Localization of IgG in the normal and dystrophic rat retina after laser lesions

Retinal Macroglial Responses in Health and Disease

Due to their permanent and close proximity to neurons, glial cells perform essential tasks for the normal physiology of the retina. Astrocytes and Müller cells (retinal macroglia) provide physical support to neurons and supplement them with several metabolites and growth factors. Macroglia are involved in maintaining the homeostasis of extracellular ions and neurotransmitters, are essential for information processing in neural circuits, participate in retinal glucose metabolism and in removing metabolic waste products, regulate local blood flow, induce the blood-retinal barrier (BRB), play fundamental roles in local immune response, and protect neurons from oxidative damage. In response to polyetiological insults, glia cells react with a process called reactive gliosis, seeking to maintain retinal homeostasis. When malfunctioning, macroglial cells can become primary pathogenic elements. A reactive gliosis has been described in different retinal pathologies, including age-related macular degeneration (AMD), diabetes, glaucoma, retinal detachment, or retinitis pigmentosa. A better understanding of the dual, neuroprotective, or cytotoxic effect of macroglial involvement in retinal pathologies would help in treating the physiopathology of these diseases. The extensive participation of the macroglia in retinal diseases points to these cells as innovative targets for new drug therapies.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Blood-derived macrophages infiltrate the retina and activate Muller glial cells under experimental choroidal neovascularization

Inflammation is a major mechanism in the pathogenesis of age-related macular degeneration, the most important cause of blindness in the elderly. Previous studies have focused on the role of macrophages in regulating the growth of pathological new vessels over the retina, called choroidal neovascularization (CNV). However, no research has been done to evaluate the role of inflammation as a mechanism of vision loss and retinal degeneration in the retina underlying CNV. In other neuropathological conditions, hematogenous macrophages and/or resident microglia contribute to neurodegeneration. We have combined laser-induced CNV in mice and bone marrow transplantation with GFP-labeled bone marrow to determine the relative role of recruited blood-derived macrophages versus resident microglia in the retina associated with CNV. Using these chimeric mice, we have found that many GFP-labeled cells infiltrated the retina underlying CNV but not the retina unaffected by CNV. Immunostaining for the cell adhesion molecules VCAM 1, ICAM 1, and PECAM was strongly upregulated in retinal blood vessels under CNV. All GFP-labeled cells were immunoreactive for the macrophage marker F4/80. Most (70%) of the F4/80 immunoreactive cells were GFP-labeled under CNV. The density of resident microglia did not increase. Most GFP-labeled cells were found in close proximity to activated Muller cells. Depleting circulating macrophages with clodronic acid diminished the density of F4/80 immunoreactive cells as well as the density of pERK immunoreactive Muller cells in the retina under CNV. Thus, recruitment of blood-derived macrophages more than resident microglia seems to be associated with CNV.

Photoreceptor synapses degenerate early in experimental choroidal neovascularization

Severe visual loss in patients with age-related macular degeneration is associated with the development of choroidal neovascularization (CNV). The pathogenic mechanisms for CNV formation have been extensively investigated, but remarkably little research has addressed the mechanisms for dysfunction of the retina in CNV. Using laser-induced CNV in mice, we evaluated the mechanisms of retinal dysfunction. At 3 days, 1 week, 2 weeks, and 4 weeks after laser application, retinas under experimental CNV were characterized physiologically (ERG recordings, synaptic uptake of the exocytotic marker FM1-43, and light-induced translocation of transducin), histologically, and immunohistochemically. ERG amplitudes were reduced by 20% at 1 week after CNV. Depolarization-induced FM1-43 uptake in photoreceptor synapses was selectively reduced by 45% at 1 week after CNV. Although photoreceptor outer segments were shortened by 36%, light adaptation as measured by transducin translocation was mostly preserved. Early in CNV (3 days to 1 week), Muller cells demonstrated induction of c-fos and pERK expression. Also, the density of macrophage-like, F4/80 immunoreactive cells increased approximately 3-fold. Minimal photoreceptor death occurred during the first week, and was variable thereafter. At later times in CNV formation (> or =2 weeks), expression of photoreceptor synaptic markers was reduced in the outer plexiform layer, indicating loss of photoreceptor synaptic terminals. ERG amplitudes, synaptic uptake of FM1-43, and the induction of c-fos and pERK in Muller cells were altered within 1 week of experimental CNV, suggesting that during CNV formation, deficits in retinal function, in particular photoreceptor synaptic function, precede degeneration of photoreceptor terminals and photoreceptor cell death.