaction (qPCR), we observed that NRL-/- OVs do not express NRL, or other rod photoreceptor markers directly or indirectly regulated by NRL. On the contrary, they show an abnormal number of photoreceptors positive for S-OPSIN, which define a primordial subtype of cone, and overexpress other cone genes indicating a conserved molecular switch in mammals. This study represents the first evidence in a human in vitro ESC-derived organoid system that NRL is required to define rod identity, and that in its absence S-cone-like cells develop as the default photoreceptor cell type. It shows how gene edited retinal organoids provide a useful system to investigate human photoreceptor specification, relevant for efforts to generate cells for transplantation in retinal degenerative diseases.

Fatty acid oxidation and photoreceptor metabolic needs

Photoreceptors have high energy demands and a high density of mitochondria that produce ATP through oxidative phosphorylation (OXPHOS) of fuel substrates. Although glucose is the major fuel for CNS brain neurons, in photoreceptors (also CNS), most glucose is not metabolized through OXPHOS but is instead metabolized into lactate by aerobic glycolysis. The major fuel sources for photoreceptor mitochondria remained unclear for almost six decades. Similar to other tissues (like heart and skeletal muscle) with high metabolic rates, photoreceptors were recently found to metabolize fatty acids (palmitate) through OXPHOS. Disruption of lipid entry into photoreceptors leads to extracellular lipid accumulation, suppressed glucose transporter expression, and a duel lipid/glucose fuel shortage. Modulation of lipid metabolism helps restore photoreceptor function. However, further elucidation of the types of lipids used as retinal energy sources, the metabolic interaction with other fuel pathways, as well as the cross-talk among retinal cells to provide energy to photoreceptors is not fully understood. In this review, we will focus on the current understanding of photoreceptor energy demand and sources, and potential future investigations of photoreceptor metabolism.

Involvement of the Retinal Pigment Epithelium in the Development of Retinal Lattice Degeneration

Lattice degeneration involves thinning of the retina that occurs over time. Here we performed an immunohistological study of tissue sections of human peripheral retinal lattice degeneration to investigate if retinal pigment epithelium (RPE) cells are involved in the pathogenesis of this condition. In two cases of retinal detachment with a large tear that underwent vitreous surgery, retinal lattice degeneration tissue specimens were collected during surgery. In the obtained specimens, both whole mounts and horizontal section slices were prepared, and immunostaining was then performed with hematoxylin and antibodies against glial fibrillary acidic protein (GFAP), RPE-specific protein 65 kDa (RPE65), pan-cytokeratin (pan-CK), and CK18. Hematoxylin staining showed no nuclei in the center of the degenerative lesion, thus suggesting the possibility of the occurrence of apoptosis. In the degenerative lesion specimens, GFAP staining was observed in the center, RPE65 staining was observed in the slightly peripheral region, and pan-CK staining was observed in all areas. However, no obvious CK18 staining was observed. In a monkey retina used as the control specimen of a normal healthy retina, no RPE65 or pan-CK staining was observed in the neural retina. Our findings suggest that migration, proliferation, and differentiation of RPE cells might be involved in the repair of retinal lattice degeneration.

Biomarkers in retinoblastoma

Retinoblastoma (RB) is the most common intraocular malignancy of childhood caused by inactivation of the Rb genes. The prognosis of RB is better with an earlier diagnosis. Many diagnostic approaches and appropriate clinical treatments have been developed to improve clinical outcomes. However, limitations exist when utilizing current methods. Recently, many studies have identified identify new RB biomarkers which can be used in diagnosis, as prognostic indicators and may contribute to understanding the pathogenesis of RB and help determine specific treatment strategies. This review focuses on recent advances in the discovery of RB biomarkers and discusses their clinical utility and challenges from areas such as epigenetics, proteomics and radiogenomics.

Focused ultrasound as a novel strategy for noninvasive gene delivery to retinal Müller glia

Müller glia are specialized retinal cells with stem cell properties in fish and frogs but not in mammals. Current efforts to develop gene therapies to activate mammalian Müller glia for retinal repair will require safe and effective delivery strategies for recombinant adeno-associated viruses (AAVs), vectors of choice for clinical translation. Intravitreal and subretinal injections are currently used for AAV gene delivery in the eye, but less invasive methods efficiently targeting Müller glia have yet to be developed. Methods: As gene delivery strategies have been more extensively studied in the brain, to validate our vectors, we initially compared the glial tropism of AAV-PHP.eB, an AAV9 that crosses the blood-brain and blood-retinal barriers, for its ability to drive fluorescent protein expression in glial cells in both the brain and retina. We then tested the glial transduction of AAV2/8-GFAP-mCherry, a virus that does not cross blood-brain and blood-retinal barriers, for its effectiveness in transducing Müller glia in murine retinal explants ex vivo. For in vivo assays we used larger rat eyes, performing invasive intravitreal injections, and non-invasive intravenous delivery using focused ultrasound (FUS) (pressure amplitude: 0.360 - 0.84 MPa) and microbubbles (Definity, 0.2 ml/kg). Results: We showed that AAV-PHP.eB carrying a ubiquitous promoter (CAG) and green fluorescent protein (GFP) reporter, readily crossed the blood-brain and blood-retinal barriers after intravenous delivery in mice. However, murine Müller glia did not express GFP, suggesting that they were not transduced by AAV-PHP.eB. We thus tested an AAV2/8 variant, which was selected based on its safety record in multiple clinical trials, adding a glial fibrillary acidic protein (GFAP) promoter and mCherry (red fluorescent protein) reporter. We confirmed the glial specificity of AAV2/8-GFAP-mCherry, showing effective expression of mCherry in astrocytes after intracranial injection in the mouse brain, and of Müller glia in murine retinal explants. For in vivo experiments we switched to rats because of their larger size, injecting AAV2/8-GFAP-mCherry intravitreally, an invasive procedure, demonstrating passage across the inner limiting membrane, leading to Müller glia transduction. We then tested an alternative non-invasive delivery approach targeting a different barrier - the inner blood-retinal-barrier, applying focused ultrasound (FUS) to the retina after intravenous injection of AAV2/8 and microbubbles in rats, using magnetic resonance imaging (MRI) for FUS targeting. FUS permeabilized the rat blood-retinal-barrier and allowed the passage of macromolecules to the retina (Evans blue, IgG, IgM), with minimal extravasation of platelets and red blood cells. Intravenous injection of microbubbles and AAV2/8-GFAP-mCherry followed by FUS resulted in mCherry expression in rat Müller glia. However, systemic delivery of AAV2/8 also had off-target effects, transducing several murine peripheral organs, particularly the liver. Conclusions: Retinal permeabilisation via FUS in the presence of microbubbles is effective for delivering AAV2/8 across the inner blood-retinal-barrier, targeting Müller glia, which is less invasive than intravitreal injections that bypass the inner limiting membrane. However, implementing FUS in the clinic will require a comprehensive consideration of any off-target tropism of the AAV in peripheral organs, combined ideally, with the development of Müller glia-specific promoters.

Differentiation of Retinal Glial Cells From Human Embryonic Stem Cells by Promoting the Notch Signaling Pathway

Dysfunction of retinal glial cells, particularly Müller cells, has been implicated in several retinal diseases. Despite their important contribution to retinal homeostasis, a specific way to differentiate retinal glial cells from human pluripotent stem cells has not yet been described. Here, we report a method to differentiate retinal glial cells from human embryonic stem cells (hESCs) through promoting the Notch signaling pathway. We first generated retinal progenitor cells (RPCs) from hESCs then promoted the Notch signaling pathway using Notch ligands, including Delta-like ligand 4 and Jagged-1. We validated glial cell differentiation with qRT-PCR, immunocytochemistry, western blots and fluorescence-activated cell sorting as we promoted Notch signaling in RPCs. We found that promoting Notch signaling in RPCs for 2 weeks led to upregulation of glial cell markers, including glial fibrillary acidic protein (GFAP), glutamine synthetase, vimentin and cellular retinaldehyde-binding protein (CRALBP). Of these markers, we found the greatest increase in expression of the pan glial cell marker, GFAP. Conversely, we also found that inhibition of Notch signaling in RPCs led to upregulation of retinal neuronal markers including cone-rod homeobox (CRX) and orthodenticle homeobox 2 (OTX2) but with little expression of GFAP. This retinal glial differentiation method will help advance the generation of stem cell disease models to study the pathogenesis of retinal diseases associated with glial dysfunction such as macular telangiectasia type 2. This method may also be useful for the development of future therapeutics such as drug screening and gene editing using patient-derived retinal glial cells.

Viability of Mouse Retinal Explant Cultures Assessed by Preservation of Functionality and Morphology

Purpose

Retinal explant cultures provide simplified systems where the functions of the retina and the effects of ocular therapies can be studied in an isolated environment. The purpose of this study was to provide insight into long-term preservation of retinal tissue in culture conditions, enable a deeper understanding of the interdependence of retinal morphology and function, and ensure the reliability of the explant technique for prolonged experiments.

Methods

Retinal explants from adult mice were cultured as organotypic culture at the air-medium interface for 14 days in vitro (DIV). Retinal functionality was assessed by multielectrode array technique and morphology by immunohistochemical methods at several time points during culture.

Results

Retinal explants retained viability for 14 DIV, although with diminishing neuronal activity, progressing neuronal loss, and increasing reactive gliosis. We recorded spontaneous retinal ganglion cell (RGC) activity up to 14 DIV with temporally changing distribution of RGC firing rates. Light responsiveness was measurable from RGCs for 7 DIV and from photoreceptors for 2 DIV. Apoptotic cells were detected beginning at 3 DIV with their density peaking at 7 DIV. The number of RGCs gradually decreased by 70% during 14 DIV. The change was accompanied by the loss of RGC functionality, resulting in 84% loss of electrically active RGCs.

Conclusions

Retinal explants provide a valuable tool for studies of retinal functions and development of ocular therapies. However, critical for long-term use, retinal functionality was lost before structural loss, emphasizing a need for both functional and morphologic readouts to determine the overall state of the cultured retina.

HIF-1α stabilization reduces retinal degeneration in a mouse model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of inherited retinal dystrophies characterized by progressive and irreversible loss of vision due to rod and cone degeneration. Evidence suggests that an inappropriate oxygen level could contribute to its pathogenesis. Rod cell death could increase oxygen concentration, reduce hypoxia-inducible factor 1 (HIF-1α) and contribute to cone cell death. The purposes of this study were: 1) to analyze the temporal profile of HIF-1α, its downstream effectors VEGF, endothelin-1 (ET-1), iNOS, and glucose transporter 1 (GLUT1), and neuroinflammation in retinas of the murine model of rd10 ( retinal degeneration 10) mice with RP; 2) to study oxygen bioavailability in these retinas; and 3) to investigate how stabilizing HIF-1α proteins with dimethyloxaloglycine (DMOG), a prolyl hydroxylase inhibitor, affects retinal degeneration, neuroinflammation, and antioxidant response in rd10 mice. A generalized down-regulation of HIF-1α and its downstream targets was detected in parallel with reactive gliosis, suggesting high oxygen levels during retinal degeneration. At postnatal d 18, DMOG treatment reduced photoreceptor cell death and glial activation. In summary, retinas of rd10 mice seem to be exposed to a hyperoxic environment even at early stages of degeneration. HIF-1α stabilization could have a temporal neuroprotective effect on photoreceptor cell survival, glial activation, and antioxidant response at early stages of RP.-Olivares-González, L., Martínez-Fernández de la Cámara, C., Hervás, D., Millán, J. M., Rodrigo, R. HIF-1α stabilization reduces retinal degeneration in a mouse model of retinitis pigmentosa.

Identification of differentially expressed proteins in retinoblastoma tumors using mass spectrometry-based comparative proteomic approach

In India, retinoblastoma is among the top five childhood cancers. Children mostly present with extraocular extension and high risk features that results in unsatisfactory treatment and low survival rate. In addition, lack of potential therapeutic and prognostic targets is another challenge in the management of retinoblastoma. We studied comparative proteome of retinoblastoma patients (HPV positive and negative (n=4 each) and controls (n=4), in order to identify potential retinoblastoma-specific protein targets. 2D-DIGE coupled MALDI-TOF/TOF mass spectrometry identified 39 unique proteins. Highly deregulated proteins were GFAP,RBP3,APOA1,CRYAA,CRABP1,SAG and TF. Gene ontology (Panther 7.0) revealed majority of proteins to be associated with metabolic processes (26%) and catalytic activity (38%). 8 proteins were significantly upregulated in HPV positive vis-a-vis HPV negative cases. Patient group exhibited 12 upregulated and 18 downregulated proteins compared to controls. Pathway and network analysis (IPA software) revealed CTNNB1 as most significantly regulated signalling pathway in HPV positive than HPV negative retinoblastoma. The trends in transcriptional change of 9 genes were consistent with those at proteomic level. The Western blot analysis confirmed the expression pattern of RBP3,GFAP and CRABP1. We suggest GFAP,RBP3,CRABP1,CRYAAA,APOA1 and SAG as prospective targets that could further be explored as potential candidates in therapy and may further assist in studying the disease mechanism.

SIGNIFICANCE

In this study we evaluated tumor tissue specimens from retinoblastoma patients and identified 39 differentially regulated proteins compared to healthy retina. From these, we propose RBP3, CRABP1, GFAP, CRYAA, APOA1 and SAG as promising proteomic signatures that could further be explored as efficient prognostic and therapeutic targets in retinoblastoma. The present study is not only a contribution to the ongoing endeavour for the discovery of proteomic signatures in retinoblastoma, but, may also act as a starting point for future studies aimed at uncovering novel targets for further therapeutic interventions and improving patient outcomes.

Epigenetic regulation of glial fibrillary acidic protein by DNA methylation in human malignant gliomas

Glial fibrillary acidic protein (GFAP) is an intermediate filament expressed in glial cells that stabilizes and maintains the cytoskeleton of normal astrocytes. In glial tumors, GFAP expression is frequently lost with increasing grade of malignancy, suggesting that GFAP is important for maintaining glial cell morphology or regulating astrocytoma cell growth. Most permanent human glioma cell lines are GFAP negative by immunocytochemistry. Given that the GFAP gene is not mutated in human glioma specimens or glioma cell lines, we considered epigenetic mechanisms, such as promoter methylation, as a cause of silencing of GFAP in these tumors. In this study, we treated known GFAP-negative glioma cell lines with 5-aza-2'-deoxycytidine to examine GFAP promoter hypermethylation. Additionally, we performed bisulfite sequencing on primary glioma samples and glioma cell lines and showed an inverse relationship between GFAP promoter methylation status and GFAP expression. Using a gene reporter assay with the GFAP promoter cloned upstream of a luciferase gene, we showed that methylation of the GFAP promoter downregulates the expression of the luciferase gene. Our results suggest that epigenetic silencing of the GFAP gene through DNA methylation of its promoter region may be one mechanism by which GFAP is downregulated in human gliomas and glioma cell lines.

A conditional immortalized mouse muller glial cell line expressing glial and retinal stem cell genes

PURPOSE

Müller glia have multiple functions in the retina, including synthesis of neurotrophic factors, uptake and metabolism of neurotransmitters, spatial buffering of ions, maintenance of the blood-retinal barrier, and response to injury. A population of Müller glia has some stem cell-like characteristics both in vivo and in vitro. The purpose of this study was to generate and characterize novel Müller glial cell lines from the postnatal mouse retina.

METHODS

Cells were cultured from postnatal day (P) 10 double heterozygous transgenic (H-2K(b)-tsA58/+; HRhoGFP/+) or C57BL/6 mice after papain dissociation. Interferon gamma (IFNγ) induction of the SV40 T-antigen (TAg) was assayed by immunohistochemistry and Western blot analysis. Proliferation was assayed by BrdU uptake and cell counts of calcein AM/ethidium bromide-stained cells. Gene expression was analyzed by RT-PCR and immunohistochemistry.

RESULTS

Conditionally immortalized (ImM10 [Immortmouse Müller P10]) and spontaneously immortalized (C57M10 [C57BL/6 Müller P10]) Müller glial cell lines were selected by differential adherence to laminin; both consisted of adherent flat cells with large, diffusely staining nuclei and an epithelial morphology. TAg induction stimulated BrdU uptake by Müller glia in mixed retinal cultures from H-2K(b)-tsA58/+; HRhoGFP/+ mice and increased the proliferation of ImM10 cells. ImM10 and C57M10 cells expressed genes characteristic of Müller glia but not genes characteristic of differentiated retinal neurons. ImM10 cells also expressed retinal stem cell genes.

CONCLUSIONS

The ImM10 cell line is a novel, conditionally immortalized Müller glial cell line isolated from the P10 mouse retina that expresses genes characteristic of Müller glial and retinal stem cells.

Hypoxia inducible factor-1α (HIF-1α) and some HIF-1 target genes are elevated in experimental glaucoma

Low levels of hypoxia have been suggested to be a mechanism of retinal damage in glaucoma. To test the hypothesis that the activation of the hypoxia-responsive transcription factor hypoxia inducible factor-1alpha (HIF-1alpha) is involved in the pathophysiology of glaucoma, we used a rat model of glaucoma to study (1) HIF-1alpha retinal protein levels by immunoblot analysis, (2) cellular localization of HIF-1alpha in the retina by immunohistochemistry, and (3) expression of retinal HIF-1 gene targets by quantitative real-time polymerase chain reaction. Glaucoma was unilaterally induced in rats by injecting hypertonic saline in episcleral veins. We find that HIF-1alpha protein was increased in the retina following elevation of intraocular pressure, specifically in Müller glia and astrocytes but not in activated microglia. Eight established HIF-1 target genes were measured in experimental glaucoma. Retinal Epo, Flt-1, Hsp-27, Pai-1, and Vegfa mRNA levels were increased and Et-1, Igf2, and Tgfbeta3 levels were decreased in the glaucomatous retinas. Thus, the increase in HIF-1alpha levels in Müller glia and astrocytes is accompanied by a marked up regulation of some, but not all, HIF-1 transcriptional targets. These data support the hypothesis that HIF-1alpha becomes transcriptionally active in astrocytes and Müller cells but not microglia or neurons in glaucoma, arguing against a global hypoxia stimulus to the retina.

Profiling of dynamically changed gene expression in dorsal root ganglia post peripheral nerve injury and a critical role of injury-induced glial fibrillary acidic protein in maintenance of pain behaviors [corrected]

To explore cellular changes in sensory neurons after nerve injury and to identify potential target genes contributing to different stages of neuropathic pain development, we used Affymetrix oligo arrays to profile gene expression patterns in L5/6 dorsal root ganglia (DRG) from the neuropathic pain model of left L5/6 spinal nerve ligation at different stages of neuropathic pain development. Our data indicated that nerve injury induced changes in expression of genes with similar biological functions in a temporal specific manner that correlates with particular stages of neuropathic pain development, indicating dynamic neuroplasticity in the DRG in response to peripheral nerve injury and during neuropathic pain development. Data from post-array validation indicated that there was a temporal correlation between injury-induced expression of the glial fibrillary acidic protein (GFAP), a marker for activated astrocytes, and neuropathic pain development. Spinal nerve ligation injury in GFAP knockout mice resulted in neuropathic pain states with similar onset, but a shortened duration compared with that in age, and gender-matched wild-type littermates. Intrathecal GFAP antisense oligonucleotide treatment in injured rats with neuropathic pain states reversed injury-induced behavioral hypersensitivity and GFAP upregulation in DRG and spinal cord. Together, these findings indicate that injury-induced GFAP upregulation not only serves as a marker for astrocyte activation, but it may also play a critical, but yet identified, role in the maintenance of neuropathic pain states.

Blood–retinal barrier disruption and ultrastructural changes in the hypoxic retina in adult rats: the beneficial effect of melatonin administration

Reactive changes in astrocytes and Müller cells in the retina of adult rats subjected to hypoxia were investigated. Along with this, the integrity of the blood-retinal barrier (BRB) was assessed using fluorescent and electron-dense tracers. In hypoxic rats, mRNA and protein expression of glial fibrillary acidic protein (GFAP) and aquaporin-4 (AQ4) were significantly increased. AQ4 immunoreactive cells were identified as astrocytes and Müller cells by double immunofluorescence labelling. Another alteration in the hypoxic retina was marked reduction in melatonin content compared to controls. In this connection, administration of exogenous melatonin reduced the tissue concentration of vascular endothelial growth factor (VEGF) and nitric oxide (NO); both were elevated in hypoxic rats. A major structural change in the hypoxic retina was swelling of astrocyte and Müller cell processes but this was noticeably attenuated after melatonin administration. Following an intraperitoneal or intravenous injection of rhodamine isothiocyanate (RhIC) or horseradish peroxidase (HRP), leakage of both tracers was observed in the retina in hypoxic rats but not in the controls, indicating that the functional integrity of the BRB is compromised in hypoxia/reoxygenation. It is suggested that enhanced tissue concentration of VEGF and NO production in the hypoxic retina contribute to increased permeability of the retinal blood vessels. The concurrent up-regulation of AQ4, a water-transporting protein, in astrocytes and Müller cells in hypoxia suggests its involvement in oedema formation. Since melatonin effectively reduced the vascular permeability in the retina of hypoxic rats, as evidenced by reduced leakage of RhIC, we suggest that its administration may be of potential benefit in the management of retinal oedema associated with retinal hypoxia.