Microglia activation in retinal degeneration
`Thomas Langmann1
`Institute of Human Genetics, University of Regensburg, Regensburg, Germany
`
`Abstract: Microglia cells are phagocytic sentinels
`in the CNS and in the retina required for neuronal
`homeostasis and innate immune defense. Accumu-
`lating experimental evidence suggests that chronic
`microglia activation is associated with various neu-
`rodegenerative diseases including retinal dystro-
`phies. Endogenous triggers alert microglia cells
`rapidly in the degenerating retina, leading to local
`proliferation, migration, enhanced phagocytosis,
`and secretion of cytokines, chemokines, and neu-
`rotoxins. 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 func-
`tions of microglia in the healthy retina and dis-
`cusses early signaling events and transcriptional
`networks of microglia activation in retinal degen-
`eration. The characterization of activation path-
`ways at the molecular level may lead to innovative,
`therapeutic options in degenerative retinal dis-
`eases based on a selective, pharmacological in-
`terference with the neurotoxic activities of micro-
`glia cells, without compromising their homeostastic
`functions. J. Leukoc. Biol. 81: 1345–1351; 2007.
`
`Key Words: neuronal homeostasis 䡠 Toll-like receptors 䡠 early
`growth response factor 1
`
`SCOPE OF THIS REVIEW
`
`The concept of microglia activation in neuropathological con-
`ditions has changed profoundly over the last few years. Initially
`thought as bystander cells with only marginal causal effects on
`neurodegeneration, an exaggerated microglia reaction is now
`thought to contribute significantly or even trigger neuronal
`apoptosis in several diseases including retinal dystrophies [1].
`Furthermore, physiological, neurotrophic microglia function
`and microglia-neuron cross-talk are now known as prerequi-
`sites for maintaining the immune privilege of the CNS and the
`retina. Loss of this regulation could also be a major cause for
`tissue damage. Based on these findings, microglia-targeted
`treatment could be envisioned to inhibit overactivation and
`support neuronal survival and tissue regeneration.
`Several excellent review articles provide a comprehensive
`overview over microglia physiology and pathology in the brain,
`especially related to Alzheimer’s disease, Parkinson’s disease,
`and multiple sclerosis [2–5]. In contrast, the homeostatic func-
`tions of retinal microglia and the contribution of overactivated
`
`0741-5400/07/0081-1345 © Society for Leukocyte Biology
`
`resident microglia to retinal neurodegeneration are less well
`covered. Therefore, this review will focus on the role of micro-
`glia-related mechanisms in the healthy retina and will empha-
`size the involvement of microglia in the process of retinal
`degeneration.
`
`MICROGLIA CELLS IN THE DEVELOPING CNS
`AND THE RETINA
`
`It is now generally accepted that microglia cells are part of the
`mononuclear phagocyte system in the parenchyma of the CNS
`including the retina [6]. As shown by bone marrow transplan-
`tation experiments and in vivo labeling assays, microglia pre-
`cursors migrate through immature blood vessels into the retinal
`tissue during late embryonic development and in the early
`postnatal period [7]. The colonization of microglia precursors is
`coordinated by a spatially and temporally regulated expression
`of
`the chemokines MCP-1 and RANTES [8]. Besides the
`remodeling of extracellular matrix (ECM) components during
`the formation of the neuron-glia network, the main functions of
`microglia in the developing retina are phagocytosis and elim-
`ination of cellular debris from apoptotic neurons in the gan-
`glion cell layer and inner nuclear layer.
`Using an antibody against the macrophage marker F4/80,
`Hume et al. [9] could demonstrate the migration of amoeboid-
`shaped microglia cells with short and broad processes and their
`colocalization with pyknotic nuclei of dying cells shortly after
`birth. Studies in embryonic chicken retina demonstrated that
`the developmental neuronal cell death is reduced strongly in
`retinal explants lacking microglia cells and that microglia-
`derived nerve growth factor (NGF) is required to induce apo-
`ptosis [10]. Furthermore, microglia are able to initiate a cell
`death program in vascular cells of the developing eye by
`activating the canonical wingless pathway [11] and also to
`regulate synaptogenesis in early CNS development [12]. The
`signaling adaptor protein, DNAX-activating protein of 12 kDa
`(DAP12)/killer cell-activating receptor-associated protein/ty-
`rosine kinase-binding protein,
`is expressed specifically on
`amoeboid microglia, and its loss of function impairs the brain-
`derived neurotrophic factor (BDNF)/tyrosine kinase receptor B
`synaptic pathway, leading to developmental defects in synap-
`togenesis [13]. It
`is interesting that DAP12 together with
`
`1 Correspondence: Institute of Human Genetics, University of Regensburg,
`Franz-Josef-Strauss-Allee
`11,
`93053 Regensburg, Germany. E-mail:
`thomas.langmann@klinik.uni-regensburg.de
`Received February 15, 2007; accepted March 25, 2007.
`doi: 10.1189/jlb.0207114
`
`Journal of Leukocyte Biology Volume 81, June 2007 1345
`CYAN EXHIBIT 1067
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`

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`triggering receptor expressed on myeloid cells 2 are also re-
`quired to potentiate phagocytosis, to clear apoptotic neurons,
`and to block an exaggerated response to TLR signaling after
`microbial challenge or CNS injury [14, 15]. The scavenger
`receptors CD36, SR-A, and SR-BI are further microglia recep-
`tors involved in the engulfment and uptake of cell corpses and
`membrane debris from developmentally dying neurons [16].
`As development is completed, these activated phagocytes
`are transformed into resting microglia cells, constituting
`5–20% of the overall glial population. Because of their unique
`neuronal microenvironment, microglia differ from other tissue
`macrophages by their high capability of proliferation in situ
`and their unique, characteristic ramified morphology. Specific
`interactions with astrocytes, their ECM components, and their
`receptor-mediated responses to astrocyte-secreted factors in-
`cluding GM-CSF and M-CSF favor microglia proliferation,
`survival, and differentiation [17, 18].
`In the adult retina, ramified microglia are found in regular,
`ordered structures in inner and outer plexiform layers as shown
`by F4/80 immunohistochemistry [9] and cellular tracing exper-
`iments with fluorescent dyes [19]. When retinal ganglion cells
`(RGCs) were labeled retrogradely with the lipophilic dye 4Di-
`10ASP, microglia cells containing fluorescent RGC debris
`could still be observed after 10 months, indicating a long
`period of survival of the local microglia population [19 –21].
`Further support for microglial stability and local repopulation
`in the retina comes from bone marrow transplantation experi-
`ments using chimeric (Y3 X) Lewis rats. In this model, a
`rapid, chimeric repopulation of hematopoietic cells in the
`spleen and the lung of lethally irradiated and transplanted
`female recipient rats has been observed, whereas no chimerism
`could be detected in the brain parenchyma and the retina until
`52 weeks of transplantation [22]. Similarly, engraftment studies
`with lacZ-expressing murine bone marrow cells indicate a slow
`turnover of microglia cells compared with other resident tissue
`macrophages with local proliferation capacity such as alveloar
`macrophages and Langerhans cells [23, 24]. These studies also
`show that bone marrow-derived monocytes do not typically
`cross the blood-retina barrier in the healthy retina.
`
`CROSS-TALK BETWEEN RESTING MICROGLIA
`AND RETINAL CELLS
`
`Different types of glia cells with neuro-supporting function are
`present in the CNS, including oligodendrocytes, astrocytes,
`Mu¨ller cells, and microglia cells, which are cells with a role in
`immunocompetent defense and also play an active part in the
`support of surrounding cells [3]. The immunological potential
`of microglia is comparable with blood monocytes and other
`tissue macrophages in terms of secretory functions. However,
`resting microglia express lower levels of costimulatory mole-
`cules and possess relatively low phagocytic activity [25]. The
`maintenance of the normal retinal immune regulation seems to
`actively involve cytokines from the retinal pigment epithelium
`such as TGF-␤ (Fig. 1). This cytokine contributes to the
`immune privilege by predisposing microglia to the preferential
`production of IL-10, which in turn down-regulates antigen-
`presenting molecules including MHC-II, CD80, and CD86
`
`[26]. TGF-␤ also directs TNF/IFN-␥-stimulated microglia cells
`to an anti-inflammatory phenotype by broadly blocking inflam-
`matory gene expression [27].
`Another important control mechanism to limit microglia
`activation in the healthy retina relies on the Ig superfamily
`domain-containing molecule CD200, which is recognized by
`the well-known mAb OX2 and its receptor on microglia [28]
`(Fig. 1). CD200 is a transmembrane glycoprotein with a short
`cytoplasmic tail without signaling motifs and is expressed on
`many different cell types including neurons, endothelial cells,
`lymphocytes, and dendritic cells [29, 30]. The CD200R is
`expressed exclusively on myeloid cells, and ligand-binding via
`cellular contact triggers signaling events, sustaining the basal,
`deactivated state with mainly homeostatic functions of macro-
`phages and microglia cells [31, 32]. Strong evidence for potent
`inhibition of microglia activity comes from CD200 knockout
`mice, which display an expansion of the myeloid population in
`several
`tissues and increased expression of
`the activation
`markers DAP12, CD11b, CD45, CD68, and inducible NO
`synthase (iNOS) on brain microglia [31]. A broad, constitutive
`expression of CD200 on retinal vessel endothelium and neu-
`rons and the induction of CD200R expression during retinal
`inflammation also strongly implicate CD200/CD200R interac-
`tions in the retina [33]. Furthermore, the potential of microglia
`to migrate and to secrete the anti-inflammatory cytokine IL-10
`following LPS/IFN-␥ stimulation requires CD200R, as shown
`by the potent inhibitory activity of CD200-Fc fusion proteins
`when added to retinal explants [34].
`In the quiescent state and following activation, microglia
`cells secrete several polypeptide neurotrophic factors, which
`impact the physiology and survival of neurons. Among these
`factors, BDNF, ciliary neurotrophic factor (CNTF), glial cell
`line-derived neurotrophic factor (GDNF), NGF, neurotrophin-3
`(NT3), and basic fibroblast growth factor (bFGF) have been
`shown to protect and regulate the survival of photoreceptors
`[35] (see Table 1). It is important that a functional microglia-
`Mu¨ller glia cell interaction is required for these trophic factors
`to work in an autocrine and paracrine manner [36]. Additional
`microglia-derived factors that stimulate the survival and regen-
`eration of retinal ganglion cells after nerve injury have been
`identified using coculture of neurons with microglia-condi-
`tioned medium [37]. One of
`these neuronal growth factor
`proteins secreted by microglia is the small Ca2⫹-binding pro-
`tein oncomodulin, which acts through a Ca2⫹/calmodulin ki-
`nase-dependent signaling pathway to stimulate neurite regen-
`eration of RGCs [38].
`
`IMMUNE SURVEILLANCE FUNCTION OF
`RESTING MICROGLIA
`
`In contrast to an initially prevailing view that mature microglia
`cells are in a dormant state when deactivated in the healthy
`CNS, these “resting” cells monitor their environment with
`highly motile protrusions to clear metabolic products and tis-
`sue debris [39]. Microglia can sense their microenvironment
`through various surface proteins including receptors for im-
`mune components such as complement, cytokines, chemo-
`kines, antibodies, and adhesion molecules [3] (see Table 1). It
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`Fig. 1. Mechanisms of microglia quiescence and activation related to neuronal degeneration. Ramified microglia are kept in a resting, stand-by mode to regulate
`retinal homeostasis by intraretinal cell contacts and soluble factors from neurons, astrocytes, and the retinal pigment epithelium. Several triggers from retinal
`degeneration can initiate TLR signaling leading to microglia activation, proliferation, and migration. Transformed amoeboid microglia secrete various bioactive
`molecules, initially active in tissue repair. Chronic activation may lead to exaggerated microglia responses, leading to retinal damage and neuronal apoptosis. Egr1,
`Early growth response factor 1; CD220R, CD200 receptor; ROS, reactive oxygen species.
`
`is important that the cells also possess receptors for the che-
`mokine fractalkine (CX3CL1) and for purine nucleotides [4]. A
`constitutive release of fractalkine from healthy neurons, which
`bind to CX3CR1 on microglia cells, seems to regulate micro-
`glia homeostasis, and loss of CX3CR1 leads to neurotoxicity
`and degeneration [40]. Fractalkine may also serve as a neuro-
`nal stress signal to induce intimate neuron-glia cross-talk,
`resulting in the release of trophic molecules such as TGF-␤1,
`which facilitates neuronal tissue regeneration [41]. As fractal-
`kine also exists as a membrane-bound molecule on neurons,
`direct cell-cell contact and interaction with the ECM could
`additionally restrain microglia activation via this pathway [42].
`The Gi-coupled P2Y12 purinergic receptor is highly ex-
`pressed on resting microglia in the brain but not other tissue
`macrophages. Membrane-anchored P2Y12 allows detection of
`ATP and ADP release from damaged neurons and is required
`for subsequent early activation mechanisms [43]. Mice lacking
`P2Y12 show normal prevalence, distribution, and morphology
`of resting microglia; however, no membrane ruffling and filo-
`podia extension occur upon stimulation with ADP or ATP.
`P2Y12 is down-regulated actively during microglial transfor-
`mation from the resting to the amoeboid state, and thus, the
`P2Y12 receptor may be a novel molecular marker for visual-
`izing microglia in their ramified state [43]. These mechanisms
`observed in the brain likely occur also in retinal microglia, as
`several P2Y receptor subtypes have been detected in the retina
`at the mRNA and protein level [44].
`
`MICROGLIA ACTIVATION IN RETINAL
`DEGENERATION MODELS
`
`Early microglia activation in the retina is a common response
`to ocular infections, autoimmune mechanisms, neuronal injury,
`ischemia, and metabolic as well as hereditary retinopathies,
`which are all associated with progressive neurodegeneration
`[1]. Activated microglia exhibit strongly enhanced prolifera-
`tion, migration, phagocytosis, and production of many different
`bioactive molecules (Table 1 and Fig. 1). The morphological
`change from ramified cells to amoeboid phagocytes is accom-
`panied by the expression of several surface markers such as
`F4/80, complement receptor 3 (CD11b/CD18, OX42), MHC-II
`(OX6), CD68, and Griffonia simplicifolia isolectin B4, which
`have been used classically to detect microglia activity by
`immunohistochemistry and immunofluorescence-staining pro-
`cedures [2, 46].
`There are now several studies [47, 48], including work from
`my own group [49], demonstrating early microglia activation in
`animal models of inherited photoreceptor degeneration. Zeiss
`and Johnson [47] could show prominent microglia migration
`and proliferation in the outer nuclear layer of retinal degener-
`ation mice. Furthermore, increased expression of the micro-
`glia-activating chemokines MCP-1, MCP-3, and RANTES as
`well as high levels of microglia-secreted TNF were observed in
`the retina of these mice well before the onset of photoreceptor
`apoptosis [48]. A recent investigation of our group, reported on
`
`Langmann Microglia quiescence and activation in the retina 1347
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`TABLE 1. Selection of Microglia-Expressed Receptors and Secreted Factors
`
`Molecule class
`
`Scavenger receptors
`Ig receptors
`Ig-like receptors
`
`Complement receptors
`
`Growth factor receptors
`Chemokine and cytokine receptors
`
`Purinergic receptors
`
`TLRs
`Antigen-presenting proteins
`Cytokines and chemokines
`
`Neurotransmitters and neurotoxins
`Neurotrophins
`
`Molecules
`
`Functions in microglia
`
`SR-A, SR-BI, CD36
`Fc␥RI, Fc␥RII, Fc␥RIII, Fc⑀RI
`CD200R
`
`C3bi-R (CR3, CD11b, CD18), C1q-R,
`C3a-R, C5a-R
`M-CSF-R, GM-CSF-R
`CX3CR1 (Fractalkine-R), TGF-␤-R,
`CCR2 (MCP-1-R), CCR5 (RANTES-R)
`Receptors for P2X4, P2X7, P2Y1, P2Y2,
`P2Y4, P2Y6, P2Y12
`TLR1–9
`MHC-II, CD80, CD86
`TNF, TGF-␤, IL-1␣/␤, IL-3, IL-6, IL-8,
`IL-10, IL-12, IL-15, IL-18, GRO␣,
`IP-10, MCP-1, MDC, M-CSF, MIP-1␣/
`␤, MIP-2, MIP-3␤, RANTES
`Glutamate, NO, ROS
`BDNF, CNTF, GDNF, NGF, NT3, bFGF,
`oncomodulin
`
`Uptake of cell debris and apoptotic corpses
`Uptake of Ig-opsonized particles
`Deactivation of resting cells upon ligation
`with CD200
`Uptake of complement components and
`opsonized particles
`Proliferation and survival
`Neuron-microglia signaling and immune
`signaling
`Purinergic sensing and early activation
`
`Pathogen and damage-associated activation
`Presentation of phagocytosed material
`Immunostimulation
`
`Cellular toxicity
`Neurotrophic signals
`
`Summarized and adapted from refs. [3, 4, 28, 45]. GRO␣, Growth-related oncogene ␣; IP-10, IFN-inducible protein 10; MDC, macrophage-derived chemokine.
`
`the retinal gene expression profile in the murine model of
`X-linked juvenile retinoschisis, was analyzed [49, 50]. A hall-
`mark of the retinoschisin-deficient murine retina is the pro-
`gressive loss of cone and rod photoreceptor cells [50] as a
`result of apoptotic events peaking at approximately Postnatal
`Day 18 [51]. Using DNA-microarray analyses, we could iden-
`tify several transcripts from activated microglia cells preceding
`gene expression patterns related to apoptosis in the diseased
`retina. Besides detrimental proinflammatory cytokines, chemo-
`kines, complement components, and scavenger receptors, we
`have also identified self-regulatory mechanisms by prominent
`expression of the macrophage deactivation gene DAP12 (see
`above) and caspase 11, which have been shown to mediate
`overactivation-induced microglial cell death [52, 53]. Related
`to our study, partially overlapping gene expression patterns
`reflecting activated microglia and indicating common tran-
`scriptional mechanisms have been identified in light-induced
`retinal degeneration [54], glaucoma models [55, 56], retinal
`needle injury [57], and neurodegeneration in the brain of
`Sandhoff disease [58] and mucopolysaccharidoses [59].
`Based on these data of early microglia responses, it can be
`hypothesized that activated microglia cells are not simply
`bystanders of neurodegeneration but instead, may be in-
`volved significantly in the initiation and perpetuation of the
`degenerative process.
`As an important tool to analyze ex vivo microglia character-
`istics in early stages of retinal dystrophies, Roque and Cald-
`well [60] developed an isolation and culture model of retinal
`microglia cells from Royal College of Surgeons (RCS) rats, an
`established model of genetic photoreceptor dystrophy. The
`phagocytic potential of the isolated cells, monitored by the
`ingestion of latex beads and the ability to proliferate in vitro,
`was dependent on the presence of recombinant M-CSF in the
`culture medium [60]. Our group has adapted this protocol for
`the isolation of early postnatal microglia cells from retinoschi-
`
`is interesting that we
`sin-deficient and wild-type mice. It
`observed the preservation of the activated microglia phenotype
`defined by amoeboid morphology and specific gene expression
`patterns only in growth medium supplemented with M-CSF
`(unpublished observations). Thus, M-CSF, which acts via the
`Fms tyrosine kinase receptor on myeloid cells, seems to rep-
`resent a key factor for the initiation and maintenance of mi-
`croglia activation, independent of the initial trigger. Supporting
`this view, overexpression of M-CSF-R in microglia cell lines
`results in proliferation and strongly increased expression of
`iNOS, IL-1␤, MIP-1␣, IL-6, and M-CSF itself [61]. As another
`consequence of M-CSF-R ligation, the myeloid-specific down-
`stream signaling molecule Iba1 causes Rac-dependent, dy-
`namic remodeling of the actin cytoskeleton, resulting in mem-
`brane ruffling, which is required for efficient phagocytosis [62].
`To study whether retinal microglia cells from RCS rats can
`induce photoreceptor apoptosis in vitro, coculture experiments
`with microglia-conditioned medium and the 661w photorecep-
`tor cell line were performed by Roque et al. [63]. In contrast to
`medium from isolated Mu¨ller cells, addition of cell culture
`supernatant from activated microglia cells resulted in a signif-
`icant increase of apoptosis-related cell death in 661w cells
`[63]. This effect was dependent on microglia-derived NGF and
`p75 neurotrophin receptors (p75NTR) on photoreceptor cells.
`It is noteworthy that augmented NGF and p75NTR mRNA
`levels were also detected in dystrophic compared with normal
`retinae [64]. These data clearly show that microglia activation
`can trigger neuronal cell death. It remains to be determined,
`however, whether the same factors are also involved in apo-
`ptosis induction in other retinal dystrophies. Furthermore, as
`immortalized 661w cells only express a subset of cone-specific
`photoreceptor markers [65], the observed in vitro effects need
`to be verified in vivo with cultures of purified, primary rods and
`cones.
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`EARLY SIGNALING EVENTS AND
`TRANSCRIPTIONAL NETWORKS IN
`MICROGLIA ACTIVATION
`
`To identify triggers in early microglia activation, mainly in
`vitro culture models with primary cells and several immortal-
`ized cell lines such as BV-2, HMO6, and CHME were used
`[66]. After stimulation, the read-out parameters for enhanced
`microglia activation were cell proliferation, phagocytosis, mi-
`gration, expression of surface receptors, and the release of
`cytokines, chemokines, prostaglandins, NO, superoxide an-
`ions, and glutamate [53]. The majority of these microglia-
`secreted molecules can cause progressive neurodegeneration
`upon chronic exposition. The most potent natural triggers for
`microglia activation include LPS, proinflammatory cytokines
`including IFN-␥ and TNF, complement components, thrombin,
`and aggregated, insoluble peptides [1]. It is important that LPS
`seems to be the strongest inducer of microglia cells, and LPS
`signaling can be initiated, even in the absence of exogenous,
`infectious agents.
`It is becoming increasingly recognized that the pattern rec-
`ognition receptors of the TLR family, which are expressed
`broadly on microglia, can react to aberrant endogenous ligands
`in neuronal tissues [67]. Gangliosides, hyaluronic acid, hepa-
`ran sulfate, and heat shock proteins carry damage-associated
`molecular patterns and thereby can elicit microglia activation
`[68]. It is thus tempting to speculate that early alarm signals
`from degenerating neuronal tissues might initiate TLR-depen-
`dent activation and hyperactivation of microglia, which may
`even lead to attacks against healthy neurons (Fig. 1). In line
`with this hypothesis, we have detected in the retina of retinos-
`chisin-deficient mice early microglial TLR4 induction and a
`strong up-regulation of activation-related transcripts, likely
`leading to neurotoxicity and consecutively, photoreceptor ap-
`optosis [49]. It is interesting that TLR4-dependent pathways in
`microglia activation have also been connected to other models
`of neuronal injury including spinal nerve transection and pain-
`ful neuropathy [69, 70]. It is noteworthy that TLR4 and TLR2
`also control autoregulatory mechanisms by triggering the in-
`duction of microglia apoptosis in vitro [71]; however, the con-
`tribution of this pathway to prevent latent microglia overacti-
`vation remains to be determined in vivo.
`A novel candidate for downstream signaling of TLR4 in
`overactivated microglia is Egr1, which is also required for
`ocular tissue development [72, 73]. Egr1 is strongly up-regu-
`lated in several retinal dystrophies including retinoschisin
`deficiency [49]. Egr1 expression is LPS-responsive in the mi-
`croglia cell line BV-2 and ex vivo-isolated retinal primary
`microglia (own unpublished observations). Furthermore, sev-
`eral microglia genes induced upon activation contain predicted
`binding sites for Egr1 in their promoter regions. To study the in
`vivo effects of Egr1 in the retina, mice lacking retinoschisin
`and Egr1 were generated [49]. It is unexpected that no major
`differences in retinal microglia activation could be detected in
`these animals compared with retinoschisin-deficient mice [49].
`A likely explanation for this finding is the functional redun-
`dance of the Egr family members. Indeed, a recent study by
`Laslo et al. [74] showed that Egr1 and Egr2 are exchangeable
`in activating macrophage genes and that only Egr1⫺/⫺Egr2-/⫹
`
`hematopoietic progenitors are defective in M-CSF-dependent
`macrophage differentiation.
`
`CONCLUSIONS AND PERSPECTIVES
`
`Similar to the CNS, microglia cells in the retina are sensors for
`disturbances in their neuronal environment. Their balanced
`activities initially contribute to neuronal protection and tissue
`regeneration. Continuous stimulation with alarm signals from
`exogenous and endogenous sources can lead to chronic over-
`activation and loss of autoregulatory mechanisms, which are
`detrimental for neurons. In vitro culture systems and animal
`models of retinal degeneration have provided a first insight into
`specific gene expression profiles associated with microglia
`activation. To understand further the processes and triggers of
`microglia activation, molecular characterization of distinct mi-
`croglia populations using cell isolation and sorting techniques
`combined with large-scale gene expression studies might be
`useful. This should enhance the identification of novel biolog-
`ical markers specific for resting, activated, or overactivated
`cells and may allow unraveling of genetic networks and key
`transcription factors controlling the different steps of microglia
`transformation. Furthermore, these studies may provide novel
`treatment options for selective inhibition of overshooting mi-
`croglia activity and preserving the trophic and homeostatic
`functions of these important immune cells.
`
`ACKNOWLEDGMENT
`
`The author thanks Prof. Bernhard Weber for discussions and
`critical reading of the manuscript.
`
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