Muller glial cells span the entire thickness of the tissue, and ensheath all retinal neurons, in vertebrate retinae of all species. This morphological relationship is reflected by a multitude of functional interactions between neurons and Muller cells, including a 'metabolic symbiosis' and the processing of visual information. Muller cells are also responsible for the maintenance of the homeostasis of the retinal extracellular milieu (ions, water, neurotransmitter molecules, and pH). In vascularized retinae, Muller cells may also be involved in the control of angiogenesis, and the regulation of retinal blood flow. Virtually every disease of the retina is associated with a reactive Muller cell gliosis which, on the one hand, supports the survival of retinal neurons but, on the other hand, may accelerate the progress of neuronal degeneration: Muller cells protect neurons via a release of neurotrophic factors, the uptake and degradation of the excitotoxin, glutamate, and the secretion of the antioxidant, glutathione. However, gliotic Muller cells display a dysregulation of various neuron-supportive functions. This contributes to a disturbance of retinal glutamate metabolism and ion homeostasis, and causes the development of retinal edema and neuronal cell death. Moreover, there are diseases evoking a primary Muller cell insufficiency, such as hepatic retinopathy and certain forms of glaucoma. Any impairment of supportive functions of Muller cells, primary or secondary, must cause and/or aggravate a dysfunction and loss of neurons, by increasing the susceptibility of neurons to stressful stimuli in the diseased retina. On the contrary, Muller cells may be used in the future for novel therapeutic strategies to protect neurons against apoptosis (somatic gene therapy), or to differentiate retinal neurons from Muller/stem cells. Meanwhile, a proper understanding of the gliotic responses of Muller cells in the diseased retina, and of their protective vs. detrimental effects, is essential for the development of efficient therapeutic strategies that use and stimulate the neuron-supportive/protective-and prevent the destructive-mechanisms of gliosis.

https://www.springer.com/cda/content/document/productFlyer/productFlyer_978-1-4419-1671-6.pdf?SGWID=0-0-1297-173938007-0

Müller Cells in the Healthy and Diseased Retina

Müller cells in the healthy and diseased retina

Blood–retinal barrier in hypoxic ischaemic conditions: Basic concepts, clinical features and management

Purpose To evaluate the impact of photodynamic therapy (PDT) on expression and distribution of vascular endothelial growth factor (VEGF), VEGF receptor (VEGFR)-3, and pigment epithelium-derived factor (PEDF). Methods Eyes of patients scheduled for enucleation due to untreatable malignancy served as study eyes (n = 4), age-matched donor eyes were used as the control (n = 4). PDT using verteporfin with the recommended standard parameters was applied to intact areas of the perimacular region. Lesions were classified by ophthalmoscopy, fluorescein angiography (FA), and indocyanine green angiography (ICGA), as well as light and electron microscopic (LM/EM) histology. Immunolabeling using specific antibodies against VEGF, VEGFR-3, and PEDF was performed in PDT-treated areas, untreated collateral areas in study eyes, and untreated areas of control eyes. Specimens were fixed in 4% paraformaldehyde and 1% glutaraldehyde and embedded in paraffin. Four-micrometer-thick sections were stained using the peroxidase-labeled streptavidin-biotin method. Results All PDT-treated areas demonstrated characteristic choroidal hypofluorescence by FA and ICGA. LM/EM histology revealed selective damage of choriocapillary endothelial cells. VEGF was expressed in the endothelial layer of choriocapillaries and focally within larger choroidal vessels in treated areas, but not in untreated areas. Sites with positive VEGF labeling also demonstrated upregulation of VEGFR-3. PEDF expression was localized to retinas in all eyes; however, PEDF staining of choroidal endothelial cells was specific for treated areas of study eyes. Conclusions PDT using verteporfin induces a reproducible angiogenic response in elderly human eyes. VEGF, VEGFR-3, and PEDF expression is enhanced after PDT. Choroidal endothelial cells appear to be the primary site of angiogenic stimulation.

Influence of Photodynamic Therapy on Expression of Vascular Endothelial Growth Factor (VEGF), VEGF Receptor 3, and Pigment Epithelium–Derived Factor

Under normal conditions, Muller cells support neuronal activity and the integrity of the blood-retinal barrier, whereas gliotic alterations of Muller cells under pathological conditions may contribute to retinal degeneration and edema formation. A major function of Muller cells is the fluid absorption from the retinal tissue, which is mediated by transcellular water transport coupled to currents through potassium channels. Alterations of retinal Muller cells under pathological conditions were investigated by immunohistochemistry and recording their behavior under osmotic stress. In animal models of various retinopathies, e.g., retinal ischemia, ocular inflammation, retinal detachment, and diabetes, it was found that Muller cells decrease the expression of their major potassium channel (Kir4.1). This alteration is associated with an impairment of the rapid water transport across Muller cell membranes, as recognizable in the induction of cellular swelling under hypoosmolar conditions. Osmotic swelling of Muller cells is also induced by oxidative stress and by inflammatory mediators such as arachidonic acid and prostaglandins. The data suggest that a disturbed fluid transport through Muller cells is (in addition to vascular leakage) a pathogenic factor contributing to the development of retinal edema. Pharmacological re-activation of the retinal water clearance by Muller cells may represent an approach to the development of new edema-resolving drugs. Triamcinolone acetonide, which is clinically used to resolve edema, prevents osmotic swelling of Muller cells as it induces the release of endogenous adenosine and subsequent A1 receptor activation which results in the opening of ion channels. Apparently, triamcinolone resolves edema by both inhibition of vascular leakage and stimulation of retinal fluid clearance by Muller cells.

Müller cells as players in retinal degeneration and edema.

PURPOSE To determine the effect of pigment epithelium-derived factor (PEDF) in a mouse model of ischemia-induced retinal neovascularization and on vascular endothelial growth factor (VEGF)--induced migration and growth of cultured microvascular endothelial cells. METHODS Human recombinant PEDF was expressed in the human embryonic kidney 293 cell line and purified by ammonium sulfate precipitation and cation exchange chromatography. C57BL/6 mice were exposed to 75% oxygen from postnatal day (P)7 to P12 and then returned to room air. Mice received intravitreal injections of 2 microg PEDF in one eye and vehicle in the contralateral eye on P12 and P14. At P17, mice were killed and eyes enucleated for quantitation of retinal neovascularization. The mitogenic and motogeneic effects of VEGF on cultured bovine retinal and adrenal capillary endothelial cells were examined in the presence or absence of PEDF, using cell counts and migration assays. RESULTS Two species of human recombinant PEDF, denoted A and B, were purified to apparent homogeneity. PEDF B appeared to comigrate on SDS-PAGE with PEDF from human vitreous samples. Changes in electrophoretic mobility after peptide-N-glycosidase F (PNGase F) digestion suggest that both PEDF forms contain N-linked carbohydrate. Analyses of the intact proteins by liquid chromatography-electrospray mass spectrometry (LC-ESMS) revealed the major molecular weight species for PEDF A (47,705 +/- 4) and B (46,757 +/- 5). LC-ESMS analysis of tryptic peptides indicated that PEDF A and B exhibit differences in glycopeptides containing N-acetylneuraminic acid (NeuAc) and N-acetylhexosamine (HexNAc). Intravitreal administration of either species of PEDF significantly inhibited retinal neovascularization (83% for PEDF A and 55% for PEDF B; P = 0.024 and 0.0026, respectively). PEDF A and B (20 nM) suppressed VEGF-induced retinal microvascular endothelial cell proliferation by 48.8% and 41.4%, respectively, after 5 days (P < 0.001) and VEGF-induced migration by 86.5% +/- 16.7% and 78.1% +/- 22.3%, respectively, after 4 hours (P = 0.004 and P = 0.008, respectively). CONCLUSIONS These data indicate that elevated concentrations of PEDF inhibit VEGF-induced retinal endothelial cell growth and migration and retinal neovascularization. These findings suggest that localized administration of PEDF may be an effective approach for the treatment of ischemia-induced retinal neovascular disorders.

Pigment epithelium-derived factor suppresses ischemia-induced retinal neovascularization and VEGF-induced migration and growth.

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Pigment epithelium-derived factor suppresses ischemia-induced retinal neovascularization and VEGF-induced migration and growth.