Modification of glial-neuronal cell interactions prevents photoreceptor apoptosis during light-induced retinal degeneration

Müller glia factors induce survival and neuritogenesis of peripheral and central neurons

We have examined the trophic effects of conditioned media obtained from purified murine Müller glia cells on chick purified sympathetic or dorsal root ganglia (DRG) neurons and on Retinal Ganglion Cells (RGC) from postnatal mice. Purified murine Müller glia cultures stained positively for vimentin, GFAP or S-100, but were negative for neuronal markers. Murine Müller glial conditioned medium (MMG) was concentrated and at 1:1 dilution supported 100% survival of chick or rat sympathetic neurons after 48 h compared to <5% in controls. Partial purification of the MMG using centriprep concentrators showed that trophic activity is from molecules above 10 kDa. MMG stimulated AKT, ERK and pStat3 in sympathetic neurons. Sympathetic or DRG neuronal survival induced by MMG was blocked by anti-human NGF, but not by anti-human CNTF (sympathetic) or by anti-BDNF (DRGs) neutralizing antibodies. MMG also induced neurite outgrowth in P4 mice retinal explants and on isolated RGC. RGCs plated on top of Müller glia cells had a much better survival rate (>80%, 96 h) compared to laminin+poly-L-lysine substrates. In conclusion, we show that purified mice Müller glia cultures secrete NGF that support peripheral neuronal survival and other unidentified trophic molecules that induce RGC survival and neuritogenesis.

Orally administered epigallocatechin gallate attenuates light-induced photoreceptor damage

EGCG, a major component of green tea, has a number of properties which includes it being a powerful antioxidant. The purpose of this investigation was to deduce whether inclusion of EGCG in the drinking water of albino rats attenuates the effect of a light insult (2200lx, for 24h) to the retina. TUNEL-positive cells were detected in the outer nuclear layer of the retina, indicating the efficacy of the light insult in inducing photoreceptor degeneration. Moreover, Ret-P1 and the mRNA for rhodopsin located at photoreceptors were also significantly reduced as well as the amplitude of both the a- and b-waves of the electroretinogram was also reduced showing that photoreceptors in particular are affected by light. An increase in protein/mRNA of GFAP located primarily to Müller cells caused by light shows that other retinal components are also influenced by the light insult. However, antigens associated with bipolar (alpha-PKC), ganglion (Thy-1) and amacrine (GABA) cells, in contrast, appeared unaffected. The light insult also caused a change in the content of various proteins (caspase-3, caspase-8, PARP, Bad, and Bcl-2) involved in apoptosis. A number of the changes to the retina caused by a light insult were significantly attenuated when EGCG was in the drinking water. The reduction of the a- and b-waves and photoreceptor specific mRNAs/protein caused by light were significantly less. In addition, EGCG attenuated the changes caused by light to certain apoptotic proteins (especially at after 2 days) but did not appear to significantly influence the light-induced up-regulation of GFAP protein/mRNA. It is concluded that orally administered EGCG blunts the detrimental effect of light to the retina of albino rats where the photoreceptors are primarily affected.

Real-time in vivo imaging of retinal cell apoptosis after laser exposure

PURPOSE

To investigate whether the detection of apoptosing retinal cells (DARC) could detect cells undergoing apoptosis in a laser model of retinal damage.

METHODS

Laser lesions were placed, with the use of a frequency-doubled Nd:YAG laser, on the retina in 34 eyes of anesthetized Dark Agouti rats. Lesion size and laser-induced retinal elevation were analyzed using in vivo reflectance imaging. Development of retinal cell apoptosis was assessed using intravitreal fluorescence-labeled annexin 5 in vivo with DARC technology from baseline until 90 minutes after laser application. Histologic analysis of retinal flat mounts and cross-sections was performed.

RESULTS

The lateral and anteroposterior depth extension of the zone of laser damage was significantly larger for higher exposure settings. A strong diffuse signal, concentrated at the outer retina, was seen with DARC for low exposures (<300 ms and <300 mW). In comparison, higher exposures (>300 ms and >300 mW) resulted in detectable hyperfluorescent spots, mainly at the level of the inner retinal layers. Dose-dependent effects on spot density and positive correlation of spot density between lesion size (P < 0.0001) and retinal elevation (P < 0.0001) were demonstrated. Histology confirmed the presence of apoptosing retinal cells in the inner nuclear and the ganglion cell layers.

CONCLUSIONS

This is the first time that DARC has been used to determine apoptotic effects in the inner nuclear layer. The ability to monitor changes spatially and temporally in vivo promises to be a major advance in the real-time assessment of retinal diseases and treatment effects.

Müller glia as an active compartment modulating nervous activity in the vertebrate retina: neurotransmitters and trophic factors

Müller cells represent the main type of glia present in the retina interacting with most, if not all neurons in this tissue. Müller cells have been claimed to function as optic fibers in the retina delivering light to photoreceptors with minimal distortion and low loss [Franze et al (2007) Proc Natl Acad Sci 104:8287-8292]. Most of the mediators found in the brain are also detected in the retinal tissue, and glia cells are active players in the synthesis, release, signaling and uptake of major mediators of synaptic function. Müller glia trophic factors may regulate many different aspects of neuronal circuitry during synaptogenesis, differentiation, neuroprotection and survival of photoreceptors, Retinal Ganglion Cells (RGCs) and other targets in the retina. Here we review the role of several transmitters and trophic factors that participate in the neuron-glia loop in the retina.

Characterization of Wnt signaling during photoreceptor degeneration

PURPOSE

The Wnt pathway is an essential signaling cascade that regulates multiple processes in developing and adult tissues, including differentiation, cellular survival, and stem cell proliferation. The authors recently demonstrated altered expression of Wnt pathway genes during photoreceptor death in rd1 mice, suggesting an involvement for Wnt signaling in the disease process. In this study, the authors investigated the role of Wnt signaling in retinal degeneration.

METHODS

The Wnt signaling reporter mouse line Tcf-LacZ was crossed with retinal degeneration rd1 mice, and beta-galactosidase expression was used to localize Wnt signaling during photoreceptor death. To analyze the role of Wnt signaling activation, primary mixed retinal cultures were prepared, and XTT and TUNEL assays were used to quantify cell death. Luciferase reporter assays were used to measure Wnt signaling.

RESULTS

The canonical Wnt signaling pathway was activated in Müller glia and the ganglion cell layer during rod photoreceptor degeneration in rd1/Tcf-LacZ mice. Wnt signaling was confirmed in cultured primary Müller glia. Furthermore, Wnt signaling activators protected photoreceptors in primary retinal cultures from H(2)O(2)-induced oxidative stress. The Wnt ligands Wnt5a, Wnt5b, Wnt10a, and Wnt13 were expressed in the degenerating retina and are candidate Wnt signaling activators in vivo.

CONCLUSIONS

This study is the first demonstration that Wnt signaling is activated in the degenerating retina and that it protects retinal cultures from oxidative stress. These data suggest that Wnt signaling is a component of the glial protective response during photoreceptor injury. Therefore, inducing Wnt activation, alone or in combination with growth factors, may increase the threshold for apoptosis and halt or delay further photoreceptor degeneration.

Identification of two novel activities of the Wnt signaling regulator Dickkopf 3 and characterization of its expression in the mouse retina

BACKGROUND

The Wnt signaling pathway is a cellular communication pathway that plays critical roles in development and disease. A major class of Wnt signaling regulators is the Dickkopf (Dkk) family of secreted glycoproteins. Although the biological properties of Dickkopf 1 (Dkk1) and Dickkopf 2 (Dkk2) are well characterized, little is known about the function of the related Dickkopf 3 (Dkk3) protein in vivo or in cell lines. We recently demonstrated that Dkk3 transcripts are upregulated during photoreceptor death in a mouse model of retinal degeneration. In this study, we characterized the activity of Dkk3 in Wnt signaling and cell death.

RESULTS

Dkk3 was localized to Müller glia and retinal ganglion cells in developing and adult mouse retina. Western blotting confirmed that Dkk3 is secreted from Müller glia cells in culture. We demonstrated that Dkk3 potentiated Wnt signaling in Müller glia and HEK293 cells but not in COS7 cells, indicating that it is a cell-type specific regulator of Wnt signaling. This unique Dkk3 activity was blocked by co-expression of Dkk1. Additionally, Dkk3 displayed pro-survival properties by decreasing caspase activation and increasing viability in HEK293 cells exposed to staurosporine and H2O2. In contrast, Dkk3 did not protect COS7 cells from apoptosis.

CONCLUSION

These data demonstrate that Dkk3 is a positive regulator of Wnt signaling, in contrast to its family member Dkk1. Furthermore, Dkk3 protects against apoptosis by reducing caspase activity, suggesting that Dkk3 may play a cytoprotective role in the retina.

Rapid and Stable Knockdown of an Endogenous Gene in Retinal Pigment Epithelium

The selective silencing of target genes in specific cell types by RNA interference (RNAi) represents a powerful approach both to gene therapy of dominantly active mutant alleles, and to the investigation of normal gene function in animal models in vivo. We established a simple and versatile in vitro method for screening the efficacy of DNA-based short hairpin RNAs (shRNAs), and identified a highly effective shRNA targeting basic fibroblast growth factor (bFGF), a gene thought to play important roles in endogenous neuroprotective responses in the rat retina. We used two viral vectors, based on lentivirus and adeno-associated virus (AAV), to deliver shRNAs and silence bFGF in retinal pigment epithelial cells in vivo. The AAV experiments made use of a "stabilized double-stranded" version of these vectors with rapid onset of gene expression. In the rat retinal pigment epithelium, shRNAs delivered by either vector reduced bFGF immunoreactivity to undetectable levels in transduced cells, whereas a nonfunctional control construct incorporating a two-base pair mutation had no measurable effect on bFGF expression. Silencing commenced within a few days after injection of virus and remained stable throughout the period of observation, as long as 60 days. Viral delivery of RNAi constructs offers a powerful and versatile approach for both gene therapy and the analysis of fundamental questions in retinal biology.

The potential role of glutamate transporters in the pathogenesis of normal tension glaucoma

Glaucoma, a progressive optic neuropathy due to retinal ganglion cell (RGC) degeneration, is one of the leading causes of irreversible blindness. Although glaucoma is often associated with elevated intraocular pressure (IOP), IOP elevation is not detected in a significant subset of glaucomas, such as normal tension glaucoma (NTG). Moreover, in some glaucoma patients, significant IOP reduction does not prevent progression of the disease. Thus, understanding IOP-independent mechanisms of RGC loss is important. Here, we show that mice deficient in the glutamate transporters GLAST or EAAC1 demonstrate spontaneous RGC and optic nerve degeneration without elevated IOP. In GLAST-deficient mice, the glutathione level in Müller glia was decreased; administration of glutamate receptor blocker prevented RGC loss. In EAAC1-deficient mice, RGCs were more vulnerable to oxidative stress. These findings suggest that glutamate transporters are necessary both to prevent excitotoxic retinal damage and to synthesize glutathione, a major cellular antioxidant and tripeptide of glutamate, cysteine, and glycine. We believe these mice are the first animal models of NTG that offer a powerful system for investigating mechanisms of neurodegeneration in NTG and developing therapies directed at IOP-independent mechanisms of RGC loss.

Inactivation of NF1 in CNS causes increased glial progenitor proliferation and optic glioma formation

The gene responsible for neurofibromatosis type 1 (NF1) encodes a tumor suppressor that functions as a negative regulator of the Ras proto-oncogene. Individuals with germline mutations in NF1 are predisposed to the development of benign and malignant tumors of the peripheral and central nervous system (CNS). Children with this disease suffer a high incidence of optic gliomas, a benign but potentially debilitating tumor of the optic nerve; and an increased incidence of malignant astrocytoma, reactive astrogliosis and intellectual deficits. In the present study, we have sought insight into the molecular and cellular basis of NF1-associated CNS pathologies. We show that mice genetically engineered to lack NF1 in CNS exhibit a variety of defects in glial cells. Primary among these is a developmental defect resulting in global reactive astrogliosis in the adult brain and increased proliferation of glial progenitor cells leading to enlarged optic nerves. As a consequence, all of the mutant optic nerves develop hyperplastic lesions, some of which progress to optic pathway gliomas. These data point to hyperproliferative glial progenitors as the source of the optic tumors and provide a genetic model for NF1-associated astrogliosis and optic glioma.

New therapeutic targets in atrophic age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. There is no effective treatment for the most prevalent atrophic (dry) form of AMD. Atrophic AMD is triggered by abnormalities in the retinal pigment epithelium (RPE) that lies beneath the photoreceptor cells and normally provides critical metabolic support to these light-sensing cells. Secondary to RPE dysfunction, macular rods and cones degenerate leading to the irreversible loss of vision. Oxidative stress, formation of drusen, accumulation of lipofuscin, local inflammation and reactive gliosis represent the pathologic processes implicated in pathogenesis of atrophic AMD. This review discusses potential target areas for small-molecule and biologic intervention, which may lead to development of new therapeutic treatments for atrophic AMD.

Molecular regulation of visual system development: more than meets the eye

Vertebrate eye development has been an excellent model system to investigate basic concepts of developmental biology ranging from mechanisms of tissue induction to the complex patterning and bidimensional orientation of the highly specialized retina. Recent advances have shed light on the interplay between numerous transcriptional networks and growth factors that are involved in the specific stages of retinogenesis, optic nerve formation, and topographic mapping. In this review, we summarize this recent progress on the molecular mechanisms underlying the development of the eye, visual system, and embryonic tumors that arise in the optic system.

Targeted transgene expression in muller glia of normal and diseased retinas using lentiviral vectors

PURPOSE

Müller glia play crucial roles in retinal homeostasis and function. Genetic modification of Müller cells by viral gene delivery would be valuable for studies of their normal physiology and roles in retinal disease states. However, stable and efficient transgene expression in Müller cells after delivery of gene transfer vectors has remained elusive. Transcriptional and transductional targeting approaches were used to engineer recombinant HIV-1-based lentiviral (LV) vectors capable of highly efficient and sustained Müller cell transgene expression in healthy and diseased rodent retinas.

METHODS

Expression cassettes containing glia-specific promoters (CD44, glial fibrillary acidic protein, and vimentin) and an enhanced green fluorescent protein (eGFP) cDNA were cloned into LV backbones, which were packaged into infectious vector particles displaying either the vesicular stomatitis virus (VSV) or Ross River virus (RRV) envelope surface glycoproteins. Vectors were injected by intravitreal and subretinal approaches in wild type Sprague-Dawley (SD) and retinal degenerate S334Ter(+/-) transgenic rats aged 1 to 180 days. In vivo fluorescent fundus imaging and immunofluorescent confocal microscopy were used for comparison of expression efficiency, cell type specificity, and temporal expression characteristics.

RESULTS

The choice of viral pseudotype, regulatory promoter, and surgical delivery site each had a measurable effect on the level of eGFP transgene expression in Müller cells. The highest expression levels in SD retinas were attained with subretinal injection of VSV-G pseudotyped LV vectors containing the CD44 promoter. With these vectors, persistent eGFP expression in Müller glia was observed for more than 6 months, covering 25% to 30% of the retinal surface area after a single subretinal injection. Immunohistochemistry (alpha-glutamine synthetase) revealed that approximately 95% of the Müller cells were transduced in the region near the injection site. Delivery of these viral vectors and subsequent Müller cell eGFP expression had no negative impact on visual function, as assessed by electroretinography (ERG).

CONCLUSIONS

Pseudotyped LV vectors containing glia-specific promoters efficiently transduce and direct sustained transgene expression in retinal Müller glia. Vectors of this type will be useful for experimental treatment of retinal disease, as well as for physiological and developmental investigations of the retina.

Dopaminergic signaling in the developing retina

The role of dopamine in the retina has been studied for the last 30 years and there is now increasing evidence that dopamine is used as a developmental signal in the embryonic retina. Dopamine is the main catecholamine found in the retina of most species, being synthesized from the L-amino acid tyrosine. Its effects are mediated by G protein coupled receptors constituting the D(1) (D(1) and D(5)) and D(2) (D(2), D(3) and D(4)) receptor subfamilies that can be coupled to adenylyl cyclase in opposite manners. Dopamine-mediated cyclic AMP (cAMP) accumulation, via D(1)-like receptors, is observed very early during retina ontogeny, before synaptogenesis and, in some species, before the expression of tyrosine hydroxylase (TH), the enzyme that characterizes the neuronal dopaminergic phenotype. D(2)-like receptors appear in the tissue days after D(1)-like activity is detected. In the embryonic avian retina, before the tissue is capable of synthesizing its own dopamine via TH, dopamine synthesis is observed from L-DOPA supplied to the neuroretina from retina pigmented epithelium which results in dopaminergic communication in the embryonic tissue before TH expression. Müller cells, the main glia type found in the retina, seem to actively contribute to dopaminergic activity in the retinal tissue. Understanding the dopaminergic role during retina development may contribute to novel strategies approaching certain visual dysfunctions such as those found in ocular albinism.

CNTF+BDNF treatment and neuroprotective pathways in the rd1 mouse retina

The rd1 mouse is a relevant model for studying the mechanisms of photoreceptor degeneration in retinitis pigmentosa. Treatment with ciliary neurotrophic factor (CNTF) in combination with brain derived neurotrophic factor (BDNF) is known to rescue photoreceptors in cultured rd1 retinal explants. To shed light on the underlying mechanisms, we studied the effects of 9 days (starting at postnatal day 2) in vitro CNTF+BDNF treatment on the endogenous production of CNTF, BDNF, fibroblast growth factor 2 (FGF2), or the activation of extracellular signal-regulated kinase (ERK), Akt and cAMP-response-element-binding protein (CREB) in retinal explants. In rd1 explants, CNTF+BDNF decreased the number of TUNEL-positive photoreceptors. The treatment also increased endogenous rd1 levels of CNTF and BDNF, but lowered the level of FGF2 expression in rd1 explants. When wild-type explants were treated, endogenous CNTF was similarly increased, while BDNF and FGF2 levels remained unaffected. In addition, treatment of rd1 retinas strongly increased the phosphorylation of ERK, Akt and CREB. In treated wild-type explants, the same parameters were either unchanged (ERK) or decreased (Akt and CREB). The results suggest a role for Akt, ERK and CREB in conveying the neuroprotective effect of CNTF+BDNF treatment in rd1 retinal explants.

Role of monocyte chemotactic protein-1 and nuclear factor kappa B in the pathogenesis of proliferative diabetic retinopathy

Intraocular concentrations of monocyte chemotactic protein-1 (MCP-1) are increased in proliferative diabetic retinopathy (PDR). Nuclear factor kappa B (NF-kappaB) is a transcription factor, and NF-kappaB binding site is located in gene promoter of MCP-1. This study was conducted to investigate the potential role of MCP-1 and NF-kappaB in the pathogenesis of PDR. Epiretinal membrane (ERM) samples were obtained during vitrectomy from 19 eyes with PDR and 16 eyes with idiopathic ERM. They were processed for RT-PCR analysis. Four PDR ERMs were processed for immunohistochemical analysis. In addition, cultured Müller glial cells were stimulated with glycated albumin or high glucose. After the stimulation, we examined nuclear localization of NF-kappaB p50, MCP-1 promoter activity, and MCP-1 concentration in culture media. MCP-1 mRNA expression was significantly higher in PDR (74%) than in idiopathic ERMs (38%) (P < 0.05). Immunohistochemical analysis revealed that MCP-1 protein is colocalized with active form of NF-kappaB p50. In vitro studies demonstrated that glycated albumin or high glucose induces NF-kappaB activation followed by up-regulation of MCP-1 promoter activity and protein production in glial cells. These results suggest that MCP-1, under the regulation of NF-kappaB, is involved in the pathogenesis of PDR.

Differential role of Jak-STAT signaling in retinal degenerations

Retinal degeneration is a major cause of severe visual impairment or blindness. Understanding the underlying molecular mechanisms is a prerequisite to develop therapeutic approaches for human patients. We show in three mouse models that induced and inherited retinal degeneration induces LIF and CLC as members of the interleukin (IL)-6 family of proteins, activates proteins of the Jak-STAT signaling pathway, and up-regulates suppressors of cytokine signaling as a negative feedback loop. Inhibition of Jak2 leads to protection of photoreceptors in a model of induced but not in a model of inherited retinal degeneration. Differential activation of Akt suggests alternative pathways for cell death and/or survival in different models. Proteins induced during photoreceptor degeneration are not mainly expressed in photoreceptors but in cells of other retinal layers. This suggests a model in which photoreceptor injury is signaled to cells of the inner retina, which in turn initiate a response either to support viability or accelerate death of injured cells.

Multiple, parallel cellular suicide mechanisms participate in photoreceptor cell death

Photoreceptor degeneration in human photoreceptor dystrophies and in the relevant animal models has been thought to be executed by one common mechanism -- caspase-mediated apoptosis. However, recent experiments have challenged this concept. In previous experiments, analyzing gene expression in the degenerating rd/rd mouse retina, we have suggested that the gene defect leads to oxidative stress and altered metabolism, which may induce caspase-dependent and caspase-independent cell death mechanisms such as the activation of cystein-proteases, lysosomal proteases, autophagy and complement-mediated lysis. In this study we asked two questions. First, whether a temporal analysis of these different mechanisms during the course of degeneration would enable us to establish a causal relationship between these events; and second, whether photoreceptor degeneration in different models of photoreceptor dystrophies occurs by activating the same mechanisms. Three models of photoreceptor degeneration were chosen in which photoreceptor degeneration is caused by different events: the rd/rd mouse (calcium overload); the rds/rds mouse (structural defect); and light-damage (LD; oxidative stress). Marker genes were selected for the identified processes. PCR-analysis on laser capture microdissection samples was used to verify the expression of these genes in the rod photoreceptor layer. A temporal relationship between the processes was established at the mRNA level, using quantitative RT-PCR. The time course of gene expression was compared to that of cell loss (loss of rows of photoreceptor nuclei) and apoptosis (TUNEL labeling). Apoptosis and autophagy was analyzed using enzymatic assays. The time course of apoptosis and TUNEL labeling coincide in all three models. Complement-activated lysis was found to either parallel (rd/rd and rds/rds) or precede (LD) the development of TUNEL-positive cells. Autophagy was determined to parallel (rd/rd and LD) or lag (rds/rds) behind the development of TUNEL-positive cells. In all three models, glucose metabolism was found to be increased significantly prior to the onset of cell death, but then dropped in parallel with the loss of cells. The presence of the marker genes was verified by laser capture microdissection, and apoptosis (caspase activity) and autophagy (lysozyme and cathepsin activity) were verified in retina extracts. These results provide evidence that irrespective of whether photoreceptor degeneration is triggered by gene defects (lack of beta-PDE or rds/peripherin) or environmental stress (light-damage), a number of pro-apoptotic mechanisms are triggered leading to the degeneration of the photoreceptor cells. The temporal pattern of the different pathways suggests that the non-caspase-dependent mechanisms may actively participate in the demise of the photoreceptors, rather than represent a passive response of the retina to the presence of dying cells. Thus, unless the common upstream initiator for a given photoreceptor dystrophy is found, multiple rescue paradigms need to be used to target all active pathways.

GDNF family ligands trigger indirect neuroprotective signaling in retinal glial cells

Apoptotic cell death of photoreceptors is the final event leading to blindness in the heterogeneous group of inherited retinal degenerations. GDNF (glial cell-line-derived neurotrophic factor) was found to rescue photoreceptor function and survival very effectively in an animal model of retinal degeneration (M. Frasson, S. Picaud, T. Leveillard, M. Simonutti, S. Mohand-Said, H. Dreyfus, D. Hicks, and J. Sahel, Investig. Ophthalmol. Vis. Sci. 40:2724-2734, 1999). However, the cellular mechanism of GDNF action remained unresolved. We show here that in porcine retina, GDNF receptors GFRalpha-1 and RET are expressed on retinal Mueller glial cells (RMG) but not on photoreceptors. Additionally, RMG express the receptors for the GDNF family members artemin and neurturin (GFRalpha-2 and GFRalpha-3). We further investigated GDNF-, artemin-, and neurturin-induced signaling in isolated primary RMG and demonstrate three intracellular cascades, which are activated in vitro: MEK/ERK, stress-activated protein kinase (SAPK), and PKB/AKT pathways with different kinetics in dependence on stimulating GFL. We correlate the findings to intact porcine retina, where GDNF induces phosphorylation of ERK in the perinuclear region of RMG located in the inner nuclear layer. GDNF signaling resulted in transcriptional upregulation of FGF-2, which in turn was found to support photoreceptor survival in an in vitro assay. We provide here a detailed model of GDNF-induced signaling in mammalian retina and propose that the GDNF-induced rescue effect on mutated photoreceptors is an indirect effect mediated by retinal Mueller glial cells.

Role of apoptosis signal-regulating kinase 1 in stress-induced neural cell apoptosis in vivo

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that plays an important role in oxidative stress-induced apoptosis. In the present study, we used ASK1 knockout (KO) mice to examine the possibility that ASK1 is involved in the neural cell apoptosis that occurs during retinal development and ischemic injury. ASK1 was expressed in retinal neurons, including retinal ganglion cells (RGCs), but retinal structure and extent of cell death during development were normal in ASK1 KO mice. On the other hand, the strain was less susceptible to ischemic injury, and the number of surviving retinal neurons was significantly increased compared with that in wild-type mice. Interestingly, ischemia-induced phosphorylation of p38 mitogen-activated protein kinase (p38), which mediates RGC apoptosis, was almost completely suppressed in ASK1 KO mice. In such retinas, the numbers of cleaved caspase-3- and TUNEL-positive neurons were apparently decreased compared with those in wild-type mice. Furthermore, cultured RGCs from ASK1 KO mice were resistant to H(2)O(2)-induced apoptosis. Our findings suggest that ASK1 is involved in the neural cell apoptosis after various kinds of oxidative stress. Thus, inhibition of the ASK1-p38 pathway could be useful for the treatment of neurodegenerative diseases including glaucoma.

The role of cytokines and trophic factors in epiretinal membranes: Involvement of signal transduction in glial cells

Idiopathic epiretinal membranes (ERMs) in the macular region can cause a reduction in vision and sometimes recurs after surgical removal, but its pathogenic mechanisms are still unknown. On the other hand, the presence of secondary ERMs has been associated with various clinical conditions including proliferative diabetic retinopathy (PDR) and proliferative vitreoretinopathy (PVR). Recent studies have shown a significant association between clinical grades of PDR or PVR, and the expression levels of specific cytokines and/or growth factors in the vitreous fluid. Expression of these factors and their receptors are also observed in secondary ERMs. ERMs are composed of many cell types such as retinal pigment epithelial cells and vascular endothelial cells, however the role of glial cells is yet unclear. Interestingly, glial cells in ERMs express some trophic factor receptors and transcription factors, such as NF-kappaB, suggesting an involvement of glial signal transduction in the pathogenesis of ERMs. In this review, we summarize recent progress regarding the clinical and laboratory findings of ERMs.

Effect of p75NTR on the regulation of naturally occurring cell death and retinal ganglion cell number in the mouse eye

Neurotrophins induce neural cell survival and differentiation during retinal development and regeneration through the high-affinity tyrosine kinase (Trk) receptors. On the other hand, nerve growth factor (NGF) binding to the low-affinity neurotrophin receptor p75 (p75(NTR)) might induce programmed cell death (PCD) in the early phase of retinal development. In the present study, we examined the retinal cell types that experience p75(NTR)-induced PCD and identify them to be postmitotic retinal ganglion cells (RGCs). However, retinal morphology, RGC number, and BrdU-positive cell number in p75(NTR) knockout (KO) mouse were normal after embryonic day 15 (E15). In chick retina, migratory RGCs express p75(NTR), whereas layered RGCs express the high-affinity NGF receptor TrkA, which may switch the pro-apoptotic signaling of p75(NTR) into a neurotrophic one. In contrast to the chick model, migratory RGCs express TrkA, while stratified RGCs express p75(NTR) in mouse retina. However, RGC number in TrkA KO mouse was also normal at birth. We next examined the expression of transforming growth factor beta (TGFbeta) receptor, which modulates chick RGC number in combination with p75(NTR), but was absent in mouse RGCs. p75(NTR) and TrkA seem to be involved in the regulation of mouse RGC number in the early phase of retinal development, but the number may be later adjusted by other molecules. These results suggest the different mechanism of RGC number control between mouse and chick retina.

[Survival factors in the treatment of hereditary retinal degeneration]

Hereditary retinal degeneration is characterized by apoptotic photoreceptor loss, a process governed by intricate molecular interplay and initiated when proapoptotic signals predominate in the individual cell. Identification of molecules involved and their actions has paved the way for testing the ones with anti-apoptotic functions in models of inherited retinal degeneration. Many of these factors are able to slow the course of the degeneration. However, to date no such treatment has been able to stop or even prevent the devolution of the disorder. Moreover, preservation of morphology does not necessarily correlate with preservation of ERG function. Deepened understanding of the pro- and anti-apoptotic networks is clearly needed for survival factors to be feasible for therapy in humans. In comparison, in a dog model of Leber's congenital amaurosis gene therapy could establish retinal function, thus supplying proof of efficacy of the method.

The Wnt signaling pathway in retinal degenerations

The retina is a complex tissue composed of multiple interconnected cell layers, highly specialized for transforming light and color into electrical signals perceived by the brain. Damage or death of the primary light-sensing cells, the photoreceptors, results in devastating effects on vision. Despite the identification of numerous mutations that cause inherited retinal degenerations, the cellular and molecular mechanisms leading from the primary mutations to photoreceptor apoptosis are not understood. Wnt signaling has essential regulatory functions in a wide variety of critical developmental processes. Our research and others' have suggested that the Wnt pathway may be involved in retinal degeneration. Wnt ligands regulate developmental death of Drosophila photoreceptors, dysregulated Wnt signaling is involved in neuronal degeneration elsewhere in the central nervous system and Wnts control the expression of pro-survival growth factors in mammalian tissues. Additionally, altered expression of Wnt pathway genes, including the anti-apoptotic Wnt signaling regulator Dickkopf 3 (Dkk3), were observed during photoreceptor loss. This review examines the evidence and develops a model proposing a pro-survival role for Wnt signaling during photoreceptor injury. Because manipulating Wnt signaling has been demonstrated to have therapeutic potential for the treatment of Alzheimers disease, understanding the involvement of Wnts in photoreceptor death will determine whether targeting the Wnt pathway should also be considered as a possible therapeutic strategy for retinal degenerations.

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

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

Phosphorylation of extracellular signal-regulated kinase and p27(KIP1) after retinal detachment

PURPOSE

The roles of the extracellular signal-regulated kinase (ERK) pathway in the expression of cyclin D1 and p27(KIP1), the phosphorylation of p27(KIP1), and proliferation activity were examined after retinal detachment.

METHODS

Normal eyes and eyes at 15 min, 2 and 4 days after retinal detachment in C57Bl6 mice were examined by immunohistochemistry using anti-phosphorylated (p) ERK1/2, anti-cyclin D1, anti-p27(KIP1), anti-p27(KIP1) phosphorylated at serine 10 (S10-phospho-p27), and anti-proliferating cell nuclear antigen (PCNA) antibodies with or without treatment with a specific ERK inhibitor, PD98059. Mouse Müller cells were isolated and examined for alteration of p27(KIP1) and cyclin D1 after exposure of basic fibroblast growth factor (bFGF) with and without treatment of PD98059 using Western blotting.

RESULTS

In the normal retina, nuclear immunoreactivity for p27(KIP1), but not S10-phospho-p27 or pERK1/2, was observed in the middle sublayer of the inner nuclear layer (INL), where Müller glial cells are situated. At 15 min after the retinal detachment, p27(KIP1), S10-phospho-p27 and pERK1/2-positive nuclei were noted in the INL, whereas immunoreactivity for pERK1/2 or S10-phospho-p27 was not observed after treatment with PD98095. Cyclin D1 was induced in the INL 2 days after the retinal detachment, and the induction was inhibited by PD98059. At 4 days after the detachment, p27(KIP1) immunoreactivity was not observed, and cyclin D1 and PCNA were expressed. The disappearance of p27(KIP1) was suppressed, whereas expression of cyclin D1 and PCNA was not observed in mice treated with PD98059. Exposure of bFGF relatively decreased the expression level of p27(KIP1) and increased the level of cyclin D1 in mouse Müller cells, compared with control level. Induction of cyclin D1 and decrease in p27(KIP1) were inhibited with treatment of PD98059.

CONCLUSION

Phosphorylation of ERK and expression of p27(KIP1) and cyclin D1 are involved in the proliferation of Müller cells after retinal detachment.

Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization and adhesion during light exposure

Loss of Crumbs homologue 1 (CRB1) function causes either the eye disease Leber congenital amaurosis or progressive retinitis pigmentosa, depending on the amount of residual CRB1 activity and the genetic background. Crb1 localizes specifically to the sub-apical region adjacent to the adherens junction complex at the outer limiting membrane in the retina. We show that it is associated here with multiple PDZ protein 1 (Mupp1), protein associated with Lin-7 (Pals1 or Mpp5) and Mpp4. We have produced Crb1(-/-) mice completely lacking any functional Crb1. Although the retinas are initially normal, by 3-9 months the Crb1(-/-) retinas develop localized lesions where the integrity of the outer limiting membrane is lost and giant half rosettes are formed. After delamination of the photoreceptor layer, neuronal cell death occurs in the inner and outer nuclear layers of the retina. On moderate exposure to light for 3 days at 3 months of age, the number of severe focal retinal lesions significantly increases in the Crb1(-/-) retina. Crb2, Crb3 and Crb1 interacting proteins remain localized to the sub-apical region and therefore are not sufficient to maintain cell adhesion during light exposure in Crb1(-/-) retinas. Thus we propose that during light exposure Crb1 is essential to maintain, but not assemble, adherens junctions between photoreceptors and Müller glia cells and prevents retinal disorganization and dystrophy. Hence, light may be an influential factor in the development of the corresponding human diseases.

Light-induced retinal damage involves tyrosine 33 phosphorylation, mitochondrial and nuclear translocation of WW domain-containing oxidoreductase in vivo

WW domain-containing oxidoreductase WOX1, also named WWOX or FOR, is a known proapoptotic protein and a candidate tumor suppressor. Stress stimuli activate WOX1 via tyrosine 33 (Tyr33) phosphorylation and translocation to the mitochondria and nuclei in vitro. Here, the potential role of WOX1 in light-induced retinal degeneration in vivo was investigated. WOX1 is expressed primarily in the inner retina at perinatal stages, whereas an enhanced expression of WOX1, along with its Tyr33 phosphorylation (p-WOX1), is shown specifically in the retinal ganglion cells in adults. Prolonged exposure of mature rats to constant, low-intensity light (500 lux) for 1-2 months resulted in substantial death of photoreceptors and the presence of activated microglia, astrocytes and Muller glial in the outer retina. However, the inner retina was not or barely affected. In the damaged inner and outer nuclear layers of rat retina, WOX1 and p-WOX1 were overly expressed. Also, WOX1 colocalized with fragments of opsin-positive cones. In rd mice with an inherited retinal deficiency, upregulation of WOX1 and p-WOX1 in degenerated retina was observed with age. By electron microscopy, a large number of immunogold particles of WOX1 and p-WOX1 were found in the damaged mitochondria and condensed nuclei of degenerating photoreceptors, indicating that WOX1 undergoes activation and translocation to these organelles. In contrast, little or no WOX1-positive particles were found in the Golgi apparatus. In conclusion, activated WOX1 is likely to exert apoptosis of neuronal cells in the outer retina during the light-induced injury and in mice with an inherited retinal defect.

Glial cell-mediated spread of retinal degeneration during detachment: a hypothesis based upon studies in rabbits

In human subjects with peripheral retinal detachments, visual deficits are not restricted to the detached retina but are also present in the non-detached tissue. Based upon studies on a rabbit model of rhegmatogenous retinal detachment, we propose a glial cell-mediated mechanism of spread of retinal degeneration into non-detached retinal areas which may also have importance for the understanding of alterations in the human retina. Both detached and attached portions of the rabbit retina display photoreceptor cell degeneration and cystic degeneration of the innermost layers. An inverse mode of photoreceptor cell degeneration in the attached tissue suggests a disturbed support of the photoreceptor cells by Müller cells which show various indications of gliosis (increased expression of intermediate filaments, cell hypertrophy, decreased plasma membrane K(+) conductance, increased Ca(2+) responsiveness to purinergic stimulation) in both detached and attached tissues. We propose that gliotic alterations of Müller cells contribute to the degeneration of the attached retina, via disturbance of glial homeostasis mechanisms. A down-regulation of the K(+) conductance of Müller cells may prevent effective retinal K(+) and water clearance, and may favor photoreceptor cell degeneration and edema development.

Effect of steroidal and non-steroidal drugs on the microglia activation pattern and the course of degeneration in the retinal degeneration slow mouse

BACKGROUND

In hereditary retinal degeneration, microglia cells become activated, migrate through the outer nuclear layer (ONL) and accumulate in the subretinal space. Although this inflammatory process is not likely to be responsible for the onset of photoreceptor apoptosis, cytotoxic substances secreted by activated microglia could potentially accelerate and perpetuate the degenerative process. Anti-inflammatory drugs have been shown to modulate the microglia response in neurodegenerative disorders and potentially ameliorate the disease progression in various animal model systems. In this study we wanted to test the impact of the most commonly used anti-inflammatory drugs (acetylsalicylate and prednisolone) on the microglia activation pattern, the rate of caspase-3-dependent photoreceptor apoptosis and the course of the degeneration in the retinal degeneration slow (rds) mouse retina.

METHODS

169 pigmented rds mice and 30 CBA wild-type mice were used for this study. The treatment groups were injected daily with either acetylsalicylate (200 mg/kg) or prednisolone (2 mg/kg) i.p. from day 0 up to 3 months. Animals were sacrificed at days 10, 14, 16, 18, 20, 30, 40, 60 and 90. Cryoprotected frozen sections were immunostained with F4/80 and cleaved caspase-3 antibodies. The main outcome measures were the total microglia count in the subretinal space, the total cleaved caspase-3-positive cells in the ONL and the averaged number of photoreceptor rows in the midperipheral retina.

RESULTS

Neither acetylsalicylate nor prednisolone reduced subretinal microglia accumulation in the rds mouse degeneration model. Moreover, they aggravated migration and accumulation in the early time course. The apoptotic cascade started earlier and was more pronounced in both treatment groups compared to the control group. The pace of retinal degeneration was not reduced in the treatment groups compared to the untreated control. In contrast, acetylsalicylate did significantly accelerate the photoreceptor cell degeneration in comparison to the prednisolone (p < 0.001) and to the control group (p < 0.001).

CONCLUSIONS

Acetylsalicylate and prednisolone do not decrease the microglia response in the rds mouse and are not neuroprotective. More research is needed to clarify the molecular mechanisms which lead to photoreceptor cell death and to elucidate the complex role of microglia in inherited retinal degeneration.

The role of tumour necrosis factor (TNF-alpha) in experimental autoimmune uveoretinitis (EAU)

The pleiotropic cytokine tumour necrosis factor-alpha (TNF-alpha) is released from cells that include macrophages and T-cells during inflammatory responses, orchestrating the initiation of further leucocytic infiltration via adhesion molecule upregulation, dendritic cell maturation and survival, macrophage activation and driving Th1 T-cells responses within tissues. Exposure to TNF also plays a role in maintaining tissue homeostasis, particularly relating to resident cell responses of both microglia and retinal pigment epithelium. Depending on the balance between duration and dose of TNF exposure, an environment where full expression of inflammatory and autoimmune responses within tissues may occur. In experimental autoimmune uveoretinitis (EAU), increased tissue concentrations of TNF facilitate the on-going T-cell effector responses and macrophage activation. These are responsible for targeted and bystander tissue damage and can be suppressed by anti-TNF therapies, in particular, those directed at the p55 TNF receptor. The ability to suppress disease experimentally has led to the successful translation of anti-TNF therapy for treatment of uveitis in cohort studies and phase I/II trials where, additionally, altered peripheral blood CD4(+) T-cell profiles can be demonstrated following each treatment.

Morphological changes in the Royal College of Surgeons rat retina during photoreceptor degeneration and after cell-based therapy

There are concomitant morphological and functional changes in the inner retina during the course of photoreceptor degeneration in a range of animal models of retina degeneration and in humans with eye disease. One concern that has been raised is that the changes occurring in the inner retina might compromise attempts to rescue or restore visual input by various interventional approaches. It is known that cell-based therapy can preserve significant visual capability for many months. In this study, we examine the overall changes in the Royal College of Surgeons (RCS) rat during degeneration and the effects of cell transplantation by means of immunohistochemistry and confocal microscopy. The degenerative changes are complex, and they progress with age. They involve the neurons with which both rods and cones interconnect--retinal second- and third-order neurons underwent dramatic modification, including sprouting, retraction as photoreceptor loss progressed--as well as Müller glia and secondary vascular changes, which were associated at later times with neuronal migration. The pathological vascular changes led to major disruption of inner retina. After introducing a retinal pigment epithelial cell line to the subretinal space early in the progress of photoreceptor degeneration, most inner retinal changes were held in abeyance for up to at least 10 months of age. Given the concern that has been raised regarding whether inner retinal changes might compromise any graft-related benefit, this is an encouraging finding.

Molecular mechanisms of light-induced photoreceptor apoptosis and neuroprotection for retinal degeneration

Human retinal dystrophies and degenerations and light-induced retinal degenerations in animal models are sharing an important feature: visual cell death by apoptosis. Studying apoptosis may thus provide an important handle to understand mechanisms of cell death and to develop potential rescue strategies for blinding retinal diseases. Apoptosis is the regulated elimination of individual cells and constitutes an almost universal principle in developmental histogenesis and organogenesis and in the maintenance of tissue homeostasis in mature organs. Here we present an overview on molecular and cellular mechanisms of apoptosis and summarize recent developments. The classical concept of apoptosis being initiated and executed by endopeptidases that cleave proteins at aspartate residues (Caspases) can no longer be held in its strict sense. There is an increasing number of caspase-independent pathways, involving apoptosis inducing factor, endonuclease G, poly-(ADP-ribose) polymerase-1, proteasomes, lysosomes and others. Similarly, a considerable number and diversity of pro-apoptotic stimuli is being explored. We focus on apoptosis pathways in our model: light-damage induced by short exposures to bright white light and highlight those essential conditions known so far in the apoptotic death cascade. In our model, the visual pigment rhodopsin is the essential mediator of the initial death signal. The rate of rhodopsin regeneration defines damage threshold in different strains of mice. This rate depends on the level of the pigment epithelial protein RPE65, which in turn depends on the amino acid (leucine or methionine) encoded at position 450. Activation of the pro-apoptotic transcription factor AP-1 constitutes an essential death signal. Inhibition of rhodopsin regeneration as well as suppression of AP-1 confers complete protection in our system. Furthermore, we describe observations in other light-damage systems as well as characteristics of animal models for RP with particular emphasis on rescue strategies. There is a vast array of different neuroprotective cytokines that are applied in light-damage and RP animal models and show diverging efficacy. Some cytokines protect against light damage as well as against RP in animal models. At present, the mechanisms of neuroprotective/anti-apoptotic action represent a "black box" which needs to be explored. Even though acute light damage and RP animal models show different characteristics in many respects, we hope to gain insights into apoptotic mechanisms for both conditions by studying light damage and comparing results with those obtained in animal models. In our view, future directions may include the investigation of different apoptotic pathways in light damage (and inherited animal models). Emphasis should also be placed on mechanisms of removal of dead cells in apoptosis, which appears to be more important than initially recognized. In this context, a stimulating concept concerns age-related macular degeneration, where an insufficiency of macrophages removing debris that results from cell death and photoreceptor turnover might be an important pathogenetic event. In acute light damage, the appearance of macrophages as well as phagocytosis by the retinal pigment epithelium are a consistent and conspicuous feature, which lends itself to the study of removal of cellular debris in apoptosis. We are aware of the many excellent reviews and the earlier work paving the way to our current knowledge and understanding of retinal degeneration, photoreceptor apoptosis and neuroprotection. However, we limited this review mainly to work published in the last 7-8 years and we apologize to all the researchers which have contributed to the field but are not cited here.

Functionally intact glutamate-mediated signaling in bipolar cells of the TRKB knockout mouse retina

In the juvenile trkB knockout (trkB−/−) mouse, retina synaptic communication from rods to bipolar cells is severely compromised as evidenced by a complete absence of electroretinogram (ERG)b-wave, even though the inner retina appears anatomically normal (Rohrer et al., 1999). Since it is well known that theb-wave reflects light-dependent synaptic activation of ON bipolar cellsviatheir metabotropic glutamate receptor, mGluR6, we sought to analyze the anatomical and functional integrity of the glutamatergic synapses at these and other bipolar cells in thetrkB−/−mouse. Although rod bipolar cells from wild-type juvenile mice were determined to be immunopositive for trkB, postsynaptic metabotropic and ionotropic glutamate receptor-mediated pathways in ON and OFF bipolar cells were found to be functionally intact, based on patch electrode recordings, using brief applications (“puffs”) of glutamate or its analog, 2-amino-4-phosphonobutyric acid (APB), a selective agonist for mGluR6 receptors. Ionotropic glutamate receptor function was assayed in OFF-cone bipolar and horizontal cells by applying exogenous glutamatergic agonists in the presence of the channel-permeant guanidinium analogue, 1-amino-4-guanidobutane (AGB). Electron-microscopic analysis revealed that the ribbon synapses between rods and postsynaptic rod bipolar and horizontal cells were formed at the appropriate age and appear to be structurally intact, and immunohistochemical analysis did not detect profound defects in the expression of excitatory amino acid transporters involved in glutamate clearance from the synaptic cleft. These data indicate that there does not appear to be evidence for postsynaptic deficits in glutamatergic signaling in the ON and OFF bipolar cells of mice lacking trkB.

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.

Effects of brain-derived neurotrophic factor on cell survival, differentiation and patterning of neuronal connections and Muller glia cells in the developing retina

The aim of the present study was to determine the influence of brain-derived neurotrophic factor (BDNF) on survival, phenotype differentiation and network formation of retinal neurons and glia cells. To achieve a defined concentration and constant level of BDNF over several days, experiments were performed in an organotypic culture of the developing rat retina. After 6 days in vitro, apoptosis in the different cell layers was determined by TUNEL staining and cell-type-specific antibodies were used to identify distinct neuronal cell types and Müller cells. Cultured retinas treated with BDNF (100 ng BDNF/mL medium) were compared with untreated as well as with age-matched in vivo retinas. Quantitative morphometry was carried out using confocal microscopy. BDNF promoted the in vitro development and differentiation of the retina in general, i.e. the number of cells in the nuclear layers and the thickness of the plexiform layers were increased. For all neurons, the number of cells and the complexity of arborizations in the synaptic layers were clearly up-regulated by BDNF. In control cultures, the synaptic stratification of cone bipolar cells within the On- and Off-layer of the inner plexiform layer was disturbed and a strong reactivity of Müller cell glia was observed. These effects were not present in BDNF-treated cultures. Our data show that BDNF promotes the survival of retinal interneurons and plays an important role in establishing the phenotypes and the synaptic connections of a large number of neuronal types in the developing retina. Moreover, we show an effect of BDNF on Müller glia cells.

Role of ubiquitin carboxy terminal hydrolase-L1 in neural cell apoptosis induced by ischemic retinal injury in vivo

Ubiquitin is thought to be a stress protein that plays an important role in protecting cells under stress conditions; however, its precise role is unclear. Ubiquitin expression level is controlled by the balance of ubiquitinating and deubiquitinating enzymes. To investigate the function of deubiquitinating enzymes on ischemia-induced neural cell apoptosis in vivo, we analyzed gracile axonal dystrophy (gad) mice with an exon deletion for ubiquitin carboxy terminal hydrolase-L1 (UCH-L1), a neuron-specific deubiquitinating enzyme. In wild-type mouse retina, light stimuli and ischemic retinal injury induced strong ubiquitin expression in the inner retina, and its expression pattern was similar to that of UCH-L1. On the other hand, gad mice showed reduced ubiquitin induction after light stimuli and ischemia, whereas expression levels of antiapoptotic (Bcl-2 and XIAP) and prosurvival (brain-derived neurotrophic factor) proteins that are normally degraded by an ubiquitin-proteasome pathway were significantly higher. Consistently, ischemia-induced caspase activity and neural cell apoptosis were suppressed approximately 70% in gad mice. These results demonstrate that UCH-L1 is involved in ubiquitin expression after stress stimuli, but excessive ubiquitin induction following ischemic injury may rather lead to neural cell apoptosis in vivo.

Adeno-associated viral vectors for retinal gene transfer

Vectors derived from adeno-associated viruses (AAV) represent a promising tool for retinal gene transfer in pre-clinical and clinical settings. AAV vectors efficiently transduce dividing and non-dividing cells, escape cellular immunity and result in long-non-term transduction. In addition, they may be targeted to specific retinal cell types by taking advantage of surface proteins from various AAV serotypes thus limiting transfer of therapeutic genes to those cells requiring correction. This review will provide an overview of the properties of AAV vectors followed by a detailed report of their use in retinal gene transfer for mendelian and non-mendelian disorders.

Changes in retinal expression of neurotrophins and neurotrophin receptors induced by ocular hypertension

Open angle glaucoma is defined as a progressive and time-dependent death of retinal ganglion cells concomitant with high intraocular pressure, leading to loss of visual field. Because neurotrophins are a family of growth factors that support neuronal survival, we hypothesized that quantitative and qualitative changes in neurotrophins or their receptors may take place early in ocular hypertension, preceding extensive cell death and clinical features of glaucoma. We present molecular, biochemical, and phenotypic evidence that significant neurotrophic changes occur in retina, which correlate temporally with retinal ganglion cell death. After 7 days of ocular hypertension there is a transient up-regulation of retinal NGF, while its receptor TrkA is up-regulated in a sustained fashion in retinal neurons. After 28 days of ocular hypertension there is sustained up-regulation of retinal BDNF, but its receptor TrkB remains unchanged. Throughout, NT-3 levels remain unchanged but there is an early and sustained increase of its receptor TrkC in Müller cells but not in retinal ganglion cells. These newly synthesized glial TrkC receptors are truncated, kinase-dead isoforms. Expression of retinal p75 also increases late at day 28. Asymmetric up-regulation of neurotrophins and neurotrophin receptors may preclude efficient neurotrophic rescue of RGCs from apoptosis. A possible rationale for therapeutic intervention with Trk receptor agonists and p75 receptor antagonists is proposed.

Potential role of glial cell line-derived neurotrophic factor receptors in Müller glial cells during light-induced retinal degeneration

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and their receptors (GFRalpha1, GFRalpha2 and Ret) play an important role in the survival of neurons in the central and peripheral nervous system. For example, GDNF as well as other trophic factors promotes photoreceptor survival during retinal degeneration. Recent studies have proposed that part of neurotophic rescue of photoreceptors may be indirect, mediated by interaction of the neurotrophic factors with other cell types, that in turn release secondary factors that act directly on photoreceptors. In the present study, we examined the GDNF receptor expression in control and light-damaged retina, and found that GFRalpha2 protein is upregulated in retina-specific Müller glial cells during photoreceptor degeneration. We also examined the effect of GDNF or NTN on cultured Müller cells. Exogenous GDNF increased brain-derived neurotrophic factor, basic fibroblast growth factor and GDNF, but not NTN mRNA production. On the other hand, NTN increased NTN, but not GDNF mRNA production in cultured Müller cells. These observations suggest that GDNF, NTN and their receptors are involved in the regulation of trophic factor production in retinal glial cells, and that functional glia-neuron network may utilize GDNF family for the protection of neural cells during retinal degeneration.

Location of CNTFRalpha on outer segments: evidence of the site of action of CNTF in rat retina

Ciliary neurotrophic factor (CNTF) is an important factor in the retina's mechanisms of self-protection. It is generated by retinal glial cells in response to stress, and has a significant protective effect on retinal neurones. In this study we have identified the location of the alpha component of the CNTF receptor complex (CNTFRalpha) in rat retina, using immunohistochemistry and high-resolution confocal microscopy. The major location of CNTFRalpha is on photoreceptor outer segments. More scattered, granular forms of CNTFRalpha were identified in association with Müller cell processes in other retinal layers. Colocalisation of CNTF with CNTFRalpha, suggestive of ligand-receptor binding, was detected on outer segments, and in both normal retinas and retinas stressed by light or oxygen. Results provide evidence that the principal site of CNTF action is the outer segments of photoreceptors. This confirms the known ability of CNTF to protect photoreceptors against stress, and suggest that it acts by modulating mechanisms specific to the outer segment, such as the phototransduction cascade or the membrane channels, which control dark current.

Anti-apoptotic effects of CNTF gene transfer on photoreceptor degeneration in experimental antibody-induced retinopathy

Autoantibodies against recoverin are found in the sera of patients with cancer-associated retinopathy syndrome, a paraneoplastic disease associated with retinal degeneration. We have previously shown that anti-recoverin autoantibodies induced photoreceptor apoptotic cell death after injection into the vitreous of Lewis rats. Ciliary neurotrophic factor (CNTF) has been shown to promote the survival of a number of neuronal cell types, including photoreceptors. In this study, we examined whether an adeno-associated virus (AAV)-mediated delivery of gene encoding the human CNTF protected photoreceptor cells from anti-recoverin antibody-induced death. One month after subretinal injection of the AAV-CNTF gene into one eye and a control vector into the other eye, an anti-recoverin antibody was injected to induce retinal cell death in Lewis rats. Subretinal administration of the virus led to an efficient transduction of photoreceptors, as indicated by immunostaining of retinas with anti-CNTF. Histological examination of the corresponding retinas showed that photoreceptor cells were significantly protected from apoptotic death in the CNTF-treated eyes. CNTF treatment of the retinas resulted in a time-dependent activation of STAT 3. The present study shows that an AAV-mediated delivery of CNTF may protect photoreceptors from antibody-induced cell death through the activation of STAT3 and the suppression of caspase 3 activity, a key caspase leading to apoptosis. Thus, CNTF may be a useful treatment for human antibody-mediated retinal degeneration.

Control of myeloid activity during retinal inflammation

Combating myeloid cell-mediated destruction of the retina during inflammation or neurodegeneration is dependent on the integrity of homeostatic mechanisms within the tissue that may suppress T cell activation and their subsequent cytokine responses, modulate infiltrating macrophage activation, and facilitate healthy tissue repair. Success is dependent on response of the resident myeloid-cell populations [microglia (MG)] to activation signals, commonly cytokines, and the control of infiltrating macrophage activation during inflammation, both of which appear highly programmed in normal and inflamed retina. The evidence that tissue CD200 constitutively provides down-regulatory signals to myeloid-derived cells via cognate CD200-CD200 receptor (R) interaction supports inherent tissue control of myeloid cell activation. In the retina, there is extensive neuronal and endothelial expression of CD200. Retinal MG in CD200 knockout mice display normal morphology but unlike the wild-type mice, are present in increased numbers and express nitric oxide synthase 2, a macrophage activation marker, inferring that loss of CD200 or absent CD200R ligation results in "classical" activation of myeloid cells. Thus, when mice lack CD200, they show increased susceptibility to and accelerated onset of tissue-specific autoimmunity.

Increased expression of brain-derived neurotrophic factor preserves retinal function and slows cell death from rhodopsin mutation or oxidative damage

There are no effective treatments for inherited retinal degenerations, which are prevalent causes of visual disability. Several proteins promote the survival of various types of neurons, and increasing expression of one or more of these survival factors is a promising strategy for a new treatment. Studies examining the effects of intravitreous injections of brain-derived neurotrophic factor (BDNF) in models of inherited retinal degenerations have suggested that BDNF has little survival-promoting activity for photoreceptors. In this study, we generated double transgenic mice with doxycycline-inducible expression of BDNF in the retina. In a model of primary rod photoreceptor degeneration, expression of BDNF resulted in significant delay in photoreceptor cell death and maintenance of retinal function assessed by electroretinogram recordings. Expression of BDNF also caused strong protection of photoreceptors from oxidative damage-induced cell death. These data suggest that continuous expression of BDNF, unlike intravitreous injections, results in morphologic and functional benefit in animal models of inherited retinal degeneration. Double transgenic mice with inducible expression of survival factors provide valuable tools for selection of survival factor candidates for gene therapy.

NF-kappaB in epiretinal membranes after human diabetic retinopathy

AIMS/HYPOTHESIS

Formation of epiretinal membranes (ERMs) in the posterior fundus results in progressive deterioration of vision. ERMs have been associated with numerous clinical conditions including proliferative diabetic retinopathy (PDR), but its pathogenic mechanisms are still unknown. This study was conducted to examine whether or not nuclear factor kappa B (NF-kappaB), a transcription factor that can be activated by various pathological conditions, is involved in the formation of ERMs after PDR.

METHODS

ERM samples were obtained by vitrectomy from 22 cases with PDR aged 56+/-11 years with 18+/-10 years of diabetes and 15 cases with idiopathic ERM. They were processed for reverse transcription-polymerase chain reaction (RT-PCR) analysis. In addition, 5 ERM samples from PDR patients aged 51+/-16 years with 15+/-6 years of diabetes were processed for immunohistochemical analysis.

RESULTS

NF-kappaB mRNA expression levels were higher (20 out of 22 cases vs. 9 out of 15 subjects in idiopathic ERM, p<0.05) in PDR subjects. Immunohistochemical analysis showed NF-kappaB protein expression in all the 5 ERMs derived from PDR patients, and that region was partially double-labelled with interleukin-8 (IL-8) and von Willebrand factor (vWF).

CONCLUSIONS/INTERPRETATION

These results suggest a possibility that NF-kappaB is involved in the formation of ERMs after PDR, especially for the development of vascular endothelial cell component.

Lack of p75 receptor does not protect photoreceptors from light-induced cell death

Rod photoreceptors are susceptible to light-induced cell death. Previous results have suggested that the neurotrophin receptor p75 in Müller cells controls photoreceptor cell death during light-exposure by suppressing trophic factor release; and consequently, if p75 is blocked or eliminated during light-exposure, apoptosis is delayed. We explored this question by examining photoreceptor cell survival in albino p75(-/-) mice as well as their heterozygous and homozygous littermates. Photoreceptor cell death was examined in semi-thin sections by counting the remaining rows of photoreceptors. No difference in the amount of cell death was found between p75(+/+) and p75(-/-) animals, whereas the single copy of p75 in the heterozygous p75(+/-) mice provided significant neuroprotection. Cell death in the wild-type animals may indeed be mediated by p75, whereas other known apoptosis pathways may be activated in the p75(-/-) mice. The pro-apoptotic activity of the p75 receptor may have been partially suppressed in the heterozygous p75(+/-) mice by the silencing effect of the Trk receptor. Thus, our results suggest that p75 signaling does not mediate the main apoptosis pathway activated during light-damage.

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.