Isolation and culture of retinal microglia

PPAR-γ promotes the polarization of rat retinal microglia to M2 phenotype by regulating the expression of CD200-CD200R1 under hypoxia

BACKGROUND

Recent reports suggest that peroxisome proliferator-activated receptor-γ (PPAR-γ) could promote microglial M2 polarization to inhibit inflammation. However, the specific molecular mechanisms that trigger PPAR-γ's anti-inflammatory ability in microglia are yet to be expounded. Thus, in this study, we aimed to explore the molecular mechanisms behind the anti-inflammatory effects of PPAR-γ in hypoxia-stimulated rat retinal microglial cells.

METHODS AND RESULTS

We used shRNA expressing lentivirus to knock down PPAR-γ and CD200 genes, and we assessed hypoxia-induced polarization markers release - M1 (iNOS, IL-1β, IL-6, and TNF-α) and M2 (Arg-1, YM1, IL-4, and IL-10) by RT-PCR. We also monitored PPAR-γ-related signals (PPAR-γ, PPAR-γ in cytoplasm or nucleus, CD200, and CD200Rs) by Western blot and RT-PCR. Our results showed that hypoxia enhanced PPAR-γ and CD200 expressions in microglial cells. Moreover, PPAR-γ agonist 15d-PGJ2 elevated CD200 and CD200R1 expressions, whereas sh-PPAR-γ had the opposite effect. Following hypoxia, expressions of M1 markers increased significantly, while those of M2 markers decreased, and the above effects were attenuated by 15d-PGJ2. Conversely, knocking down PPAR-γ or CD200 inhibited the polarization of microglial cells to M2 phenotype.

CONCLUSION

Our findings demonstrated that PPAR-γ performed an anti-inflammatory function in hypoxia-stimulated microglial cells by promoting their polarization to M2 phenotype via the CD200-CD200R1 pathway.

Ginsenoside Rg1 attenuates cerebral ischemia-reperfusion injury through inhibiting the inflammatory activation of microglia

It is recognized that the cerebral ischemia/reperfusion (I/R) injury triggers inflammatory activation of microglia and supports microglia-driven neuronal damage. Our previous studies have shown that ginsenoside Rg1 had a significant protective effect on focal cerebral I/R injury in middle cerebral artery occlusion (MCAO) rats. However, the mechanism still needs further clarification. Here, we firstly reported that ginsenoside Rg1 effectively suppressed the inflammatory activation of brain microglia cells under I/R conditions depending on the inhibition of Toll-likereceptor4 (TLR4) proteins. In vivo experiments showed that the ginsenoside Rg1 administration could significantly improve the cognitive function of MCAO rats, and in vitro experimental data showed that ginsenoside Rg1 significantly alleviated neuronal damage via inhibiting the inflammatory response in microglia cells co-cultured under oxygen and glucose deprivation/reoxygenation (OGD/R) condition in gradient dependent. The mechanism study showed that the effect of ginsenoside Rg1 depends on the suppression of TLR4/MyD88/NF-κB and TLR4/TRIF/IRF-3 pathways in microglia cells. In a word, our research shows that ginsenoside Rg1 has great application potential in attenuating the cerebral I/R injury by targeting TLR4 protein in the microglia cells.

The role of NAD+ metabolism in macrophages in age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of legal blindness and moderate and severe vision impairment (MSVI) in people older than 50 years. It is classified in various stages including early, intermediate, and late stage. In the early stages, innate immune system, especially macrophages, play an essential part in disease onset and progression. NAD⁺ is an essential coenzyme involved in cellular senescence and immune cell function, and its role in age-related diseases is gaining increasing attention. The imbalance between the NAD⁺ synthesis and consumption causes the fluctuation of intracellular NAD⁺ level which determines the polarization fate of macrophages. In AMD, the over-expression of NAD⁺-consuming enzymes in macrophages leads to declining of NAD⁺ concentrations in the microenvironment. This phenomenon triggers the activation of inflammatory pathways in macrophages, positive feedback aggregation of inflammatory cells and accumulation of reactive oxygen species (ROS). This review details the role of NAD⁺ metabolism in macrophages and molecular mechanisms during AMD. The selected pathways were identified as potential targets for intervention in AMD, pending further investigation.

Inhibition of vascular endothelial growth factor alleviates neovascular retinopathy with regulated neurotrophic/proinflammatory cytokines through the modulation of DBI-TSPO signaling

Diazepam binding inhibitor (DBI)-translocator protein (18kDa) (TSPO) signaling in the retina was reported to possess coordinated macroglia-microglia interactions. We investigated DBI-TSPO signaling and its correlation with vascular endothelial growth factor (VEGF), neurotrophic or inflammatory cytokines in neovascular retinopathy, and under hypoxic conditions. The vitreous expression of DBI, VEGF, nerve growth factor (NGF), and interleukin-1beta (IL-1β) were examined in proliferative diabetic retinopathy (PDR) patients with or without anti-VEGF therapy and nondiabetic controls. Retinal DBI-TSPO signaling and the effect of the anti-VEGF agent were evaluated in a mouse model of oxygen-induced retinopathy (OIR). Interactions between Müller cell-derived VEGF and DBI, as well as cocultured microglial cells under hypoxic conditions, were studied, using Western blot, real-time RT-PCR, enzyme-linked immunosorbent assay (ELISA), flow cytometry, and immunofluorescent labeling. Results showed that vitreous levels of DBI, VEGF, NGF, and IL-1β were significantly higher in PDR patients compared with controls, which further changed after anti-VEGF therapy. A statistical association was found between vitreous DBI and VEGF, NGF, IL-1β, and age. The application of the anti-VEGF agent in the OIR model induced retinal expression of DBI and NGF, and attenuated inflammation and microglial cell activation. Inhibition of Müller cell-derived VEGF could increase its DBI expression under hypoxic conditions, while the DBI-TSPO signaling pathway is essential for anti-VEGF agents exerting anti-inflammatory and neuroprotective effects, as well as limiting inflammatory magnitude, promoting its neurotrophin production and anti-inflammatory (M2) polarization in microglial cells. These findings suggest the beneficial effect of anti-VEGF therapy on inflammation and neurotrophy of retinal glial cells through modulation of the DBI-TSPO signaling pathway.

Rapid morphologic changes to microglial cells and upregulation of mixed microglial activation state markers induced by P2X7 receptor stimulation and increased intraocular pressure

BACKGROUND

The identification of endogenous signals that lead to microglial activation is a key step in understanding neuroinflammatory cascades. As ATP release accompanies mechanical strain to neural tissue, and as the P2X7 receptor for ATP is expressed on microglial cells, we examined the morphological and molecular consequences of P2X7 receptor stimulation in vivo and in vitro and investigated the contribution of the P2X7 receptor in a model of increased intraocular pressure (IOP).

METHODS

In vivo experiments involved intravitreal injections and both transient and sustained elevation of IOP. In vitro experiments were performed on isolated mouse retinal and brain microglial cells. Morphological changes were quantified in vivo using Sholl analysis. Expression of mRNA for M1- and M2-like genes was determined with qPCR. The luciferin/luciferase assay quantified retinal ATP release while fura-2 indicated cytoplasmic calcium. Microglial migration was monitored with a Boyden chamber.

RESULTS

Sholl analysis of Iba1-stained cells showed retraction of microglial ramifications 1 day after injection of P2X7 receptor agonist BzATP into mouse retinae. Mean branch length of ramifications also decreased, while cell body size and expression of Nos2, Tnfa, Arg1, and Chil3 mRNA increased. BzATP induced similar morphological changes in ex vivo tissue isolated from Cx3CR1+/GFP mice, suggesting recruitment of external cells was unnecessary. Immunohistochemistry suggested primary microglial cultures expressed the P2X7 receptor, while functional expression was demonstrated with Ca2+ elevation by BzATP and block by specific antagonist A839977. BzATP induced process retraction and cell body enlargement within minutes in isolated microglial cells and increased Nos2 and Arg1. While ATP increased microglial migration, this required the P2Y12 receptor and not P2X7 receptor. Transient elevation of IOP led to microglial process retraction, cell body enlargement, and gene upregulation paralleling changes observed with BzATP injection, in addition to retinal ATP release. Pressure-dependent changes were reduced in P2X7-/- mice. Death of retinal ganglion cells accompanied increased IOP in C57Bl/6J, but not P2X7-/- mice, and neuronal loss showed some association with microglial activation.

CONCLUSIONS

P2X7 receptor stimulation induced rapid morphological activation of microglial cells, including process retraction and cell body enlargement, and upregulation of markers linked to both M1- and M2-type activation. Parallel responses accompanied IOP elevation, suggesting ATP release and P2X7 receptor stimulation influence the early microglial response to increased pressure.

KATP Opener Attenuates Diabetic-Induced Müller Gliosis and Inflammation by Modulating Kir6.1 in Microglia

Purpose

This study aimed to determine the effect of pinacidil, a nonselective K_ATP channel opener, on diabetes-induced retinal gliosis and inflammation.

Methods

Primary and immortalized cell lines of retinal microglia and Müller cells were used to set up a coculture model. In the trans-well system, microglia were seeded in the upper chamber and Müller cells in the bottom chamber. Microglia were polarized into proinflammatory (M1, with lipopolysaccharide and INF-γ) with or without different pinacidil concentrations before coculturing with Müller cells. The expression of inflammatory or anti-inflammatory genes and protein in microglia, and the expression of glial fibrillary acidic protein (GFAP), Kir4.1, and AQP4 in Müller cells were examined by real-time polymerase chain reaction and Western blot. Pinacidil was injected intravitreally into streptozotocin-induced diabetic rats. Retinal gliosis and inflammation were examined by immunohistochemistry and Western blot.

Results

Intravitreal injection of pinacidil alleviated diabetes-induced Müller cell gliosis and microglial activation and reduced vascular endothelial growth factor expression. In vitro study demonstrated that pinacidil inhibited tumor necrosis factor and interleukin-1β expression in M1-type microglia and alleviated the M1 microglia-induced GFAP expression in the Müller cells. Furthermore, we found that pinacidil on its own, or in combination with IL-4, can upregulate arginase-1 (Arg-1) and Kir6.1 expression in microglial cells.

Conclusions

Our results suggest that potassium channels are critically involved in diabetes-induced gliosis and microglial activation. The K_ATP opener, pinacidil, can reduce microglial activation by upregulating Kir6.1 expression.

Glycolysis links reciprocal activation of myeloid cells and endothelial cells in the retinal angiogenic niche

The coordination of metabolic signals among different cellular components in pathological retinal angiogenesis is poorly understood. Here, we showed that in the pathological angiogenic vascular niche, retinal myeloid cells, particularly macrophages/microglia that are spatially adjacent to endothelial cells (ECs), are highly glycolytic. We refer to these macrophages/microglia that exhibit a unique angiogenic phenotype with increased expression of both M1 and M2 markers and enhanced production of both proinflammatory and proangiogenic cytokines as pathological retinal angiogenesis-associated glycolytic macrophages/microglia (PRAGMs). The phenotype of PRAGMs was recapitulated in bone marrow-derived macrophages or retinal microglia stimulated by lactate that was produced by hypoxic retinal ECs. Knockout of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (PFKFB3; Pfkfb3 for rodents), a glycolytic activator in myeloid cells, impaired the ability of macrophages/microglia to acquire an angiogenic phenotype, rendering them unable to promote EC proliferation and sprouting and pathological neovascularization in a mouse model of oxygen-induced proliferative retinopathy. Mechanistically, hyperglycolytic macrophages/microglia produced large amount of acetyl-coenzyme A, leading to histone acetylation and PRAGM-related gene induction, thus reprogramming macrophages/microglia into an angiogenic phenotype. These findings reveal a critical role of glycolytic metabolites as initiators of reciprocal activation of macrophages/microglia and ECs in the retinal angiogenic niche and suggest that strategies targeting the metabolic communication between these cell types may be efficacious in the treatment of pathological retinal angiogenesis.

A modified protocol for isolation of retinal microglia from the pig

Microglia are the resident immune cells in the retina. To investigate their properties and behaviour, a reliable and yielding procedure to culture them is necessary. We here describe a way of isolation of microglial cells from the porcine retina, as pig eyes are similar to human eyes in size, structure and vasculature, including similarities in proteins and pathways. Retina was isolated from fresh pig eyes, dissociated by a mixture of collagenase, hyaluronidase and DNAse, and passed through a cell strainer. After triple centrifugation with decreasing velocity and re-suspension, cells were seeded into poly-d-lysine coated culture flasks and cultured using DMEM and macrophage-colony stimulating factor (M-CSF). Number of cells increased gradually during the first 10-14 days, till they could be split and used for experiments. Identity of isolated cells as microglia was assessed by immunostaining against the microglia/macrophage markers Iba1, CD11b, CD68, CD45 and TMEM119. Phagocytic function of microglia could be demonstrated by phagocytosis of fluorescence beads and their response to lipopolysaccharide (LPS). As a conclusion, we developed a protocol for isolation and cultivation of pig retinal microglial cells that are suitable for research in the laboratory.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Characterization of a functionally active primary microglial cell culture from the pig retina

Retinal inflammation is an integral component of many retinal diseases including diabetic retinopathy (DR), age-related macular degeneration (AMD) and retinopathy of prematurity (ROP). Inflammation is commonly initiated and perpetuated by myeloid-derived immune cells. In the retina, microglial cells are resident macrophages with myeloid origins, which acts as the first responders involved in the innate immune system. To understand the disease pathogenesis, the use of isolated retinal cell culture model is vital for the examination of multiple cellular responses to injury or trauma. The pig retina resembles human retina in terms of tissue architecture, vasculature, and topography. Additionally, it is a better model than the rodent retina because of the presence of the pseudomacula. In the present study, we sought to establish and characterize pig retinal primary microglial cell (pMicroglia) culture. We used pig eyes from the local abattoir and optimized pMicroglia cultures using multiple cell culture conditions and methods. The best results were obtained by seeding cells in DMEM-high glucose media for 18 days followed by shaking of the culture plate. The resulting pMicroglia were characterized by cellular morphology, phenotype, and immunostaining with Iba-1, CD68, P2Y12, CD163, CD14, and Isolectin GS-IB4. Generated pMicroglia were found functionally active in phagocytosis assay and responsive to lipopolysaccharides (LPS) in dose-dependent production of IL-1β. Furthermore, they showed increased secretion of pro-inflammatory cytokines with LPS treatment. Thus, we report a novel and reproducible method for the isolation of primary microglial cells from pig eyes, which may be useful for studying retinal diseases.

Microglia mediate non-cell-autonomous cell death of retinal ganglion cells

Excitotoxicity is well known in the neuronal death in the brain and is also linked to neuronal damages in the retina. Recent accumulating evidence show that microglia greatly affect excitotoxicity in the brain, but their roles in retina have received only limited attention. Here, we report that retinal excitotoxicity is mediated by microglia. To this end, we employed three discrete methods, that is, pharmacological inhibition of microglia by minocycline, pharmacological ablation by an antagonist for colony stimulating factor 1 receptor (PLX5622), and genetic ablation of microglia using Iba1-tTA::DTA^tetO/tetO mice. Intravitreal injection of NMDA increased the number of apoptotic retinal ganglion cells (RGCs) followed by reduction in the number of RGCs. Although microglia did not respond to NMDA directly, they became reactive earlier than RGC damages. Inhibition or ablation of microglia protected RGCs against NMDA. We found up-regulation of proinflammatory cytokine genes including Il1b, Il6 and Tnfa, among which Tnfa was selectively blocked by minocycline. PLX5622 also suppressed Tnfa expression. Tumor necrosis factor α (TNFα) signals were restricted in microglia at very early followed by spreading into other cell types. TNFα up-regulation in microglia and other cells were significantly attenuated by minocycline and PLX5622, suggesting a central role of microglia for TNFα induction. Both inhibition of TNFα and knockdown of TNF receptor type 1 by siRNA protected RGCs against NMDA. Taken together, our data demonstrate that a phenotypic change of microglia into a neurotoxic one is a critical event for the NMDA-induced degeneration of RGCs, suggesting an importance of non-cell-autonomous mechanism in the retinal neuronal excitotoxicity.

Microglia activation is essential for BMP7-mediated retinal reactive gliosis

Background

Our previous studies have shown that BMP7 is able to trigger activation of retinal macroglia. However, these studies showed the responsiveness of Müller glial cells and retinal astrocytes in vitro was attenuated in comparison to those in vivo, indicating other retinal cell types may be mediating the response of the macroglial cells to BMP7. In this study, we test the hypothesis that BMP7-mediated gliosis is the result of inflammatory signaling from retinal microglia.

Methods

Adult mice were injected intravitreally with BMP7 and eyes harvested 1, 3, or 7 days postinjection. Some mice were treated with PLX5622 (PLX) to ablate microglia and were subsequently injected with control or BMP7. Processed tissue was analyzed via immunofluorescence, RT-qPCR, or ELISA. In addition, cultures of retinal microglia were treated with vehicle, lipopolysaccharide, or BMP7 to determine the effects of BMP7-isolated cells.

Results

Mice injected with BMP7 showed regulation of various inflammatory markers at the RNA level, as well as changes in microglial morphology. Isolated retinal microglia also showed an upregulation of BMP-signaling components following treatment. In vitro treatment of retinal astrocytes with conditioned media from activated microglia upregulated RNA levels of gliosis markers. In the absence of microglia, the mouse retina showed a subdued gliosis and inflammatory response when exposed to BMP7.

Conclusions

Gliosis resulting from BMP7 is mediated through an inflammatory response from retinal microglia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0855-0) contains supplementary material, which is available to authorized users.

A review of the mechanisms of cone degeneration in retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited condition that features degeneration of rod and cone photoreceptors. In all forms of RP, the genetic mutation is expressed exclusively in rods; however, cones die too. The secondary death of cones in RP remains somewhat mysterious. A better understanding of the mechanisms that cause cone degeneration in RP could lead to novel treatments that preserve cones. There are a number of prevailing theories that attempt to explain cone degeneration in RP. One concept is that cone survival is dependent on trophic factors produced by rods. Another hypothesis is that cones suffer from a nutrient shortage after rods have been lost. Additionally, oxidative stress and pro-inflammatory microglial activation have also been suggested to play a role in cone death. The present review evaluates the evidence supporting these theories and provides an update on the mechanisms of cone degeneration in RP.

Treatment with polyamine oxidase inhibitor reduces microglial activation and limits vascular injury in ischemic retinopathy

Retinal vascular injury is a major cause of vision impairment in ischemic retinopathies. Insults such as hyperoxia, oxidative stress and inflammation contribute to this pathology. Previously, we showed that hyperoxia-induced retinal neurodegeneration is associated with increased polyamine oxidation. Here, we are studying the involvement of polyamine oxidases in hyperoxia-induced injury and death of retinal vascular endothelial cells. New-born C57BL6/J mice were exposed to hyperoxia (70% O2) from postnatal day (P) 7 to 12 and were treated with the polyamine oxidase inhibitor MDL 72527 or vehicle starting at P6. Mice were sacrificed after different durations of hyperoxia and their retinas were analyzed to determine the effects on vascular injury, microglial cell activation, and inflammatory cytokine profiling. The results of this analysis showed that MDL 72527 treatment significantly reduced hyperoxia-induced retinal vascular injury and enhanced vascular sprouting as compared with the vehicle controls. These protective effects were correlated with significant decreases in microglial activation as well as levels of inflammatory cytokines and chemokines. In order to model the effects of polyamine oxidation in causing microglial activation in vitro, studies were performed using rat brain microvascular endothelial cells treated with conditioned-medium from rat retinal microglia stimulated with hydrogen peroxide. Conditioned-medium from activated microglial cultures induced cell stress signals and cell death in microvascular endothelial cells. These studies demonstrate the involvement of polyamine oxidases in hyperoxia-induced retinal vascular injury and retinal inflammation in ischemic retinopathy, through mechanisms involving cross-talk between endothelial cells and resident retinal microglia.

Culture and characterization of microglia from the adult murine retina

Purpose. To develop a protocol for isolating and culturing murine adult retinal microglia and to characterize the phenotype and function of the cultured cells. Method. Retinal single-cell suspensions were prepared from adult MF1 mice. Culture conditions including culture medium, growth factors, seeding cell density, and purification of microglia from the mixed cultures were optimised. Cultured retinal microglial cells were phenotyped using the surface markers CD45, CD11b, and F4/80. Their ability to secrete proinflammatory cytokines in response to lipopolysaccharide (LPS) stimulation was examined using cytometric bead array (CBA) assay. Results. Higher yield was obtained when retinal single-cell suspension was cultured at the density of 0.75 × 10⁶ cells per cm² in Dulbecco's modified Eagle medium (DMEM)/F12 + Glutamax supplement with 20% fetal calf serum (FCS) and 20% L929 supernatant. We identified day 10 to be the optimum day of microglial isolation. Over 98% of the cells isolated were positive for CD45, CD11b, and F4/80. After stimulating with LPS they were able to secrete proinflammatory cytokines such as IL-6 and TNF-α and express CD86, CD40, and MHC-II. Conclusion. We have developed a simple method for isolating and culturing retinal microglia from adult mice.

Neuroprotective effect of melatonin against hypoxia-induced retinal ganglion cell death in neonatal rats

The purpose of this study was to determine whether melatonin treatment would mitigate retinal ganglion cell (RGC) death in the developing retina following a hypoxic insult. Lipid peroxidation (LPO), glutathione (GSH), tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) concentrations, expression of vascular endothelial growth factor receptors, Flt-1 and Flk-1, release of cytochrome c from mitochondria, and caspase-3 expression were examined in the retinas of 1-day-old rats at 3 hr to 14 days after a hypoxic exposure. The mRNA and protein expression of Flt-1 and Flk-1 and the tissue concentration of LPO, TNF-α, and IL-1β were upregulated significantly after the hypoxic exposure, whereas the content of GSH was decreased significantly. RGC cultures also showed increased LPO and decreased GSH levels after hypoxic exposure but these effects were reversed in cells treated with melatonin. TNF-α and IL-1β expression was specifically located on microglial cells, whereas Flt-1 and Flk-1 was limited to RGCs as confirmed by double immunofluorescence labeling. Cultures of hypoxic microglial cells treated with melatonin showed a significant reduction in the release of these cytokines as compared to untreated hypoxic cells. Hypoxia induced increase in the cytosolic cytochrome c and caspase-3 in RGCs was attenuated with melatonin treatment. The results suggest that, in hypoxic injuries, melatonin is neuroprotective to RGCs in the developing retina through its antioxidative, anti-inflammatory, and anti-apoptotic effects. Melatonin suppressed Flt-1 and Flk-1 expression in retinal blood vessels, which may result in reduced retinal vascular permeability and it also preserved mitochondrial function as shown by a reduction in cytochrome c leakage into the cytosol. The results may have therapeutic implications for the management of retinopathy of prematurity.

Pro-inflammatory cytokines modulate iron regulatory protein 1 expression and iron transportation through reactive oxygen/nitrogen species production in ventral mesencephalic neurons

Both inflammatory processes associated with microglia activation and abnormal iron deposit in dopaminergic neurons are involved in the pathogenesis of Parkinson's disease (PD). However, the relationship between neuroinflammation and iron accumulation was not fully elucidated. In the present study, we aimed to investigate whether the pro-inflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) released by microglia, could affect cellular iron transportation in primary cultured ventral mesencephalic (VM) neurons. The results showed that IL-1β or TNF-α treatment led to increased ferrous iron influx and decreased iron efflux in these cells, due to the upregulation of divalent metal transporter 1 with the iron response element (DMT1+IRE) and downregulation of ferroportin1 (FPN1). Increased levels of iron regulatory protein 1 (IRP1), transferrin receptor 1 (TfR1) and hepcidin were also observed in IL-1β or TNF-α treated VM neurons. IRP1 upregulation could be fully abolished by co-administration of radical scavenger N-acetyl-l-cysteine and inducible NO synthetase inhibitor Nω-nitro-l-arginine methyl ester hydrochloride. Further experiments demonstrated that IL-1β and TNF-α release was remarkably enhanced by iron load in activated microglia triggered by lipopolysaccharide or 1-methyl-4-phenylpyridinium (MPP(+)). In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice, salicylate application could not block DMT1+IRE upregulation in dopaminergic neurons of substantia nigra. These results suggested that IL-1β and TNF-α released by microglia, especially under the condition of iron load, might contribute to iron accumulation in VM neurons by upregulating IRP1 and hepcidin levels through reactive oxygen/nitrogen species production. This might provide a new insight into unraveling that microglia might aggravate this iron mediated neuropathologies in PD.

Localization and developmental expression patterns of CSPG-cs56 (aggrecan) in normal and dystrophic retinas in two rat strains

Proteoglycans have a number of important functions in the central nervous system. Aggrecan (hyaluronan-binding proteoglycan, CSPG-cs56) is found in the extracellular matrix of cartilage as well as in the developing brain. We compared the postnatal distribution of CSPG-cs56 in Long Evans (LE) and Royal College of Surgeons (RCS) rat retinas to determine if this proteoglycan played a role in the development of dystrophic retinas. CSPG-cs56 expression was examined in rat retinas aged between birth (postnatal day 0, P0) and P150 using immunofluorescence and Western-blots. Immunofluorescence was quantified using ImageJ. GFAP staining was used to compare Müller cell labeling and the distribution of CSPG-cs56. Both rat strains showed a significant rise in total retinal CSPG-cs56 between P0 and P21; values peaked on P21 in LE rats and P14 in RCS rats. CSPG-cs56 then significantly decreased to lower levels (P35) in both strains before reaching significantly higher levels by P90-P150. CSPG-cs56 positive staining was present in the ganglion cell layer at birth and clear layering of the inner plexiform layer was seen between P7 and P21 due to dendritic staining of retinal ganglion cells. Staining was less intense and diffuse within the outer plexiform over a similar time-course. Light CSPG-cs56 labeling in the region of the outer segments was present at (P14) and became more intense as the retina approached maturity. CSPG-cs56 in the outer segments was the main contributor to the higher expression in older animals. Substantial differences in CSPG-cs56 labeling were not seen between LE and RCS rats. There was no evidence to suggest that Müller cells were the source of CSPG-cs56 in either rat strain, although their staining distributions had a degree of overlap. The lack of significant differences between LE and RCS rats indicates that CSPG-cs56 may not be involved in the degenerative process or the reorganization of the RCS rat retina. We suggest that the main role of CPSG-cs56 is to maintain retinal ganglion cell dendritic structure in the inner plexiform layer and is closely related to providing adequate support and flexibility for the photoreceptor outer segments, which is necessary to maintain their function.

Blockade of VEGFR1 and 2 suppresses pathological angiogenesis and vascular leakage in the eye

Objective

VEGFR1 and 2 signaling have both been increasingly shown to mediate complications of ischemic retinopathies, including retinopathy of prematurity (ROP), age-related macular degeneration (AMD), and diabetic retinopathy (DR). This study evaluates the effects of blocking VEGFR1 and 2 on pathological angiogenesis and vascular leakage in ischemic retinopathy in a model of ROP and in choroidal neovascularization (CNV) in a model of AMD.

Materials and Methods

Neutralizing antibodies specific for mouse VEGFR1 (MF1) and VEGFR2 (DC101) were administrated systemically. CNV was induced by laser photocoagulation and assessed 14d after laser treatment. Retinal NV was generated in oxygen-induced ischemic retinopathy (OIR) and assessed at p17. NV quantification was determined by measuring NV tufts and vascular leakage was quantified by measuring [³H]-mannitol leakage from blood vessels into the retina. Gene expression was measured by real-time quantitative (Q)PCR.

Results

VEGFR1 and VEGFR2 expressions were up-regulated during CNV pathogenesis. Both MF1 and DC101 significantly suppressed CNV at 50 mg/kg: DC101 suppressed CNV by 73±5% (p<0.0001) and MF1 by 64±6% (p = 0.0002) in a dosage-dependent manner. The combination of MF1 and DC101 enhanced the inhibitory efficacy and resulted in an accumulation of retinal microglia at the CNV lesion. Similarly, both MF1 and DC101 significantly suppressed retinal NV in OIR at 50 mg/kg: DC101 suppressed retinal NV by 54±8% (p = 0.013) and MF1 by 50±7% (p<0.0002). MF1 was even more effective at inhibiting ischemia-induced BRB breakdown than DC101: the retina/lung leakage ratio for MF1 was reduced by 73±24%, p = 0.001 and for DC101 by 12±4%, p = 0.003. The retina/renal leakage ratio for MF1 was reduced by 52±28%, p = 0.009 and for DC101 by 13±4%, p = 0.001.

Conclusion

Our study provides further evidence that both VEGFR1 and 2 mediate pathological angiogenesis and vascular leakage in these models of ocular disease and suggests that antagonist antibodies to these receptor tyrosine kinases (RTKs) are potential therapeutic agents.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Macrophage colony-stimulating factor and its receptor signaling augment glycated albumin-induced retinal microglial inflammation in vitro

Background

Microglial activation and the proinflammatory response are controlled by a complex regulatory network. Among the various candidates, macrophage colony-stimulating factor (M-CSF) is considered an important cytokine. The up-regulation of M-CSF and its receptor CSF-1R has been reported in brain disease, as well as in diabetic complications; however, the mechanism is unclear. An elevated level of glycated albumin (GA) is a characteristic of diabetes; thus, it may be involved in monocyte/macrophage-associated diabetic complications.

Results

The basal level of expression of M-CSF/CSF-1R was examined in retinal microglial cells in vitro. Immunofluorescence, real-time PCR, immunoprecipitation, and Western blot analyses revealed the up-regulation of CSF-1R in GA-treated microglial cells. We also detected increased expression and release of M-CSF, suggesting that the cytokine is produced by activated microglia via autocrine signaling. Using an enzyme-linked immunosorbent assay, we found that GA affects microglial activation by stimulating the release of tumor necrosis factor-α and interleukin-1β. Furthermore, the neutralization of M-CSF or CSF-1R with antibodies suppressed the proinflammatory response. Conversely, this proinflammatory response was augmented by the administration of M-CSF.

Conclusions

We conclude that GA induces microglial activation via the release of proinflammatory cytokines, which may contribute to the inflammatory pathogenesis of diabetic retinopathy. The increased microglial expression of M-CSF/CSF-1R not only is a response to microglial activation in diabetic retinopathy but also augments the microglial inflammation responsible for the diabetic microenvironment.

Microglia in the mouse retina alter the structure and function of retinal pigmented epithelial cells: a potential cellular interaction relevant to AMD

BACKGROUND

Age-related macular degeneration (AMD) is a leading cause of legal blindness in the elderly in the industrialized word. While the immune system in the retina is likely to be important in AMD pathogenesis, the cell biology underlying the disease is incompletely understood. Clinical and basic science studies have implicated alterations in the retinal pigment epithelium (RPE) layer as a locus of early change. Also, retinal microglia, the resident immune cells of the retina, have been observed to translocate from their normal position in the inner retina to accumulate in the subretinal space close to the RPE layer in AMD eyes and in animal models of AMD.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we examined the effects of retinal microglia on RPE cells using 1) an in vitro model where activated retinal microglia are co-cultured with primary RPE cells, and 2) an in vivo mouse model where retinal microglia are transplanted into the subretinal space. We found that retinal microglia induced in RPE cells 1) changes in RPE structure and distribution, 2) increased expression and secretion of pro-inflammatory, chemotactic, and pro-angiogenic molecules, and 3) increased extent of in vivo choroidal neovascularization in the subretinal space.

CONCLUSIONS/SIGNIFICANCE

These findings share similarities with important pathological features found in AMD and suggest the relevance of microglia-RPE interactions in AMD pathogenesis. We speculate that the migration of retinal microglia into the subretinal space in early stages of the disease induces significant changes in RPE cells that perpetuate further microglial accumulation, increase inflammation in the outer retina, and fosters an environment conducive for the formation of neovascular changes responsible for much of vision loss in advanced AMD.

[Possible role of alkylphosphocholines in retinal reattachment surgery]

BACKGROUND

Proliferative vitreoretinopathy (PVR) is a major complication after retinal detachment surgery, but there is no established pharmacotherapy available to control the cell biology of the disease. The aim of this study was to investigate the role of alkylphosphocholines [APCs; erucylphosphocholine (ErPC) was used in this study], novel pharmacologic substances with antiproliferative properties, on intraretinal proliferation initiated by experimental retinal detachment in a well-established in vivo model.

METHODS

Retinal detachments were created in adult pigmented rabbits. ErPC was injected intravitreally on either day 1 or day 2 after detachment. Bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) was injected on day 3. Following fixation, retinas were triple-labelled with anti-BrdU (proliferation marker), Isolectin B4 (retinal microglia marker), and anti-vimentin (retinal Mueller glia cell marker). The number of anti-BrdU-labelled cells per millimeter of retina was determined from sections imaged by laser scanning confocal microscopy. Toxicity was assessed by light and electron microscopy.

RESULTS

A single intravitreal injection of ErPC had a significant effect on reducing the number of proliferating non-neural retinal cells on day 3 after experimental retinal detachment in the rabbit. Injection of ErPC on day 1 was more effective than when given on day 2. No evidence of toxicity was observed in the retina on day 3 for any of the conditions.

CONCLUSIONS

APCs are novel pharmacologic substances that significantly inhibited intraretinal proliferation after experimental retinal detachment in this in vivo model. They could be considered as an adjunct therapy at the time of retinal reattachment surgery to potentially prevent proliferative vitreoretinal diseases such as PVR. However, long-term toxicity studies must be performed before APCs can be considered for clinical application.

AGEs mediated expression and secretion of TNF alpha in rat retinal microglia

Diabetic retinopathy induces an inflammatory response in the retina characterized by an increase in inflammatory cytokines and the activation of microglia. The degree of microglia activation may influence the extent of retina injury following retinal metabolic stress. We have previously shown that DR rats have elevated levels of advanced glycation end products (AGEs) in their blood. We have also suggested that AGEs might be involved in microglial activation and production of tumor necrosis factor alpha (TNF alpha). In this study, we attempted to confirm that AGEs induce the release of TNF alpha from rat retinal microglia using an in vitro microglia culture system, and concurrently to explore the mediating mechanisms. AGEs increased the protein secretion and mRNA expression of TNF alpha in cultured rat retinal microglia. These effects of AGEs were primarily mediated by reactive oxygen species (ROS). Furthermore, the inhibitors for mitogen-activated protein kinases (MAPK; p38, JNK and ERK 1/2) and nuclear factor-kB (NF-kB) could significantly decrease AGEs-induced TNF alpha release. AGEs-activated microglia showed an increase of NF-kB p65 nuclear translocation. These observations indicated that pathophysiological levels of AGEs may alter rat retinal microglia function by up-regulating TNF alpha expression and release via enhanced formation of intracellular ROS. AGEs-induced ROS subsequently activates MAPK (p38, JNK and ERK1/2) and NF-kB.

Microglia activation in retinal degeneration

Microglia cells are phagocytic sentinels in the CNS and in the retina required for neuronal homeostasis and innate immune defense. Accumulating experimental evidence suggests that chronic microglia activation is associated with various neurodegenerative diseases including retinal dystrophies. Endogenous triggers alert microglia cells rapidly in the degenerating retina, leading to local proliferation, migration, enhanced phagocytosis, and secretion of cytokines, chemokines, and neurotoxins. This amplified, immunological cascade and the loss of limiting control mechanisms may contribute significantly to retinal tissue damage and proapoptotic events. This review summarizes the developmental and immune surveillance functions of microglia in the healthy retina and discusses early signaling events and transcriptional networks of microglia activation in retinal degeneration. The characterization of activation pathways at the molecular level may lead to innovative, therapeutic options in degenerative retinal diseases based on a selective, pharmacological interference with the neurotoxic activities of microglia cells, without compromising their homeostastic functions.

Behavior of in vitro cultured ameboid microglial cells migrating on Müller cell end-feet in the quail embryo retina

Ameboid microglial cells migrate tangentially on the vitreal part of quail embryo retinas by crawling on Müller cell end-feet (MCEF) to which they adhere. These microglial cells can be cultured immediately after dissection of the eye and isolation of sheets containing the inner limiting membrane (ILM) covered by a carpet of MCEF (ILM/MCEF sheets), to which the cells remain adhered. Morphological changes of microglial cells cultured on ILM/MCEF sheets for 4 days were characterized in this study. During the first minutes in vitro, lamellipodia-bearing bipolar microglial cells became rounded in shape. From 1 to 24 h in vitro (hiv), microglial cells swept and phagocytosed the MCEF on which they were initially adhered, becoming directly adhered on the ILM. MCEF sweep was dependent on active cell motility, as shown by inhibition of sweep after cytochalasin D treatment. From 24 hiv on, after MCEF phagocytosis, microglial cells became more flattened, increasing the surface area of their adhesion to substrate, and expressed the beta1 subunit of integrins on their membrane. Morphological evidence suggested that microglial cells migrated for short distances on ILM/MCEF sheets, leaving tracks produced by their strong adhesion to the substrate. The simplicity of the isolation method, the immediate availability of cultured microglial cells, and the presence of multiple functional processes (phagocytosis, migration, upregulation of surface molecules, etc.) make cultures of microglial cells on ILM/MCEF sheets a valuable model system for in vitro experimental investigation of microglial cell functions.

Neural stem cell properties of Müller glia in the mammalian retina: regulation by Notch and Wnt signaling

The retina in adult mammals, unlike those in lower vertebrates such as fish and amphibians, is not known to support neurogenesis. However, when injured, the adult mammalian retina displays neurogenic changes, raising the possibility that neurogenic potential may be evolutionarily conserved and could be exploited for regenerative therapy. Here, we show that Müller cells, when retrospectively enriched from the normal retina, like their radial glial counterparts in the central nervous system (CNS), display cardinal features of neural stem cells (NSCs), i.e., they self-renew and generate all three basic cell types of the CNS. In addition, they possess the potential to generate retinal neurons, both in vitro and in vivo. We also provide direct evidence, by transplanting prospectively enriched injury-activated Müller cells into normal eye, that Müller cells have neurogenic potential and can generate retinal neurons, confirming a hypothesis, first proposed in lower vertebrates. This potential is likely due to the NSC nature of Müller cells that remains dormant under the constraint of non-neurogenic environment of the adult normal retina. Additionally, we demonstrate that the mechanism of activating the dormant stem cell properties in Müller cells involves Wnt and Notch pathways. Together, these results identify Müller cells as latent NSCs in the mammalian retina and hence, may serve as a potential target for cellular manipulation for treating retinal degeneration.

Microglia-derived pronerve growth factor promotes photoreceptor cell death via p75 neurotrophin receptor

Reports implicating microglia-derived nerve growth factor (NGF) during programmed cell death in the developing chick retina led us to investigate its possible role in degenerative retinal disease. Freshly isolated activated retinal microglia expressed high molecular weight forms of neurotrophins including that of nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4. Conditioned media from cultured retinal microglia (MGCM) consistently yielded a approximately 32-kDa NGF-reactive band when supplemented with bovine serum albumin (BSA) or protease inhibitors (PI); and promoted cell death that was suppressed by NGF immunodepletion in a mouse photoreceptor cell line (661w). The approximately 32 kDa protein was partially purified (MGCM/p32) and was highly immunoreactive with a polyclonal anti-pro-NGF antibody. Both MGCM/p32 and recombinant pro-NGF protein promoted cell death in 661w cultures. Increased levels of pro-NGF mRNA and protein were observed in the RCS rat model of retinal dystrophy. MGCM-mediated cell death was reversed by p75NTR antiserum in p75NTR(+)/trkA(-) 661w cells. Our study shows that a approximately 32 kDa pro-NGF protein released by activated retinal microglia promoted degeneration of cultured photoreceptor cells. Moreover, our study suggests that defective post-translational processing of NGF might be involved in photoreceptor cell loss in retinal dystrophy.

Neuronal susceptibility to damage: comparison of the retinas of young, old and old/caloric restricted rats before and after transient ischemia

Compared to young rats, old age increases susceptibility and caloric restriction decreases susceptibility for the loss of retinal ganglion cells and displaced amacrine cells following retinal ischemia/reperfusion. In retinas of old animals before ischemia, reactive gliosis, including activation of Muller cells, microglia and astrocytes, is increased compared to retinas from young and old/caloric restricted animals. Post-ischemia, the existing reactive gliosis in retinas of old animals is not neuroprotective and the reactive gliosis is even further increased in old animals compared to young or old/caloric restricted animals. In retinas from old/caloric restricted animals, inducible heat shock protein-70 and brain-derived neurotrophic factor increased more markedly after ischemia/reperfusion compared to retinas from young and old animals. Thus, compared to retinas in young animals, neurons of old animals may be more susceptible to cell death by secondary glial mechanisms after retinal ischemia/reperfusion. Caloric restriction in old animals is neuroprotective against damage in the retina following ischemia, perhaps by suppressing glial activity and by the neuroprotective effects of inducible heat shock protein-70 and brain-derived neurotrophic factor.

Microglia-Müller glia cell interactions control neurotrophic factor production during light-induced retinal degeneration

Activation of microglia commonly occurs in response to a wide variety of pathological stimuli including trauma, axotomy, ischemia, and degeneration in the CNS. In the retina, prolonged or high-intensity exposure to visible light leads to photoreceptor cell apoptosis. In such a light-reared retina, we found that activated microglia invade the degenerating photoreceptor layer and alter expression of neurotrophic factors such as nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), and glial cell line-derived neurotrophic factor (GDNF). Because these neurotrophic factors modulate secondary trophic factor expression in Müller glial cells, microglia-Müller glia cell interaction may contribute to protection of photoreceptors or increase photoreceptor apoptosis. In the present study, we demonstrate the possibility that such functional glia-glia interactions constitute the key mechanism by which microglia-derived NGF, brain-derived neurotrophic factor (BDNF), and CNTF indirectly influence photoreceptor survival, although the receptors for these neurotrophic factors are absent from photoreceptors, by modulating basic fibroblast growth factor (bFGF) and GDNF production and release from Müller glia. These observations suggest that microglia regulate the microglia-Müller glia-photoreceptor network that serves as a trophic factor-controlling system during retinal degeneration.

Cellular localization of cyclooxygenase-1 and cyclooxygenase-2 in the normal mouse, rat, and human retina

Prostaglandins, synthesized by cyclooxygenase (COX), regulate diverse neurophysiological actions such as regulation of autonomic responses, transmission of pain, generation of fever, control of sleep-wake cycle, synaptic signaling, and cross-talk between neurons and glia in the central nervous system. Although prostaglandins have been widely studied in the anterior segment tissues of the eye, relatively little is known about prostaglandins in the neural retina. By using immunohistochemistry, we have compared the cellular expression and localization of COX-1 and COX-2 in the normal mouse, rat, and human retina. In the normal mouse retina, COX-1 immunoreactivity is present in the outer segments of photoreceptor cells, horizontal cells, microglia, retinal ganglion cells, and displaced amacrine cells. In the normal rat retina, COX-1 immunoreactivity is present in microglia, retinal ganglion cells, and displaced amacrine cells. In the normal human retina, COX-1 immunoreactivity is present in microglia, astrocytes, retinal ganglion cells, and displaced amacrine cells. In the normal mouse and rat retina, COX-2 immunoreactivity is present in processes of the outer plexiform layer and in certain amacrine cells and retinal ganglion cells. In the normal human retina, COX-2 immunoreactivity is only present in processes of the outer plexiform layer. These results suggest that prostaglandins, synthesized by COX-1 or COX-2, may contribute to normal physiological and homeostatic functions in the retina.

Differential expression of protein tyrosine kinase genes during microglial activation

Protein tyrosine kinase (PTK) activity is abundant in microglia, but the PTKs that participate in their activation have not been identified. For these studies, we used three paradigms to characterize PTK expression during microglial activation: resting and activated microglia were bulk fractionated from the adult brain, cultured newborn microglia were treated with lipopolysaccharide (LPS) to model the transition from activated toward phagocytic microglia, and PTK expression was examined in activated microglia in situ after facial nerve axotomy. Two PCR-based strategies were used to show that 21 different PTK genes are expressed by rat brain microglia: 5 receptor PTKs, 10 nonreceptor PTKs, and 6 members of the src family. Seven of the 21 PTKs were examined in greater detail. Five PTK mRNAs (fgr, hck, fak, jak-2, and flk-1) increased expression across all three models of activation. We conclude that they represent key components in the cascades that participate in microglial activation. In contrast, expression of fes and fms correlated with stimuli that affect microglial proliferation. Four of the PTKs (hck, fgr, fes, and fms) are believed to be myeloid cell specific and were not expressed by cultured astrocytes. HCK and FAK protein were also not expressed in lysates of immature astrocytes and oligodendrocytes. Because of their putative specificity, these kinases represent potential targets for inhibitors of microglial activation. Because reactive microglia can exacerbate the severity of neurological diseases, the identification of specific kinases that participate in microglial activation represents an important advance toward the development of new therapeutics.

Distribution, markers, and functions of retinal microglia

Retinal microglia originate from hemopoietic cells and invade the retina from the retinal margin and the optic disc, most likely via the blood vessels of the ciliary body and iris, and the retinal vasculature, respectively. The microglial precursors that appear in the retina prior to vascularization are major histocompatibility complex (MHC) class I- and II-positive and express the CD45 marker, but lack specific macrophage markers. They differentiate into ramified parenchymal microglia in the adult retina. A second category of microglial precursors, which do express specific macrophage markers, migrate into the retina along with vascular precursors. They appear around blood vessels in the adult retina and are similar to macrophages or cells of the mononuclear phagocyte series (MPS). Microglia are distributed in the outer plexiform layer (OPL), outer nuclear layer (ONL), inner plexiform layer (IPL), ganglion cell layer (GCL), and nerve fiber layer (NFL) of the primate retina. The pattern of microglial distribution in the avascular retina of the quail indicates that blood vessels are not responsible for the final location of microglia in the retina. In the human retina, microglia express MHC class I, MHC class II, CD45, CD68, and S22 markers. In the rat and mouse retina, OX41, OX42, OX3, OX6, OX18, ED1, Mac-1, F4/80, 5D4 anti-keratan sulfate, and lectins are used to recognize microglia. Microglial cells play an important role in host defense against invading microorganisms, immunoregulation, and tissue repair. During neurodegeneration, activated microglial cells participate in the phagocytosis of debris and facilitate regenerative processes. In autoimmune disease, microglia have dual functions: initiating uveoretinitis, but also limiting subsequent inflammation. Retinal microglia may be associated with vitreoretinopathy, diabetic retinopathy, glaucoma, and age-related macular degeneration. The goal of this article was to review the present knowledge about retinal microglia and the function of retinal microglia in pathological conditions.

Retina-derived microglial cells induce photoreceptor cell death in vitro

In animals with retinal degeneration, the presence of activated microglial cells in the outer retina during the early stages of injury suggests that they may be involved in the ensuing photoreceptor cell death. In the following study, we investigated the effects of rat retina-derived microglial cells on a photoreceptor cell line (661w) using cell culture techniques. The difficulty of obtaining pure populations of photoreceptor cells necessitated our use of the 661w photoreceptor cells generated from retinas of transgenic mice. 661w Cells were incubated for 24-48 h in basal medium or basal medium conditioned by activated microglial cells (MGCM) or Müller cells (MCCM), and tested for cell death using lactate dehydrogenase (LDH) assay. The induction of apoptosis in the 661w cells by MGCM was investigated using Terminal deoxynucleotidyl Transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) and DNA laddering. Treatment of 661w cells with MGCM for 48 h resulted in approximately 73% of cells dead as compared with 19-20% of cells grown in either basal medium or MCCM. Serum supplementation or pretreatment with heat did not abolish the cytotoxicity of MGCM. More TUNEL-positive cells were observed in MGCM-treated cultures as compared with those in basal medium. Bands in multiples of approximately 180 bp formed DNA ladders in MGCM-treated but not in basal medium-treated samples. Our study shows that microglial cells release soluble product(s) that induce degeneration of cultured photoreceptor cells. Moreover, the mechanism of microglia-induced photoreceptor cell death may involve apoptosis similar to that observed in animals with retinal degeneration.

T Cell Immunity Induced by Allogeneic Microglia in Relation to Neuronal Retina Transplantation

Microglia share a lineage relationship with bone marrow-derived monocytes/macrophages and dendritic cells, and their inclusion in retinal and brain transplants may function as “passenger leukocytes.” In other solid allografts, passenger leukocytes are the primary sources of immunogenicity, triggering alloimmune rejection. We have conducted a series of in vitro and in vivo studies examining the capacity of microglia cultured from forebrain to activate alloreactive T cells and to induce and elicit alloimmunity. Cultured microglia expressed class II MHC molecules and costimulatory molecules (B7-1, B7-2, and CD40), and they secreted IL-12. Cultured microglia injected s.c. into naive recipients induced allospecific delayed hypersensitivity and elicited delayed hypersensitivity directed at alloantigens. Cultured microglia differed from conventional APCs by secreting significant amounts of mature TGF-β2, but smaller amounts of IL-12. Moreover, while both cultured microglia and conventional APC stimulated T cell proliferation in vitro, microglia directed the responding T cells toward the Th2 pathway in which IL-4, but not IL-2 and IFN-γ, was secreted. The abilities of microglia to secrete TGF-β2, to stimulate alloreactive Th2 cells, and to induce anterior chamber associated immune deviation when injected into the eye of naive allogeneic mice suggest that they are not typical passenger leukocytes. The unique functional properties of cultured microglia may account for the capacity of neonatal retinal tissue transplanted into the eye to alter the systemic alloimmune response in a manner that delays, but does not prevent, graft rejection.

Microglia in cell culture and in transplantation therapy for central nervous system disease

The central nervous system (CNS) is host to a significant population of macrophage-like cells known as microglia. In addition to these cells which reside within the parenchyma, a diverse array of macrophages are present in meningeal, perivascular, and other peripheral locations. The role that microglia and other CNS macrophages play in disease and injury is under intensive investigation, and functions in development and in the normal adult are just beginning to be explored. At present the biology of these cells represents one of the most fertile areas of CNS research. This article describes methodology for the isolation and maintenance of microglia in cell cultures prepared from murine and feline animals. Various approaches to identify microglia are provided, using antibody, lectin, or scavenger receptor ligand. Assays to confirm macrophage-like functional activity, including phagocytosis, lysosomal enzyme activity, and motility, are described. Findings regarding the origin and development of microglia and results of transplantation studies are reviewed. Based on these data, a strategy is presented that proposes to use the microglial cell lineage to effectively deliver therapeutic compounds to the CNS from the peripheral circulation.

Tumor necrosis factor and nitric oxide production by resident retinal glial cells from rats presenting hereditary retinal degeneration

The inherited retinal dystrophy observed in Royal College of Surgeons (RCS) rats is a widely used model for the study of the photoreceptor degeneration that occurs in retinitis pigmentosa and macular degeneration. The visual cell degeneration is accompanied by an abnormal accumulation of microglial cells in the retina of RCS rats presenting the dystrophy. In the present study, we show that combined stimulation of RCS dystrophic retinal Müller glial (RMG) cells with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) induced the release in culture supernatants of significantly higher amounts of tumor necrosis factor (TNF) and nitric oxide (NO) compared to nondystrophic congenic controls. In contrast, the levels of TNF and NO found in the supernatants from microglial cells were not significantly different in both strains. Interestingly, as shown by thymidine incorporation, microglial cells from RCS dystrophic rats have a prominent capacity of proliferation in culture medium compared to microglia isolated from RCS non dystrophic controls. Incubation of RMG cells and microglia with the stereoselective inhibitor of NOS, NG-monomethyl-L-arginine (L-NMMA), inhibited nitrite release in LPS + IFN-gamma-stimulated RMG cells and microglia. The addition of TGF-beta with LPS + IFN-gamma clearly inhibited TNF release in supernatants from both dystrophic and control rat RMG cells and microglia. While TGF-beta significantly inhibited nitrite synthesis in RMG cells, the effect on nitrite synthesis by microglia was very low. The retinal dystrophy observed in RCS dystrophic rats could result from an abnormal reactivity of RMG and microglial cells to release TNF and NO in response to stimulants. The immunomodulatory cytokine TGF-beta and inhibitors of NOS could be negative regulators in the cytokine network and nitrite synthesis thus interfering with the development of photoreceptor cell death.

Microglia-derived cytotoxic factors. Part I: Inhibition of tumor cell growth in vitro

The rarity of neoplasms in the adult mammalian retina has led us to hypothesize the presence or increased expression of 'tumor-inhibitory molecules' in the mature differentiated retina. We have begun to investigate the source(s) of these molecules, and the following study describes the inhibitory activity of a soluble microglia-derived cytotoxic factor on the proliferation of C6 cells, a glial tumor cell line. C6 cells were treated for 24, 48, or 72 h with basal medium or basal medium conditioned by retina-derived Muller cells (MCCM) or microglial cells (MGCM) and assayed for cell proliferation and/or cell death using various techniques involving fluorescent probes, lactate dehydrogenase release, or bromodeoxyuridine uptake. C6 cells increased in number from 24 to 72 h following incubation in basal medium or MCCM, but not in MGCM, where the cells rounded up and retracted their processes. The number of dead cells appeared to be the same in all groups at each time point. Similar findings were observed in the presence of 1-10% serum. About 25% of cells treated with basal medium for 72 h were positive for bromodeoxyuridine as compared with < 1% in MGCM-treated cultures. Our studies suggest that retina-derived microglial cells secrete soluble product(s) that inhibit the growth of C6 cells in culture. These molecules may provide protection for the mature retina against the invasion of tumor cells and may prove useful in the treatment of cancer.