Three-Dimensional Ultrastructure of the Normal Rod Photoreceptor Synapse and Degenerative Changes Induced by Retinal Detachment

The rod photoreceptor synapse is the first synapse of dim-light vision and one of the most complex in the mammalian CNS. The components of its unique structure, a presynaptic ribbon and a single synaptic invagination enclosing several postsynaptic processes, have been identified, but disagreements about their organization remain. Here, we have used EM tomography to generate high-resolution images of 3-D volumes of the rod synapse from the female domestic cat. We have resolved the synaptic ribbon as a single structure, with a single arciform density, indicating the presence of one long site of transmitter release. The organization of the postsynaptic processes, which has been difficult to resolve with past methods, appears as a tetrad arrangement of two horizontal cell and two rod bipolar cell processes. Retinal detachment severely disrupts this organization. After 7 d, EM tomography reveals withdrawal of rod bipolar dendrites from most spherules; fragmentation of synaptic ribbons, which lose their tight association with the presynaptic membrane; and loss of the highly branched telodendria of the horizontal cell axon terminals. After detachment, the hilus, the opening through which postsynaptic processes enter the invagination, enlarges, exposing the normally sequestered environment within the invagination to the extracellular space of the outer plexiform layer. Our use of EM tomography provides the most accurate description to date of the complex rod synapse and details changes it undergoes during outer segment degeneration. These changes would be expected to disrupt the flow of information in the rod pathway.SIGNIFICANCE STATEMENT Ribbon-type synapses transmit the first electrical signals of vision and hearing. Despite their crucial role in sensory physiology, the three-dimensional ultrastructure of these synapses, especially the complex organization of the rod photoreceptor synapse, is not well understood. We used EM tomography to obtain 3-D imaging at nanoscale resolution to help resolve the organization of rod synapses in normal and detached retinas. This approach has enabled us to show that in the normal retina a single ribbon and arciform density oppose a tetrad of postsynaptic processes. In addition, it enabled us to provide a 3-D perspective of the ultrastructural changes that occur in response to retinal detachment.

A farnesyltransferase inhibitor activates lysosomes and reduces tau pathology in mice with tauopathy

Tau inclusions are a shared feature of many neurodegenerative diseases, among them frontotemporal dementia caused by tau mutations. Treatment approaches for these conditions include targeting posttranslational modifications of tau proteins, maintaining a steady-state amount of tau, and preventing its tendency to aggregate. We discovered a new regulatory pathway for tau degradation that operates through the farnesylated protein, Rhes, a GTPase in the Ras family. Here, we show that treatment with the farnesyltransferase inhibitor lonafarnib reduced Rhes and decreased brain atrophy, tau inclusions, tau sumoylation, and tau ubiquitination in the rTg4510 mouse model of tauopathy. In addition, lonafarnib treatment attenuated behavioral abnormalities in rTg4510 mice and reduced microgliosis in mouse brain. Direct reduction of Rhes in the rTg4510 mouse by siRNA reproduced the results observed with lonafarnib treatment. The mechanism of lonafarnib action mediated by Rhes to reduce tau pathology was shown to operate through activation of lysosomes. We finally showed in mouse brain and in human induced pluripotent stem cell-derived neurons a normal developmental increase in Rhes that was initially suppressed by tau mutations. The known safety of lonafarnib revealed in human clinical trials for cancer suggests that this drug could be repurposed for treating tauopathies.

Assessment of Outer Retinal Remodeling in the Hibernating 13-Lined Ground Squirrel

Purpose

We examined outer retinal remodeling of the euthermic and torpid cone-dominant 13-lined ground squirrel (13-LGS) retina using optical coherence tomography (OCT) imaging and histology.

Methods

Retinas and corneas of living 13-LGSs were imaged during euthermic and torpid physiological states using OCT. Retinal layer thickness was measured at the visual streak from registered and averaged vertical B-scans. Following OCT, some retinas were collected immediately for postmortem histologic comparison using light microscopy, immunofluorescence, or transmission electron microscopy.

Results

Compared to OCT images from euthermic retinae, OCT images of torpid retinae revealed significantly thicker inner and outer nuclear layers, as well as increases in the distances between outer retinal reflectivity bands 1 and 2, and bands 3 and 4. A significant decrease in the distance between bands 2 and 3 also was seen, alongside significant thinning of the choriocapillaris and choroid. OCT image quality was reduced in torpid eyes, partly due to significant thickening of the corneal stroma during this state.

Conclusions

The torpid retina of the hibernating 13-LGS undergoes structural changes that can be detected by OCT imaging. Comparisons between in vivo OCT and ex vivo histomorphometry may offer insight to the origin of hyperreflective OCT bands within the outer retina of the cone-dominant 13-LGS.

Sox2 regulates astrocytic and vascular development in the retina

Sox2 is a transcriptional regulator that is highly expressed in retinal astrocytes, yet its function in these cells has not previously been examined. To understand its role, we conditionally deleted Sox2 from the population of astrocytes and examined the consequences on retinal development. We found that Sox2 deletion does not alter the migration of astrocytes, but it impairs their maturation, evidenced by the delayed upregulation of glial fibrillary acidic protein (GFAP) across the retina. The centro-peripheral gradient of angiogenesis is also delayed in Sox2-CKO retinas. In the mature retina, we observed lasting abnormalities in the astrocytic population evidenced by the sporadic loss of GFAP immunoreactivity in the peripheral retina as well as by the aberrant extension of processes into the inner retina. Blood vessels in the adult retina are also under-developed and show a decrease in the frequency of branch points and in total vessel length. The developmental relationship between maturing astrocytes and angiogenesis suggests a causal relationship between the astrocytic loss of Sox2 and the vascular architecture in maturity. We suggest that the delay in astrocytic maturation and vascular invasion may render the retina hypoxic, thereby causing the abnormalities we observe in adulthood. These studies uncover a novel role for Sox2 in the development of retinal astrocytes and indicate that its removal can lead to lasting changes to retinal homeostasis.

Anatomical and Gene Expression Changes in the Retinal Pigmented Epithelium Atrophy 1 (rpea1) Mouse: A Potential Model of Serous Retinal Detachment

Purpose

The purpose of this study was to examine the rpea1 mouse whose retina spontaneously detaches from the underlying RPE as a potential model for studying the cellular effects of serous retinal detachment (SRD).

Methods

Optical coherence tomography (OCT) was performed immediately prior to euthanasia; retinal tissue was subsequently prepared for Western blotting, microarray analysis, immunocytochemistry, and light and electron microscopy (LM, EM).

Results

By postnatal day (P) 30, OCT, LM, and EM revealed the presence of small shallow detachments that increased in number and size over time. By P60 in regions of detachment, there was a dramatic loss of PNA binding around cones in the interphotoreceptor matrix and a concomitant increase in labeling of the outer nuclear layer and rod synaptic terminals. Retinal pigment epithelium wholemounts revealed a patchy loss in immunolabeling for both ezrin and aquaporin 1. Anti-ezrin labeling was lost from small regions of the RPE apical surface underlying detachments at P30. Labeling for tight-junction proteins provided a regular array of profiles outlining the periphery of RPE cells in wild-type tissue, however, this pattern was disrupted in the mutant as early as P30. Microarray analysis revealed a broad range of changes in genes involved in metabolism, signaling, cell polarity, and tight-junction organization.

Conclusions

These data indicate changes in this mutant mouse that may provide clues to the underlying mechanisms of SRD in humans. Importantly, these changes include the production of multiple spontaneous detachments without the presence of a retinal tear or significant degeneration of outer segments, changes in the expression of proteins involved in adhesion and fluid transport, and a disrupted organization of RPE tight junctions that may contribute to the formation of focal detachments.

Cell-mediated remodeling of biomimetic encapsulating hydrogels triggered by adipogenic differentiation of adipose stem cells

One of the most common regenerative therapies is autologous fat grafting, which frequently suffers from unexpected volume loss. One approach is to deliver adipose stem cells encapsulated in the engineered hydrogels supportive of cell survival, differentiation, and integration after transplant. We describe an encapsulating, biomimetic poly(ethylene)-glycol hydrogel, with embedded peptides for attachment and biodegradation. Poly(ethylene)-glycol hydrogels containing an Arg–Gly–Asp attachment sequence and a matrix metalloprotease 3/10 cleavage site supported adipose stem cell survival and showed remodeling initiated by adipogenic differentiation. Arg–Gly–Asp–matrix metalloprotease 3/10 cleavage site hydrogels showed an increased number and area of lacunae or holes after adipose stem cell differentiation. Image analysis of adipose stem cells in Arg–Gly–Asp–matrix metalloprotease 3/10 cleavage site hydrogels showed larger Voronoi domains, while cell density remained unchanged. The differentiated adipocytes residing within these newly remodeled spaces express proteins and messenger RNAs indicative of adipocytic differentiation. These engineered scaffolds may provide niches for stem cell differentiation and could prove useful in soft tissue regeneration.

Strategies for bioengineered scaffolds that support adipose stem cells in regenerative therapies

Regenerative medicine possesses the potential to ameliorate damage to tissue that results from a vast range of conditions, including traumatic injury, tumor resection and inherited tissue defects. Adult stem cells, while more limited in their potential than pluripotent stem cells, are still capable of differentiating into numerous lineages and provide feasible allogeneic and autologous treatment options for many conditions. Adipose stem cells are one of the most abundant types of stem cell in the adult human. Here, we review recent advances in the development of synthetic scaffolding systems used in concert with adipose stem cells and assess their potential use for clinical applications.

Astrocyte structural reactivity and plasticity in models of retinal detachment

Although retinal neurodegenerative conditions such as age-related macular degeneration, glaucoma, diabetic retinopathy, retinitis pigmentosa, and retinal detachment have different etiologies and pathological characteristics, they also have many responses in common at the cellular level, including neural and glial remodeling. Structural changes in Müller cells, the large radial glia of the retina in retinal disease and injury have been well described, that of the retinal astrocytes remains less so. Using modern imaging technology to describe the structural remodeling of retinal astrocytes after retinal detachment is the focus of this paper. We present both a review of critical literature as well as novel work focusing on the responses of astrocytes following rhegmatogenous and serous retinal detachment. The mouse presents a convenient model system in which to study astrocyte reactivity since the Mϋller cell response is muted in comparison to other species thereby allowing better visualization of the astrocytes. We also show data from rat, cat, squirrel, and human retina demonstrating similarities and differences across species. Our data from immunolabeling and dye-filling experiments demonstrate previously undescribed morphological characteristics of normal astrocytes and changes induced by detachment. Astrocytes not only upregulate GFAP, but structurally remodel, becoming increasingly irregular in appearance, and often penetrating deep into neural retina. Understanding these responses, their consequences, and what drives them may prove to be an important component in improving visual outcome in a variety of therapeutic situations. Our data further supports the concept that astrocytes are important players in the retina's overall response to injury and disease.

Characterizing spatial distributions of astrocytes in the mammalian retina

MOTIVATION

In addition to being involved in retinal vascular growth, astrocytes play an important role in diseases and injuries, such as glaucomatous neuro-degeneration and retinal detachment. Studying astrocytes, their morphological cell characteristics and their spatial relationships to the surrounding vasculature in the retina may elucidate their role in these conditions.

RESULTS

Our results show that in normal healthy retinas, the distribution of observed astrocyte cells does not follow a uniform distribution. The cells are significantly more densely packed around the blood vessels than a uniform distribution would predict. We also show that compared with the distribution of all cells, large cells are more dense in the vicinity of veins and toward the optic nerve head whereas smaller cells are often more dense in the vicinity of arteries. We hypothesize that since veinal astrocytes are known to transport toxic metabolic waste away from neurons they may be more critical than arterial astrocytes and therefore require larger cell bodies to process waste more efficiently.

AVAILABILITY AND IMPLEMENTATION

A 1/8th size down-sampled version of the seven retinal image mosaics described in this article can be found on BISQUE (Kvilekval et al., 2010) at http://bisque.ece.ucsb.edu/client_service/view?resource=http://bisque.ece.ucsb.edu/data_service/dataset/6566968.

Viral-mediated RdCVF and RdCVFL expression protects cone and rod photoreceptors in retinal degeneration

Alternative splicing of nucleoredoxin-like 1 (Nxnl1) results in 2 isoforms of the rod-derived cone viability factor. The truncated form (RdCVF) is a thioredoxin-like protein secreted by rods that promotes cone survival, while the full-length isoform (RdCVFL), which contains a thioredoxin fold, is involved in oxidative signaling and protection against hyperoxia. Here, we evaluated the effects of these different isoforms in 2 murine models of rod-cone dystrophy. We used adeno-associated virus (AAV) to express these isoforms in mice and found that both systemic and intravitreal injection of engineered AAV vectors resulted in RdCVF and RdCVFL expression in the eye. Systemic delivery of AAV92YF vectors in neonates resulted in earlier onset of RdCVF and RdCVFL expression compared with that observed with intraocular injection using the same vectors at P14. We also evaluated the efficacy of intravitreal injection using a recently developed photoreceptor-transducing AAV variant (7m8) at P14. Systemic administration of AAV92YF-RdCVF improved cone function and delayed cone loss, while AAV92YF-RdCVFL increased rhodopsin mRNA and reduced oxidative stress by-products. Intravitreal 7m8-RdCVF slowed the rate of cone cell death and increased the amplitude of the photopic electroretinogram. Together, these results indicate different functions for Nxnl1 isoforms in the retina and suggest that RdCVF gene therapy has potential for treating retinal degenerative disease.

Robust segmentation based tracing using an adaptive wrapper for inducing priors

Segmentation based tracing algorithms detect the extent and borders of an object in a given frame IZ by propagating results from frames I1 ≤ z < Z. Although application specific tracers have been forthcoming, techniques that automatically adapt across applications have been less explored. We approach this problem by learning a prior model on topological dynamics that encourages segmentation transitions across frames that are most likely for a given application. Further, we augment a generic tracing technique with a locality sensitive prior derived from dense optic flow fields for deformation guidance. The proposed approach comprises two stages where the generic tracer initially yields multiple segmentation transitions when its parameters are perturbed, and the learnt topology prior subsequently propagates high scoring segmentations. Because the learnt topology model wraps around a generic tracer and adapts it by setting its free parameters, the need for careful parameter tuning is completely obviated. Through extensive experimental validation in surveillance, biological and medical image datasets, we verify the applicability of the proposed model while demonstrating good tracing performance under severe clutter.

Remodelling of retinal on- and off-bipolar cells following experimental retinal detachment

BACKGROUND

To study the response of ON and OFF bipolar cells in experimental retinal detachment.

METHODS

Domestic cat retinas were detached for 7 days. The retinas were prepared for immunocytochemical staining with antibodies to Go alpha (α), glutamate transporter GLT-1, protein kinase C and rod opsin, which serve as markers for ON bipolar cells, OFF bipolar cells, rod bipolar cells and rod photoreceptors, respectively. Both sections and whole-mounts were labelled with antibodies to Goα and GLT-1.

RESULTS

Following 7 days of detachment, ON bipolar cell processes extended into the outer nuclear layer and had neurites extending beyond their target layer into the inner plexiform layer. In contrast, OFF bipolar cell processes were reduced in the outer plexiform layer following detachment.

CONCLUSION

ON and OFF bipolar cells undergo significant remodelling of their processes in response to retinal detachment, and the ON and OFF pathways may be differentially affected. The remodelling may be due to morphological changes that have previously been shown to occur in photoreceptor synaptic terminals or as a result of loss of synaptic connections due to photoreceptor cell death.

Optomotor and immunohistochemical changes in the juvenile S334ter rat

The aim of this study was to examine the temporal relationship between behaviorally measured visual thresholds, photoreceptor degeneration and dysfunction, synaptic and neuronal morphology changes in the retina in the S334ter line 4 rat. Specifically, we examined the optokinetic tracking (OKT) behavior in S334ter rats daily and found that OKT thresholds reflected normal values at eye opening but quickly reduced to a non-response level by postnatal day (P) 22. By contrast, the scotopic electroretinogram (ERG) showed a much slower degeneration, with substantial scotopic function remaining after P90 as previously demonstrated for this line of rats. Photopic b-wave amplitudes revealed functional levels between 70 and 100% of normal between P30 and P90. Histological evidence demonstrated that photoreceptor degeneration occurred over many months, with an outer nuclear layer (ONL) roughly half the thickness of a normal age-matched control at P90. Immunohistochemical analysis revealed a number of changes in retinal morphology in the Tg S334ter line 4 rat that occur at or before P40 including: elevated levels of rod opsin expression in the ONL, cone photoreceptor morphology changes, glial cell activation, inner retinal neuron sprouting, and microglial cell activation. Many of these changes were evident at P30 and in some cases as early as eye opening (P15). Thus, the morphological changes occurred in concert with or before the very rapid loss of the behavioral (OKT) responses, and significantly before the loss of photoreceptors and photoreceptor function.

Protein changes in the retina following experimental retinal detachment in rabbits

PURPOSE

Retinal detachment leads to the widespread cellular remodeling of the retina. The purpose of this study was to identify protein changes that accompany these cellular alterations by comparing the proteomic profiles of sham and experimentally detached rabbit retina. Elucidation of the proteins most dramatically affected by retinal detachment would add further understanding to the pathophysiology of this condition, and potentially identify therapeutic targets useful in preventing the deleterious effects of detachment, including photoreceptor cell death and the activation of non-neuronal microglial and Müller cells.

METHODS

Retinal detachments were induced in the right eyes of six New Zealand Red pigmented rabbits. Sham surgery was performed in the right eyes of six other rabbits that were used as controls. At seven days, the eyes were enucleated and the retinal tissue was harvested. The individual retinal samples were subjected to high resolution two-dimensional polyacrylamide gel electrophoresis. Differentially expressed protein spots were processed for identification by liquid chromatography-tandem mass spectrometry. Further investigation was undertaken with western blotting, and immunocytochemical studies on a further set of four sham and four detached retinas.

RESULTS

Eighteen protein spots were found to be at least twofold differentially expressed between the sham and detached retinas. These protein spots were identified as: vimentin; tubulin β-2C; fragments of α-enolase; fructose-bisphosphate aldolase A; ATP synthase subunit β; mitochondrial creatine kinase; N-terminal fragments of albumin; prohibitin; and transducin-β(1).

CONCLUSIONS

The differentially expressed proteins determined in this study may play an important role in the cellular responses of the retina after its detachment, subsequent ability to recover following surgical reattachment, as well as in serious complications such as subretinal fibrosis and proliferative vitreoretinopathy.

Cell proliferation in human epiretinal membranes: characterization of cell types and correlation with disease condition and duration

PURPOSE

To quantify the extent of cellular proliferation and immunohistochemically characterize the proliferating cell types in epiretinal membranes (ERMS) from four different conditions: proliferative vitreoretinopathy (PVR), proliferative diabetic retinopathy, post-retinal detachment, and idiopathic ERM.

METHODS

Forty-six ERMs were removed from patients undergoing vitrectomy and immediately fixed in paraformaldehyde. The membranes were processed whole and immunolabeled with either anti-MIB-1 or anti-SP6 to detect the K(i)-67 protein in proliferating cells, in combination with anti-glial fibrillary acidic protein or anti-vimentin to identify glia, anti-ezrin to identify retinal pigment epithelial cells, Ricinus communis to identify immune cells, and Hoechst to label nuclei. Digital images were collected using a laser scanning confocal microscope. The cell types were identified, their combined proliferative indices were tabulated as the average number of anti-K(i)-67-positive cells/mm(2) of tissue, and the number of dividing cells was related to the specific ocular condition and estimated disease duration.

RESULTS

ERMs of all four types were shown to be highly cellular and contained proliferating cells identified as glia, retinal pigment epithelium, and of immune origin. In general, membranes identified as PVR had many more K(i)-67-positive cells in comparison to those in the other three categories, with the average number of K(i)-67-positive cells identified per mm(2) of tissue being 20.9 for proliferative diabetic retinopathy, 138.3 for PVR, 12.2 for post-retinal detachment, and 19.3 for idiopathic ERM. While all membrane types had dividing cells, their number was a relatively small fraction of the total number of cells present.

CONCLUSIONS

The four ERM types studied demonstrated different cell types actively dividing at the time of removal, confirming that proliferation is a common event and does continue over many months. The low number of dividing cells at the time of removal in comparison to the total number of cells present, however, is an indicator that proliferation alone may not be responsible for the problems observed with the ERMs. Treatment strategies may need to take into consideration the timing of drug administration, as well as the contractile and possibly the inflammatory characteristics of the membranes to prevent the ensuing effects on the retina.

Expression profiles of nestin and synemin in reactive astrocytes and Müller cells following retinal injury: a comparison with glial fibrillar acidic protein and vimentin

PURPOSE

To examine the expression patterns of the intermediate filament (IF) proteins nestin and synemin following retinal injury.

METHODS

Wide-scale retinal injuries were created by experimental retinal detachment of 1, 3, 7, or 30 days' duration. Injuries were induced in the right eyes of Long Evans rats, while the left eyes served as internal controls. Vibratome sections of control and injured retinas were labeled with fluorescent probes using a combination of anti-glial fibrillary acidic protein, -vimentin, -nestin, -synemin, -bromodeoxyuridine, and the lectin probe, isolectin B4. Additionally, antibody specificity, as well as protein and mRNA levels of nestin and synemin were determined and quantified using standard western blotting and real time polymerase chain reaction (RT-PCR) techniques.

RESULTS

Immunocytochemistry showed increased Müller cell labeling at 1, 3, and 7 days post injury for all four IFs, although the relative levels of nestin expression varied dramatically between individual Müller cells. Nestin was consistently observed in the foremost processes of those Müller cells that grew into the subretinal space, forming glial scars. Elevated levels of nestin expression were also observed in bromodeoxyuridine-labeled Müller cells following retinal insult. Quantitative polymerase chain reaction (qPCR) showed a twofold increase in nestin mRNA 1 day after injury, a level maintained at 3 and 7 days. Western blotting using anti-nestin showed a single band at 220 kDa and the intensity of this band increased following injury. Anti-synemin labeling of control retinas revealed faint labeling of astrocytes; this increased after injury, demonstrating an association with blood vessels. Additionally, there was an upregulation of synemin in Müller cells. qPCR and western blotting with anti-synemin showed a continuous increase in both gene and protein expression over time.

CONCLUSIONS

Retinal injury induces an upregulation of a complement of four intermediate filament proteins, including synemin and nestin, in Müller cells. The latter provides suggestive support for the concept that these cells may revert to a more developmentally immature state, since these two IF proteins are developmentally regulated and expressed, and thus may serve as cell cycle reentry markers. Nestin and its differential expression patterns with glial fibrillary acidic protein and vimentin networks, as well as its association with proliferating Müller cells and those extending into the subretinal space, suggest a significant role of this protein in glial scar formation and perhaps gliogenesis. Synemin immunopositive astrocytes demonstrate a close relationship to the retinal vasculature, and illustrate a remarkable ability to reorganize their morphology in response to injury. Further examination of the changes in the cytoskeletal signatures of both of these glial cell types may lead to a more comprehensive understanding of mechanisms underway following retinal and other central nervous system injuries.

Evidence that neurites in human epiretinal membranes express melanopsin, calretinin, rod opsin and neurofilament protein

AIMS

We have previously identified neurofilament-protein-containing neurites in human epiretinal membranes (ERMs). The aim of this study was to further characterise these neurites by examining the expression of additional specific proteins in human ERMs and to correlate this expression with various retinal disease conditions.

METHODS

Epiretinal membranes originating from 43 patients with proliferative vitreoretinopathy (PVR), proliferative diabetic retinopathy (PDR) or with no known pathology (idiopathic epiretinal membrane; iERM) were removed during vitrectomy at varying durations after diagnosis and immediately placed in fixative. The membranes were labelled immunohistochemically with different combinations of antibodies to the proteins melanopsin, calretinin and neurofilament (to identify subclasses of ganglion cells), rod opsin (to identify rod photoreceptors), synaptophysin and synaptic vesicle glycoprotein 2A (SV2) (identifies synaptic vesicles) and vimentin (identifies glial cells).

RESULTS

Anti-melanopsin-, anti-calretinin-, anti-neurofilament- and anti-rod-opsin-labelled neurites were routinely observed in the epiretinal membranes. Their presence did not appear to correlate with a specific disease condition or duration of the membrane. Generally neurites were observed in regions of glial cells.

CONCLUSIONS

Based on the expression of selected markers for neurites, we show neurite processes in human ERMs of various aetiologies originating from rod photoreceptors and different populations of retinal ganglion cells, although there was no obvious correlation with specific disease condition. In addition, synaptophysin and SV2 labelling was observed associated with all types of neurites, indicating the presence of at least one component necessary for synaptic transmission. Our data suggest that the adult human retina retains a significant capacity for neuronal remodelling under various disease conditions.

Alkylphosphocholines: a new approach to inhibit cell proliferation in proliferative vitreoretinopathy

Proliferative vitreoretinopathy represents the major complication in retinal detachment surgery and occurs in about 5-15% of cases resulting in a significant loss of vision despite multiple surgical procedures. Although successful anatomical reattachment is usually achieved, the reduction in central vision often remains permanent due to the intraretinal changes induced by retinal detachment and the subsequent proliferative response within the retina. Retinal Muller glial cells play a pivotal role in this process together with retinal pigment epithelial cells which are dispersed in the vitreous and stimulated by growth factors and serum in the vitreous after the breakdown of the blood-retinal barrier.

The effects of transient retinal detachment on cavity size and glial and neural remodeling in a mouse model of X-linked retinoschisis

PURPOSE

To determine the cellular consequences of retinal detachment in retinoschisin knockout (Rs1-KO) mice, a model for retinoschisin in humans.

METHODS

Experimental retinal detachments (RDs) were induced in the right eyes of both Rs1-KO and wild-type (wt) control mice. Immunocytochemistry was performed on retinal tissue at 1, 7, or 28 days after RD with antibodies to anti-GFAP, -neurofilament, and -rod opsin to examine cellular changes after detachment. Images of the immunostained tissue were captured by laser scanning confocal microscopy. Quantitative analysis was performed to measure the number of Hoechst-stained photoreceptor nuclei and their density, number, and size of inner retinal cavities, as well as the number of subretinal glial scars.

RESULTS

Since detachments were created with balanced salt solution, by examination, all retinas had spontaneously reattached by 1 day. Cellular responses common to many photoreceptor degenerations occurred in the nondetached retinas of Rs1-KO mice, and, of importance, RD did not appear to significantly accentuate these responses. The number of schisis cavities was not changed after detachment, but their size was reduced.

CONCLUSIONS

These data indicate that large short-term RD in Rs1-KO mice, followed by a period of reattachment may cause a slight increase in photoreceptor cell death, but detachments do not accentuate the gliosis and neurite sprouting already present and may in fact reduce the size of existing retinal cavities. This finding suggests that performing subretinal injections to deliver therapeutic agents may be a viable option in the treatment of patients with retinoschisis without causing significant cellular damage to the retina.

Muller cell reactivity and photoreceptor cell death are reduced after experimental retinal detachment using an inhibitor of the Akt/mTOR pathway

PURPOSE

To test the effect of Palomid 529, an inhibitor of the Akt/mTOR pathway, on Müller cell proliferation, subretinal glial scar formation, and photoreceptor survival after experimental retinal detachment (RD).

METHODS

Palomid 529 (600 microg) in balanced salt solution or balanced salt solution alone was injected intravitreally immediately after RD into the right eyes of 12 rabbits. Ten micrograms of BrdU was injected intravitreally on day 3. Animals were killed on day 3 or 7, at which time retinal sections were labeled with anti-BrdU to detect dividing cells, with anti-vimentin to identify Müller cells, and with the isolectin B4 to identify microglia and macrophages. Outer nuclear layer (ONL) thickness was measured from fluorescence-labeled nuclear-stained sections. Labeling was imaged using confocal microscopy. Six additional animals received either drug or balanced salt solution injections into normal eyes, and paraffin sections were stained with hematoxylin and eosin.

RESULTS

In the drug-treated eyes there was a significant decrease in the number of anti-BrdU-labeled Müller cells, the number and size of subretinal scars, and the number of isolectin B4-labeled cells. The ONL was also significantly thicker, and there was no evidence of toxic effects.

CONCLUSIONS

Palomid 529 is an effective suppressor of Müller cell proliferation, glial scar formation, and photoreceptor cell death in a rabbit model of RD. This suggests that inhibiting the Akt/mTOR signal transduction pathway may be an effective strategy to decrease proliferation and photoreceptor cell death induced by detachment and perhaps represents a novel therapy for related human diseases such as proliferative vitreoretinopathy.

Retraction and remodeling of rod spherules are early events following experimental retinal detachment: an ultrastructural study using serial sections

PURPOSE

To describe changes induced by retinal detachment in the ultrastructure and organization of rod terminals and their connections with B-type horizontal cell (HC) axon terminals and rod bipolar cell (RB) dendrites.

METHODS

Sections from control, 3 day, 7 day, and 28 day detached feline retinas were prepared for confocal immunofluorescence, light microscopy, and electron microscopy (EM). In addition, 100 mum-thick vibratome sections were immunolabeled with markers for photoreceptor terminals, HCs, and RBs. More than 40 rod spherules were studied in 90 nm-thick serial sections by transmission EM to greater detail changes in their ultrastructure and innervation.

RESULTS

Following retinal detachment, many rod terminals retracted varying distances toward their respective cell bodies in the outer nuclear layer (ONL). In retinas detached for 1 to 4 weeks, an altered synaptic vesicle population and associated ribbons were found in all retracting terminals. Many rod somata in the distal ONL seemed to lack synaptic terminal structures altogether. In a retina detached for 1 week, EM showed that less than half of the retracted terminals remain in contact with RB dendrites. In contrast, almost every surviving spherule was contacted by neurite outgrowths from the axon terminals of the B-type HC. Although retracted spherules had several presynaptic structures similar to those in normal retina, numerous changes occurred in their overall synaptic architecture. The spherule's invagination was shallower, contained fewer postsynaptic processes, and often had "opened," allowing swollen HC processes apposing the synaptic ribbon to directly contact other processes of the outer plexiform layer (OPL) neuropil. Whereas in normal cat retina each HC "lobe" comes from a different axon terminal system, after detachment, the opposing lateral elements can stem from the same terminal. The innervating RB dendrites that branched off stout RB dendritic trunks that extended up into the ONL were thinner than normal, unbranched, often electron dense, and lacked organelles. When present, most merely lay adjacent to retracting spherules rather than enter any synaptic invagination that might still occur.

CONCLUSIONS

Immunocytochemistry enabled RB and HC neurites to appear postsynaptic to retracted rod terminals. However, at the ultrastructural level, HCs seemed to more consistently retain connection with the retracted spherules than the RBs. The highly conserved architecture of the rod spherule was lost as the invagination opened and postsynaptic contacts became fewer. It would seem that the lack of RB central elements as well as the drastic alterations in the architecture of most retracted terminals would necessarily alter the physiology of this complex synapse.

Abnormal reactivity of muller cells after retinal detachment in mice deficient in GFAP and vimentin

PURPOSE

To determine the roles of glial fibrillary acidic protein (GFAP) and vimentin in Müller cell reactivity.

METHODS

Retinal detachments were created in mice deficient for GFAP and vimentin (GFAP(-/-)vim(-/-)) and age-matched wild-type (wt) mice. The reactivity of the retina was studied by immunofluorescence and electron microscopy.

RESULTS

Müller cell morphology was different and glutamine synthetase immunoreactivity was reduced in the undisturbed GFAP(-/-)vim(-/-) retinas. After retinal detachment, Müller cells formed subretinal glial scars in the wt mice. In contrast, such scars were not observed in GFAP(-/-)vim(-/-) mice. Müller cells, which normally elongate and thicken in response to detachment, appeared compressed, thin, and "spikey" in the GFAP(-/-)vim(-/-) mice. The end foot region of Müller cells in the GFAP(-/-)vim(-/-) mice often sheared away from the rest of the retina during detachment, corroborating earlier results showing decreased resistance of this region in GFAP(-/-)vim(-/-) retinas to mechanical stress. In regions with end foot shearing, ganglion cells showed intense neurite sprouting, as revealed by anti-neurofilament labeling, a response rarely observed in wt mice.

CONCLUSIONS

Müller cells are subtly different in the GFAP(-/-)vim(-/-) mouse retina before detachment. The end foot region of these cells may be structurally reinforced by the presence of the intermediate filament cytoskeleton, and our data suggest a critical role for these proteins in Müller cell reaction to retinal detachment and participation in subretinal gliosis.

[Possible role of alkylphosphocholines in retinal reattachment surgery]

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Attenuated glial reactions and photoreceptor degeneration after retinal detachment in mice deficient in glial fibrillary acidic protein and vimentin

PURPOSE

To characterize the reactions of retinal glial cells (astrocytes and Müller cells) to retinal injury in mice that lack glial fibrillary acidic protein (GFAP) and vimentin (GFAP-/-Vim-/-) and to determine the role of glial cells in retinal detachment (RD)-induced photoreceptor degeneration.

METHODS

RD was induced by subretinal injection of sodium hyaluronate in adult wild-type (WT) and GFAP-/-Vim-/- mice. Astroglial reaction and subsequent monocyte recruitment were quantified by measuring extracellular signal-regulated kinase (Erk) and c-fos activation and the level of expression of chemokine monocyte chemoattractant protein (MCP)-1 and by counting monocytes/microglia in the detached retinas. Immunohistochemistry, immunoblotting, real-time quantitative polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) were used. RD-induced photoreceptor degeneration was assessed by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) and measurement of outer nuclear layer (ONL) thickness.

RESULTS

RD-induced reactive gliosis, characterized by GFAP and vimentin upregulation, Erk and c-fos activation, MCP-1 induction, and increased monocyte recruitment in WT mice. Absence of GFAP and vimentin effectively attenuated reactive responses of retinal glial cells and monocyte infiltration. As a result, detached retinas of GFAP-/-Vim-/- mice exhibited significantly reduced numbers of TUNEL-positive photoreceptor cells and increased ONL thickness compared with those of WT mice.

CONCLUSIONS

The absence of GFAP and vimentin attenuates RD-induced reactive gliosis and, subsequently, limits photoreceptor degeneration. Results of this study indicate that reactive retinal glial cells contribute critically to retinal damage induced by RD and provide a new avenue for limiting photoreceptor degeneration associated with RD and other retinal diseases or damage.

The effect of alkylphosphocholines on intraretinal proliferation initiated by experimental retinal detachment

PURPOSE

To determine the effect of alkylphosphocholines (APCs) on intraretinal proliferation induced by experimental retinal detachment in the rabbit.

METHODS

Retinal detachments were created in adult pigmented rabbits. APCs, either liposome bound (liposome, L-APC) or unbound (free, F-APC), were injected intravitreally on either day 1 or day 2 after detachment. BrdU was injected on day 3, 4 hours before death. After fixation, retinas were triple labeled with anti-BrdU, anti-vimentin, and the isolectin B4. The number of anti-BrdU-labeled cells was counted per millimeter of retina from sections imaged by laser scanning confocal microscopy. Toxicity was examined using toluidine blue-stained sections imaged by light microscopy and by electron microscopy for ultrastructural evaluation.

RESULTS

Retinal detachment initiated proliferation of all non-neuronal cells. After intravitreal injection on day 1 or 2 after experimental induction of retinal detachment, APCs significantly reduced the number of dividing cells at day 3. Liposome-bound drug given on day 2 was more effective on Müller cell proliferation than was unbound drug. Injection of F-APC on day 1 was more effective than when given on day 2. No apparent effect was seen on Müller cell hypertrophy as indicated by vimentin expression. In addition, no evidence of toxicity was observed in the retina at day 3 for any of the conditions.

CONCLUSIONS

APCs significantly reduce the number of Müller cells that are stimulated to divide as a result of retinal detachment. The preliminary results indicate no evidence of significant toxicity; however, further studies are needed. APCs have the potential to be used as part of a therapeutic approach if they can be combined with other agents that can suppress the fibrosis that is also a critical event in the pathogenesis of proliferative vitreoretinal diseases such as proliferative vitreoretinopathy (PVR).

Identification of ganglion cell neurites in human subretinal and epiretinal membranes

AIM

To determine whether neural elements are present in subretinal and epiretinal proliferative vitreoretinopathy (PVR) membranes as well as in diabetic, fibrovascular membranes removed from patients during vitrectomy surgery.

METHODS

Human subretinal and epiretinal membranes of varying durations were immunolabelled with different combinations of antibodies to glial fibrillary acidic protein, vimentin, neurofilament protein and laminin.

RESULTS

Anti-neurofilament-labelled neurites from presumptive ganglion cells were frequently found in epiretinal membranes and occasionally found in subretinal membranes. In addition, the neurites were only observed in regions that also contained glial processes.

CONCLUSIONS

These data demonstrate that neuronal processes are commonly found in human peri-retinal cellular membranes similar to that demonstrated in animal models. These data also suggest that glial cells growing out of the neural retina form a permissive substrate for neurite growth and thus may hold clues to factors that support this growth.

Automated tool for the detection of cell nuclei in digital microscopic images: application to retinal images

PURPOSE

To develop an automated tool that provides reliable, consistent, and accurate results for counting cell nuclei in tissue sections.

METHODS

We propose a novel method based on an image processing algorithm to analyze large sets of digital micrographs. The nucleus detector design is based on a Laplacian of Gaussian filter. We use the leave-one-out cross validation method for estimating the generalization error, which is then used to choose the model and parameters of the proposed nucleus detector with both fluorescent and dye stained images. We also evaluate the performance of a nucleus detector by comparing the results with manual counts.

RESULTS

When our nucleus detector is applied to previously unanalyzed images of feline retina, it correctly counts nuclei within the outer nuclear layer (ONL) with an average error of 3.67% ranging from 0 to 6.07%, and nuclei within the inner nuclear layer (INL) with an average error of 8.55% ranging from 0 to 13.76%. Our approach accurately identifies the location of cell bodies. Even though we have a relatively large error in the INL due to the large intra-observer variation, both manual counting and nucleus detector result in the same conclusion. This is the first time that cell death in the INL in response to retinal detachment is analyzed quantitatively. We also test the proposed tool with various images and show that it is applicable to a wide range of image types with nuclei varying in size and staining intensity.

CONCLUSIONS

The proposed method is simple and reliable. It also has widespread applicability to a variety of sample preparation and imaging methods. Our approach will be immediately useful in quantifying cell number in large sets of digital micrographs and from high-throughput imaging. The tool is available as a plug-in for Image J.

Electrophysiologic and retinal penetration studies following intravitreal injection of bevacizumab (Avastin)

PURPOSE

Intravitreal bevacizumab (Avastin; Genentech Inc., San Francisco, CA) is a new treatment for age-related macular degeneration. The aim of this study was to evaluate retinal penetration and toxicity of bevacizumab.

METHODS

Ten albino rabbits were injected intravitreally with 0.1 mL (2.5 mg) of Avastin into one eye and 0.1 mL saline into the fellow eye. The electroretinogram (ERG) was recorded after 3 hours, 3 days, and 1, 2, and 4 weeks. The visual evoked potential (VEP) was recorded after 4 weeks. Confocal immunohistochemistry was used to assess retinal penetration.

RESULTS

The ERG responses of the control and experimental eyes were similar in amplitude and pattern throughout the follow-up period. The flash VEP responses of the experimental eyes were of normal pattern and amplitude and did not differ from those recorded by stimulation of the control eye alone. Full thickness retinal penetration was present at 24 hours and was essentially absent at 4 weeks.

CONCLUSIONS

Bevacizumab was found to be nontoxic to the retina of rabbits based on electrophysiologic studies. Full thickness retinal penetration may explain observed clinical effects of intravitreal bevacizumab. Although it is difficult to directly extrapolate to humans, our study supports the safe use of intravitreal bevacizumab injection.

Morphological characterization of the retinal degeneration in three strains of mice carrying the rd-3 mutation

Retinal development in 3 strains of rd-3/rd-3 mutant mice, previously shown to have different rates of degeneration, was studied using light, electron, and immunofluorescence microscopy. The time course and phenotype of the degeneration as well as details on the mechanism of massive photoreceptor cell loss are compared with other known retinal degenerations in mice. Up until postnatal day (P) 10, the retinas of all three strains (RBF, 4Bnr, In-30) develop similarly to those of pigmented and nonpigmented controls. TUNEL-positive cells appear in the outer nuclear layer (ONL) by P14, and reach a maximum in all three mutant strains around P21. Scattered rods and cones form a loose, monolayered ONL by 8 weeks in the albino RBF strain, by 10 weeks in the albino 4Bnr strain, and by 16 weeks in the pigmented In-30 strain. Though the initial degeneration begins in the central retina, there is no preferred gradient of cell death between central and peripheral photoreceptors. Rods and cones are present at all ages examined. During development, stacks of outer segments (OS) form in all three strains though they never achieve full adult lengths, and often have disorganized, atypical OS. Rod opsin is expressed in the developing OS but is redistributed into plasma membrane as OS degeneration proceeds. Retinal pigment epithelial (RPE) cells of all mutant strains contain packets of phagocytosed OS, and their apical processes associate with the distal ends of the OS. At their synaptic sites, photoreceptor terminals contain ribbons apposed to apparently normal postsynaptic triads. As photoreceptors are lost, Müller cells fill in space in the ONL but they do not appear to undergo significant hypertrophy or migration, though during the degeneration, glial fibrillary acidic protein (GFAP) expression is gradually upregulated. Macrophage-like cells are found frequently in the subretinal space after the onset of photoreceptor apoptosis. As OS disappear, the RPE apical processes revert to simple microvilli. Late in the degeneration, some RPE cells die and neighboring cells appear to flatten as if to maintain confluence. In regions of RPE cell loss that happen to lie above retina where the ONL is gone, cells of the inner nuclear layer (INL), wrapped by Müller cell processes, may front directly on Bruch's membrane.

Microglial cell activation following retinal detachment: a comparison between species

PURPOSE

To compare the activation of microglia in response to retinal detachment in four species.

METHODS

Experimental detachments were created in cats, rabbits, and ground squirrels and the retinas harvested 1, 3, 7, or 28 days later. Retinal reattachments of 28 days in duration were also performed in cats following a 3-day detachment. Human tissue was obtained during reattachment surgery. Microglia and macrophages were labeled with the lectins Griffonia simplicifolia and Ricinus communis and the antibody CD11b. Müller cell and photoreceptor responses were followed immunocytochemically on the same tissue sections labeled with microglial markers. Images were collected by laser scanning confocal microscopy.

RESULTS

Lightly labeled microglia were observed primarily in the inner retina of control tissue. In the cat and rabbit, a progressive increase in the number of labeled cells occurred in the outer retina beginning at 1 day of detachment. In both long term human and cat detachments numbers of microglia were elevated throughout the retina. This is in contrast to the rabbit and ground squirrel retinas where microglial activation was dramatically diminished in longer term detachments. Presumptive macrophages (anti-CD11b labeled cells) occurred only in the subretinal space. Retinal reattachment in cats significantly attenuated the response except in areas of poor outer segment regeneration.

CONCLUSIONS

The robust microglial response to retinal detachment is an indicator of the importance of this cell type in the overall response of the retina. Our data suggest that the feline retina is a particularly appropriate model system for understanding this response in humans. Inhibiting the microglial response in that species should help us understand more precisely its potential role in photoreceptor survival in human pathology.

Cellular remodeling in mammalian retina: results from studies of experimental retinal detachment

Retinal detachment, the separation of the neural retina from the retinal pigmented epithelium, starts a cascade of events that results in cellular changes throughout the retina. While the degeneration of the light sensitive photoreceptor outer segments is clearly an important event, there are many other cellular changes that have the potential to significantly effect the return of vision after successful reattachment. Using animal models of detachment and reattachment we have identified many cellular changes that result in significant remodeling of the retinal tissue. These changes range from the retraction of axons by rod photoreceptors to the growth of neurites into the subretinal space and vitreous by horizontal and ganglion cells. Some neurite outgrowths, as in the case of rod bipolar cells, appear to be directed towards their normal presynaptic target. Horizontal cells may produce some directed neurites as well as extensive outgrowths that have no apparent target. A subset of reactive ganglion cells all fall into the latter category. Muller cells, the radial glia of the retina, undergo numerous changes ranging from proliferation to a wholesale structural reorganization as they grow into the subretinal space (after detachment) or vitreous after reattachment. In a few cases have we been able to identify molecular changes that correlate with the structural remodeling. Similar changes to those observed in the animal models have now been observed in human tissue samples, leading us to conclude that this research may help us understand the imperfect return of vision occurring after successful reattachment surgery. The mammalian retina clearly has a vast repertoire of cellular responses to injury, understanding these may help us improve upon current therapies or devise new therapies for blinding conditions.

Glial Remodeling and Neural Plasticity in Human Retinal Detachment with Proliferative Vitreoretinopathy

PURPOSE

To investigate glial remodeling and neuronal plasticity in adult human retinal detachment complicated by proliferative vitreoretinopathy (PVR) and to grade pathologic changes with a severity scoring system.

METHODS

Sixteen full-thickness retinectomy specimens obtained at retinal relaxing surgery for PVR were fixed in 4% paraformaldehyde immediately after excision and compared to similarly processed normal donor retinas. Agarose-embedded sections (100-microm-thick) were double labeled for immunohistochemistry by confocal microscopy, with antibodies against rod opsin and GFAP; vimentin and M/L-cone opsin; calbindin D and S-cone opsin; and cytochrome oxidase and synaptophysin. These staining patterns formed the basis of a retinal pathology scoring system, and immunohistochemistry was also used to detect CD68, neurofilaments, protein kinase C, growth-associated protein-43, and a pan-cone-specific enzymatic marker. Morphology was also assessed by light microscopy of resin-embedded semithin sections.

RESULTS

Prolonged detachment was characterized by photoreceptor degeneration and intracellular redistribution of opsin proteins to the plasma membrane in the outer nuclear layer (ONL). Remodeling of rod synaptic terminals was characterized by terminal retraction and also by axon extension to the inner retina in most specimens. Rod bipolar cell dendrites extended into the ONL, as did fine, horizontal cell processes. Large ganglion cells showed upregulated neurofilament and GAP-43 expression, with neurites sprouting from somata and axon collaterals. Anti-cytochrome oxidase labeling of surviving inner segments was reduced but detectable in all specimens, as was anti-calbindin D labeling of horizontal and amacrine cells. All specimens demonstrated a marked upregulation of Muller cell and astrocyte expression of GFAP and vimentin. More severe degenerative changes correlated with trauma and prolonged detachment duration when scored according to this system.

CONCLUSIONS

The neural and glial components of detached neurosensory retina complicated by PVR exhibit pathology that changes characteristically with increasing detachment severity. Even in advanced degeneration, most of the structural motifs necessary for functional recovery are retained. Evidence of remodeling in the first-, second-, and third-order neurons of detached adult human retina may represent an attempt to re-establish synaptic connectivity.

The efficacy of delayed oxygen therapy in the treatment of experimental retinal detachment

PURPOSE

To evaluate the ability of delayed hyperoxia to slow or prevent degenerative and gliotic changes initiated by retinal detachment.

DESIGN

An experimental study.

METHODS

Rhegmatogenous detachments were produced in the right eyes of eight cats. After 1 day in room air (21% O(2)), four cats were placed in chambers with the O(2) concentration regulated at 70%; the other four were left in room air. At 7 days the retinas were harvested and examined by light and confocal microscopy. Cell specific antibodies, TUNEL and proliferation assays, outer segment length, and photoreceptor counts, were used to assess the condition of the retina. The contralateral unoperated eyes were used as controls.

RESULTS

Animals maintained in elevated O(2) showed a dramatic preservation of rod and cone outer segments as well as in the organization of the outer plexiform layer. The number of surviving photoreceptors was increased in the hyperoxia-treated animals. Neurite sprouting, a characteristic of detached retina, was rarely observed in the experimental eyes. Proliferation of non-neuronal cells was reduced, but not halted, by hyperoxia. GFAP and vimentin expression was not effected by hyperoxia; these intermediate filament proteins increased in Müller cells similar to that observed in control detachments.

CONCLUSIONS

Exposure to hyperoxia, delayed by 1 day after the onset of retinal detachment, was highly effective in preserving photoreceptor cells and in reducing proliferation within the retina. It did not, however, reduce the hypertrophy of Müller glia. There were no apparent detrimental effects of exposure to 70% O(2) for 6 days. These results suggest that human patients may benefit from breathing elevated oxygen levels while awaiting reattachment surgery, even if the hyperoxia is delayed relative to the time of detachment.

The neurons of the ground squirrel retina as revealed by immunostains for calcium binding proteins and neurotransmitters

Ground squirrel retinas were immunostained with antibodies against calcium binding proteins (CBPs) and classical neurotransmitters in order to describe neuronal phenotypes in a diurnal mammalian retina and to then compare these neurons with those of more commonly studied nocturnal retinas like cats' and rabbits'. Double immunostained tissue was examined by confocal microscopy using antibodies against the following: rhodopsin and the CBPs, calbindin, calretinin, parvalbumin, calmodulin and recoverin (CB, CR, PV, CM, RV), glycine, GABA, choline acetyltransferase (CHAT) and tyrosine hydroxylase (TOH). In ground squirrel retina, the traditional cholinergic mirror symmetric amacrine cells colocalize CHAT with PV and GABA and faintly with glycine. A second cholinergic amacrine cell type colocalizes glycine alone. CR is found in at least 3 different amacrine cell types. The CR-immunoreactive (IR) cell population is a mixture of glycinergic and GABAergic types. The dopamine cell type IR to tyrosine hydroxylase has the typical morphology of a wide field cell with dendrites in S1 but the "rings" seen in cat or rabbit retina are not as numerous. TOH-IR amacrine cells send large club-shaped processes to the outer plexiform layer. CB and CR are in bipolar cells, A- and B-type horizontal cells and several amacrine cell types. Anti-rhodopsin labels the low density rod photoreceptor population in this species. Anti-recoverin labels cones and some bipolar cells while PKC is found in several different bipolar cell types. One ganglion cell with dendritic branching in S3 is strongly CR-IR. We find no evidence for an AII amacrine cell in the ground squirrel, with either anti-CR or anti-PV. An amacrine cell with similarity to the DAP1-3 cell of rabbit is CR-IR and glycine-IR. We discuss this labeling pattern in relationship to other mammalian species. The differences in staining patterns and phenotypes revealed suggest a functional diversity in the populations of amacrine cells according to whether the retinas are rod or cone dominated.

Drusen-Associated Degeneration in the Retina

PURPOSE

Drusen are variably sized extracellular deposits that form between the retinal pigmented epithelium (RPE) and Bruch's membrane. They are commonly found in aged eyes, however, numerous and/or confluent drusen are a significant risk factor for age-related macular degeneration. The purpose of this study was to investigate the impact of drusen on overlying cells of the retina.

METHODS

Tissue containing retina and RPE/choroid was dissected from human donor eyes, embedded in agarose, and sectioned at 100 micro m using a vibratome. Sections were immunostained with a panel of antibodies that labeled glial cells, first-, second-, and third-order retinal neurons and processed for confocal microscopy.

RESULTS

Retinal cells that overlie both soft and hard drusen exhibited numerous structural and molecular abnormalities. Normally detectable only in the outer segments of rod photoreceptors, rod opsin immunolabeling was also observed in the inner segment, cell body, axon, and axon terminal of photoreceptors that overlie drusen. Labeling with this antibody also revealed the deflection and shortening of rod inner and outer segments. Cone photoreceptors displayed similar structural abnormalities, as well as a decrease in cone opsin immunoreactivity. Drusen-associated abnormalities in the synaptic terminals of photoreceptor cells were also observed. In addition, an increase in intermediate filament protein immunoreactivity (vimentin and glial fibrillary acidic protein) was observed within Müller glial cells in areas of retina overlying drusen. Both soft and hard drusen were associated with a similar spectrum of effects in both macular and extramacular regions. Second- and third-order neurons, including bipolar, horizontal, amacrine, and ganglion cells all appeared unaffected. The structural and molecular abnormalities observed in photoreceptors and Müller glial cells were confined to retinal regions directly overlying and immediately adjacent to drusen; more distant retinal regions appeared unperturbed. Remarkably, significant abnormalities were observed over small subclinical drusen.

CONCLUSIONS

Retinal cells overlying both soft and hard drusen exhibit structural and molecular abnormalities indicative of photoreceptor degeneration and Müller glial activation. These abnormalities resemble the degenerative effects common to many forms of retinal degeneration, but are confined to areas directly overlying drusen. This suggests that photoreceptor cell function is compromised as a consequence of drusen formation.

Müller cell and neuronal remodeling in retinal detachment and reattachment and their potential consequences for visual recovery: a review and reconsideration of recent data

Recent evidence suggests that the adult mammalian retina is far more plastic than was previously thought. Retinal detachment induces changes beyond the degeneration of outer segments (OS). Changes in photoreceptor synapses, second- and even third-order neurons may all contribute to imperfect visual recovery that can occur after successful reattachment. Changes that occur in Müller cells have obvious effects through subretinal fibrosis and proliferative vitreoretinopathy, but other unidentified effects seem likely as well. Reattachment of the retina induces its own set of responses aside from OS re-growth. Reattachment halts the growth of Müller cell processes into the subretinal space, but induces their growth on the vitreal surface. It also induces the outgrowth of rod axons into the inner retina.

Evidence that ganglion cells react to retinal detachment

Growth associated protein 43 (GAP 43) is involved in synapse formation and it is expressed in the retina in a very specific pattern. Although GAP 43 is downregulated at the time of synapse formation, it can be re-expressed following injury such as axotomy or ischemia. Because of this we sought to characterize the expression of GAP 43 after retinal detachment (RD). Immunoblot, immunocytochemical and quantitative polymerase chain reaction (QPCR) techniques were used to assess the level of GAP 43 expression after experimental RD. GAP 43 was localized to three sublaminae of the inner plexiform layer of the normal retina. GAP 43 became upregulated in a subset of retinal ganglion cells following at least 7 days of RD. By immunoblot GAP 43 could be detected by 3 days. QPCR shows the upregulation of GAP 43 message by 6hr of detachment. To further characterize changes in ganglion cells, we used an antibody to neurofilament 70 and 200kDa (NF) proteins. Anti-NF labels horizontal cells, ganglion cell dendrites in the inner plexiform layer, and ganglion cell axons (fasicles) in the normal retina. Following detachment it is upregulated in horizontal cells and ganglion cells. When detached retina was double labelled with anti-GAP 43 and anti-NF, some cells were labelled with both markers, while others labelled with only one. We have previously shown that second order neurons respond to detachment; here we show that third order neurons are responding as well. Cellular remodelling of this type in response to detachment may explain the slow recovery of vision that often occurs after reattachment, or those changes that are often assumed to be permanent.

Choline acetyltransferase is expressed by non-starburst amacrine cells in the ground squirrel retina

We have used immunostaining techniques to reveal a new type of amacrine cell that is immunoreactive for choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, in the Ground Squirrel (Spermophilus beecheyi) retina. Cryostat sections and double immunostained wholemount preparations were examined by confocal microscopy. This new ChAT type III cell is distinct in morphology and neurotransmitter content from the well know 'starburst' amacrine cells (types I and II) that are so well represented in the ground squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. The type III cell colocalizes glycine with the acetylcholine and does not appear to be GABAergic or exhibit calcium-binding proteins like the well-known starburst type. As well, type III cells do not occur as a mirror-symmetric pair with normally placed and displaced varieties. The type III cell is probably a small field amacrine type branching broadly in upper sublamina b of the inner plexiform layer, and is most likely A6 of the Ground Squirrel retina [J. Comp. Neurol. 365 (1996) 173-216]. Type III cells are ideally placed in the architecture of the Ground Squirrel retina to influence ON directionally selective ganglion cell types.

Cone photoreceptor recovery after experimental detachment and reattachment: an immunocytochemical, morphological, and electrophysiological study

PURPOSE

To compare the morphologic and functional recovery of the retina after detachment and reattachment in an animal with a cone-dominant retina, the ground squirrel.

METHODS

Ground squirrel (Spermophilus beecheyi) retinas were detached for 1 day and reattached for 7, 35, or 96 days (n = 2, each time point). Flicker ERGs were recorded 1 day after the detachment and at various times after reattachment. Contrast-response functions were measured for isochromatic modulation and for selective modulation of short-wavelength-sensitive (S) and middle-wavelength-sensitive (M) cones. At the end of the experiment, retinas were prepared for light microscopy or immunocytochemical staining with antibodies to rod opsin, S and M cone opsins, cytochrome oxidase, synaptophysin, glial fibrillary acidic protein (GFAP), cellular retinaldehyde-binding protein (CRALBP), interphotoreceptor-binding protein (IRBP), and peanut agglutinin lectin (PNA). Photoreceptor density maps were created from wholemount preparations labeled with biotinylated PNA and anti-S cone opsin. Cell counts of photoreceptor nuclei and cone outer segments (OS) were compared with flicker ERG data. Cell death was examined by the TUNEL method.

RESULTS

Reattachment stopped photoreceptor cell death and reversed the disruption of interphotoreceptor matrix as well as the redistribution of Müller cell proteins. It also activated some astrocytes based on anti-GFAP staining. S- and M-cone OS showed a gradual recovery in length after reattachment, and this recovery continued to the longest time points examined. ERG contrast gains also recovered after reattachment, but these reached asymptotic levels by approximately a week after reattachment. There were significant correlations between outer nuclear layer (ONL) cell counts and ERG contrast gains. No differences were noted in the indices of recovery of M and S cones.

CONCLUSIONS

The ERG can be used to follow specifically the changes in the retina that occur after retinal detachment and reattachment.

Experimental Retinal Reattachment: A New Perspective

In the feline model, retinal detachment initiates a cascade of changes that include photoreceptor- cell "deconstruction," apoptotic death of some photoreceptors, neurite outgrowth from second- and third-order neurons, remodeling of photoreceptor synaptic terminals, and Müller-cell gliosis. We have previously shown that reattachment within 24 h halts or reverses many of these presumed detrimental changes. However, in patients with retinal detachments, reattachment cannot always be performed within this 24-h window. Moreover, recovery of vision following successful reattachment surgery in the macula is often imperfect. Here, we examine the ability of relatively long-term reattachment (28 d) to stop or reverse several cellular events that occur at 3 d of detachment. In contrast to earlier studies of reattachment, which focused on the regeneration of outer segments, we focus our attention here on other cellular events such as neuronal remodeling and gliosis. Some of these changes are reversed by reattachment, but reattachment itself appears to stimulate other changes that are not associated with detachment. The implications of these events for the return of vision are unknown, but they do indicate that simply reattaching the retina does not return the retina to its pre-detachment state within 28 d.

Up-Regulation of Glial Fibrillary Acidic Protein in Response to Retinal Injury: Its Potential Role in Glial Remodeling and a Comparison to Vimentin Expression

Intermediate filament proteins are a heterogeneous group of proteins that form 10-nm-diameter filaments, a highly stable cytoskeletal component occurring in various cell types. The up-regulation of one of these intermediate filament proteins, glial fibrillary acidic protein (GFAP), historically has been an indicator of "stress" in central nervous system (CNS) astrocytes. The retina also responds similarly to "stress" but the up-regulation of intermediate filaments occurs primarily in the Müller cells, the radial glia of the retina. This is a remarkably ubiquitous response in that a similar up-regulation can be observed in numerous forms of retinal degeneration. As a consequence of retinal detachment, a "mechanical" injury to the retina, GFAP, and another intermediate filament protein, vimentin, dramatically increase in Müller cells. Concomitant with this up-regulation is the hypertrophy of these cells both within the retina and onto the photoreceptor and vitreal surfaces of the retina. The function of this distinctive intermediate filament up-regulation in glial cells is unknown, but in the retina their expression is differentially regulated in a polarized manner as the Müller cells hypertrophy, suggesting that they play some role in this process. Moreover the response of intermediate filaments and the Müller cells differs depending on whether the retina has been detached or reattached to the retinal pigment epithelium. The differential expression of these proteins may give insight into their role in the formation of glial scars in the retina and elsewhere in the CNS.