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

Retinal damage extends beyond the border of the detached retina in fovea-on retinal detachment

PURPOSE

The aim of this study was to investigate the preoperative and postoperative change in retinal sensitivity in relation to the distance to the retinal detachment (RD) in patients with fovea-on RD.

METHODS

We prospectively evaluated 13 patients with fovea-on RD and a healthy control eye. Preoperatively, OCT scans of the RD border and the macula were obtained. The RD border was highlighted on the SLO image. Microperimetry was used to assess the retinal sensitivity at the macula, the RD border and the retina around the RD border. At 6 weeks, 3 and 6 months postoperatively, follow-up examinations of OCT and microperimetry were performed in the study eye. Microperimetry was performed once in control eyes. Microperimetry data were overlaid on the SLO image. The shortest distance to the RD border was calculated for each sensitivity measurement. The change in retinal sensitivity was calculated as control-study. The relation between the change in retinal sensitivity and the distance to the RD border was assessed using a locally weighted scatterplot smoothing curve.

RESULTS

Preoperatively, the greatest loss in retinal sensitivity was 21 dB at 3° inside the RD which decreased linearly, through the RD border, and reached a plateau of 2 dB at 4°. For 6 weeks and 3 months postoperatively, the greatest retinal sensitivity loss remained at 3° inside the RD but was 4 dB and sensitivity loss decreased linearly to a plateau of 0 dB at 5° outside the RD. At 6 months postoperatively, the greatest sensitivity loss was 2 dB at 3° inside the RD, and decreased linearly to a plateau of 0 dB at 2° outside the RD.

CONCLUSIONS

Retinal damage extends beyond the detached retina. Retinal sensitivity loss of the attached retina decreased drastically as the distance to the RD increased. Postoperative recovery occurred for both attached and detached retina.

Swelling and membrane potential dynamics of glial Müller cells

Presently a detailed biophysical model describing reversible and irreversible swelling dynamics of Müller cells (MC) is reported. The model includes a biophysical block of ionic and neutral species transport via MC membrane, water transport induced by osmotic pressure and pressure generated by membrane deformations, MC membrane potential and membrane mechanical properties. The model describes reversible and irreversible MC swelling (MCS) using the same set of parameters. The model was used in fitting available experimental data, and produced numerical values of previously unknown model parameters, including those describing mechanical properties of Müller cell membrane (MCM) with respect to bending and stretching. Numerical experiments simulating MC swelling showed complex oscillation dynamics of the relevant parameters in physiological initial conditions. In particular, MC membrane potential (ΔΨ_MC) demonstrated complex oscillation dynamics, which may be described by a superposition of several oscillations with their periods in the milliseconds, 100-ms and seconds time ranges. Dynamics of reversible and irreversible MCS, and the transition criteria from reversible to irreversible MCS modes were determined in model simulations.

Association of microvasculature changes with visual outcomes after successful retinal detachment surgery

PURPOSE

To evaluate macular microvascular changes and their correlation with visual outcomes after rhegmatogenous retinal detachment (RRD) surgery.

DESIGN

Retrospective case series PARTICIPANTS: Seventy-four eyes successfully operated for RRD were included in the study.

METHODS

The area of the foveal avascular zone in the superficial capillary plexus, the vessel density in the superficial capillary plexus (SCP), deep capillary plexus (DCP), and choriocapillary plexus (CCP) were evaluated using optical coherence tomography angiography (OCTA) at 1 and 6 months postoperatively. The fellow eyes were used as controls for comparison.

MAIN OUTCOME MEASURES

The correlation between OCTA parameters and postoperative best-corrected visual acuity (BCVA) was assessed using a hybrid linear mixed model. We also analyzed the relationship between OCTA parameters and the integrity of the external limiting membrane (ELM) and ellipsoid zone (EZ) of photoreceptors.

RESULTS

The median vessel density in the DCP was significantly decreased in RRD eyes at 1 month (p=0.019), but gradually improved over time to reach the value of the fellow eyes at 6 months (p=1). There was no significant difference between the 2 groups in median foveal avascular zone area, vessel density in the SCP and in the CCP (p=0.579, p=0.618, p=0.068 and p=0.819, p=1 and p=1, respectively) at both 1 and 6 months postoperatively. Postoperative BCVA was positively correlated with the vessel density in the DCP (p=0.009) in multivariate analysis. Eyes with low 1-month vessel density in the DCP tended to have worse final BCVA (p=0.067). There was no correlation between the vessel density in the DCP and ELM and/or EZ integrity at both 1 (p=0.156) and 6 months postoperatively (p=0.161). Eyes with intact ELM and EZ at 6 months, however, had significantly higher 1-month vessel density in the DCP than those with disrupted ELM and/or EZ (p=0.027).

CONCLUSION

The vessel density in the DCP was affected in RRD eyes as well as associated with postoperative BCVA and the restoration of photoreceptor layers. It might thus serve as a predictor for visual recovery following successful RRD surgery.

Macular microcirculation changes after macula-off rhegmatogenous retinal detachment repair with silicone oil tamponade evaluated by OCT-A: preliminary results

Background:

Rhegmatogenous retinal detachment (RRD) with macular involvement is a sight-threatening condition. Silicone oil (SO) is efficacious for retinal tamponade, especially in complex cases. Whether macular detachment per se or the potential tamponading agent may affect macular microcirculation after RRD repair is a matter of research.

Objectives:

To investigate macular microcirculation changes using optical coherence tomography angiography (OCT-A) after pars plana vitrectomy (PPV) with intravitreal SO for RRD repair in the early posttreatment period.

Design:

Prospective comparative cross-sectional study.

Data sources and Methods:

Fourteen eyes of 14 patients were included in the study. All eyes underwent a single successful PPV with SO tamponade for macula-off RRD. OCT-A was performed to analyze macular microcirculation and visual outcomes at 1 month postoperatively. The fellow unaffected eye was used as control.

Results:

Vessel density (VD) in the superficial capillary plexus (SCP) was significantly lower at each macular region (fovea, parafovea, and perifovea) of SO-treated eyes compared with the fellow eyes (all p = 0.001). Similarly, perfusion density (PD) in the SCP was significantly lower at each macular region than the fellow eyes (all p = 0.001). There was enlargement of foveal avascular zone (FAZ) area and decrease of circularity at RRD eyes compared with the fellow ones (all p = 0.001). Postoperative logMAR visual acuity (VA) was significantly lower in treated eyes than fellow eyes and correlated inversely with foveal, parafoveal, and perifoveal VD and PD SCP (all p < 0.001). Postoperative VA had no correlation with FAZ parameters.

Conclusion:

Enlargement of FAZ SCP and decrease in VD and PD SCP during the short-term follow-up were possibly attributable to ischemic changes in the macular area after RRD repair with SO tamponade. In this preliminary study, the flow density in macular capillary plexus may represent an indicator of visual outcomes.

Association of OCT-A characteristics with postoperative visual acuity after rhegmatogenous retinal detachment surgery: a review of the literature

PURPOSE

Emerging evidence has suggested that macular microcirculation and microstructural changes after rhegmatogenous retinal detachment (RRD) successful reattachment surgery are currently evaluated in detail by OCT-Angiography (OCT-A). New imaging technology has revealed the existence of microscopic macular changes, even in cases that retinal morphology appears to be normal in fundus biomicroscopy. The use of OCT-A for the examination of foveal characteristics has attracted significant attention in recent years as the technique offers a potential explanation of the suboptimal recovery of visual acuity and incomplete restoration of the macula despite anatomical repair. However, the available evidence that is needed to establish the OCT-A parameters as predicting factors in clinical practice is both limited and contradictory.

METHODS

A detailed review of the literature was conducted. The association of OCT-A characteristics with postoperative visual acuity after RRD surgery, including vitrectomy with gas tamponade and in some cases scleral buckle, was extensively analyzed.

RESULTS

A comprehensive update on microcirculation and microstructural changes of the macula using OCT-A after RRD repair may indicate potential factors of functional outcomes in clinical practice.

CONCLUSION

A review of the existing literature sheds light on the microvascular changes of the macular capillary plexus that may significantly affect functional outcomes after RRD surgery. The current article discusses important aspects of key publications on the topic, highlights the importance of long-term effectiveness of these possible prognostic factors and proposes the need for further future research.

Assessment of Anatomical and Functional Macular Changes with Optical Coherence Tomography Angiography After Macula-Off Rhegmatogenous Retinal Detachment Repair

Purpose: To evaluate macular capillary plexus changes with OCT-angiography with regard to macular detachment duration in macula-off rhegmatogenous retinal detachment (RRD).Methods: 23 eyes of 23 patients were included in this retrospective case-control study and were divided into 2 groups. Group-A had a macular detachment duration of 0 to 10 days and group-B of 11 to 30 days. All eyes underwent a single successful pars plana vitrectomy (PPV) and gas tamponade. We analysed the OCT-A characteristics and visual outcomes at 12 weeks postoperatively in the 2 groups. The fellow eye was used as control.Results: Vessel density (VD) and flow density (FD) in the superficial capillary plexus (SCP) were significantly lower in group-B (p < .01) than in group-A and control. FAZ SCP in both groups was significantly larger than the control (p < .01), although it did not differ significantly between the two groups (p = 1.000). BCVA was significantly lower in group-B compared to group-A (p < .01). BCVA was positively correlated with VD SCP in group B (p = .015), but not in group-A. Postoperative BCVA had no correlation with FAZ SCP in both groups.Conclusions: The macular detachment duration could be a predicting factor of flow density in macular capillary plexus which in turn might be an indicator of visual outcomes in chronic cases.

Retinal Detachment-Induced Müller Glial Cell Swelling Activates TRPV4 Ion Channels and Triggers Photoreceptor Death at Body Temperature

Using region-specific injection of hyaluronic acid, we developed a mouse model of acute retinal detachment (RD) to investigate molecular mechanisms of photoreceptor cell death triggered by RD. We focused on the transient receptor potential vanilloid 4 (TRPV4) ion channel, which functions as a thermosensor, osmosensor, and/or mechanosensor. After RD, the number of apoptotic photoreceptors was reduced by ∼50% in TRPV4KO mice relative to wild-type mice, indicating the possible involvement of TRPV4 activation in RD-induced photoreceptor cell death. Furthermore, TRPV4 expressed in Müller glial cells can be activated by mechanical stimuli caused by RD-induced swelling of these cells, resulting in release of the cytokine MCP-1, which is reported as a mediator of Müller glia-derived strong mediator for RD-induced photoreceptor death. We also found that the TRPV4 activation by the Müller glial swelling was potentiated by body temperature. Together, our results suggest that RD adversely impacts photoreceptor viability via TRPV4-dependent cytokine release from Müller glial cells and that TRPV4 is part of a novel molecular pathway that could exacerbate the effects of hypoxia on photoreceptor survival after RD.

SIGNIFICANCE STATEMENT Identification of the mechanisms of photoreceptor death in retinal detachment is required for establishment of therapeutic targets for preventing loss of visual acuity. In this study, we found that TRPV4 expressed in Müller glial cells can be activated by mechanical stimuli caused by RD-induced swelling of these cells, resulting in release of the cytokine MCP-1, which is reported as a mediator of Müller glia-derived strong mediator for RD-induced photoreceptor death. We also found that the TRPV4 activation by the Müller glial swelling was potentiated by body temperature. Hence, TRPV4 inhibition could suppress cell death in RD pathological conditions and suggests that TRPV4 in Müller glial cells might be a novel therapeutic target for preventing photoreceptor cell death after RD.

Role of Purines in Müller Glia

Müller glia, the principal macroglia of the retina, express diverse subtypes of adenosine and metabotropic purinergic (P2Y) receptors. Müller cells of several species, including man, also express ionotropic P2X₇ receptors. ATP is liberated from Müller cells after activation of metabotropic glutamate receptors and during osmotic and mechanical induction of membrane stretch; adenosine is released through equilibrative nucleoside transporters. Müller cell-derived purines modulate the neuronal activity and have autocrine effects, for example, induction of glial calcium waves and regulation of the cellular volume. Glial calcium waves induced by neuron-derived ATP mediate functional hyperemia in the retina. Purinergic signaling contributes to the induction of Müller cell gliosis, for example, of cellular proliferation and downregulation of potassium channels, which are important for the homeostatic functions of Müller cells. Purinergic glial calcium waves may also promote the long-range propagation of gliosis and neuronal degeneration across the retinal tissue. The osmotic ATP release is inhibited under pathological conditions. Inhibition of the ATP release may result in osmotic Müller cell swelling and dysregulation of the water transport through the cells; both may contribute to the development of retinal edema. Suppression of the osmotic ATP release and upregulation of the ecto-apyrase (NTPDase1), which facilitate the extracellular degradation of ATP and the formation of adenosine, may protect neurons and photoreceptors from death due to overactivation of P2X receptors. Pharmacological inhibition of P2X₇ receptors and stimulation of adenosine receptors may represent clinical approaches to prevent retinal cell death and dysregulated cell proliferation, and to treat retinal edema.

A novel immunodeficient NOD.SCID-rd1 mouse model of retinitis pigmentosa to investigate potential therapeutics and pathogenesis of retinal degeneration

Retinitis pigmentosa (RP) is a common retinal degeneration disease caused by mutation in any gene of the photo transduction cascade and results in photoreceptor dystrophy. Over decades, several animal models have been used to address the need for the elucidation of effective therapeutics and factors regulating retinal degeneration to prohibit or renew the damaged retina. However, controversies over the immune privilege of retina during cell transplantation and the role of immune modulation during RP still remain largely uninvestigated because of the lack of suitable animal models. Here, we have developed an immunocompromised mouse model, NOD.SCID-rd1, for retinitis pigmentosa (RP) by crossing CBA/J and NOD SCID mice and selecting homozygous double mutant animals for further breeding. Characterization of the newly developed RP model indicates a similar retinal degeneration pattern as CBA/J, with a decreased apoptosis rate and rhodopsin loss. It also exhibits loss of T cells, B cells and NK cells. The NOD.SCID-rd1 model is extremely useful for allogenic and xenogenic cell-based therapeutics, as indicated by the higher cell integration capacity post transplantation. We dissect the underlying role of the immune system in the progression of RP and the effect of immune deficiency on immune privilege of the eye using comparative qPCR studies of this model and the immune-competent RP model.

Summary: NOD.SCID-rd1 is an immune compromised mouse model of retinitis pigmentosa (RP) to investigate cell-based therapeutics for retinal rescue during RP and to study immunological aspects of its pathogenesis and progression.

Elevated vitreous body glial fibrillary acidic protein in retinal diseases

Purpose

Increased expression of glial fibrillary acidic protein (GFAP) is a characteristic of gliotic activation (Müller cells and astrocytes) in the retina. This study assessed vitreous body GFAP levels in various forms of retinal pathology.

Methods

This prospective study included 82 patients who underwent vitrectomy (46 retinal detachments (RDs), 13 macular hole (MHs), 15 epiretinal glioses (EGs), 8 organ donors). An established enzyme–linked immunosorbent assay (ELISA, SMI26) was used for quantification of GFAP.

Results

The highest concentration of vitreous body GFAP in organ donors was 20 pg/mL and it was used as the cutoff. A significant proportion of patients suffering from RD (65 %) to EG (53 %) had vitreous body GFAP levels above this cutoff when compared to organ donors (0 %, p < 0.0001, p = 0.0194, respectively, Fisher’s exact test) and MH (8 %, p < 0.0001, p = 0.0157, respectively). In RD and EG, vitreous body GFAP levels were correlated with axial length (R = 0.69, R = 0.52, p < 0.05 for both).

Conclusions

The data suggest that human vitreous body GFAP is a protein biomarker for glial activation in response to retinal pathologies. Vitreous body GFAP levels may be of interest as a surrogate outcome for experimental treatment strategies in translational studies.

Purinergic signaling in retinal degeneration and regeneration

Purinergic signaling is centrally involved in mediating the degeneration of the injured and diseased retina, the induction of retinal gliosis, and the protection of the retinal tissue from degeneration. Dysregulated calcium signaling triggered by overactivation of P2X7 receptors is a crucial step in the induction of neuronal and microvascular cell death under pathogenic conditions like ischemia-hypoxia, elevated intraocular pressure, and diabetes, respectively. Overactivation of P2X7 plays also a pathogenic role in inherited and age-related photoreceptor cell death and in the age-related dysfunction and degeneration of the retinal pigment epithelium. Gliosis of micro- and macroglial cells, which is induced and/or modulated by purinergic signaling and associated with an impaired homeostatic support to neurons, and the ATP-mediated propagation of retinal gliosis from a focal injury into the surrounding noninjured tissue are involved in inducing secondary cell death in the retina. On the other hand, alterations in the glial metabolism of extracellular nucleotides, resulting in a decreased level of ATP and an increased level of adenosine, may be neuroprotective in the diseased retina. Purinergic signals stimulate the proliferation of retinal glial cells which contributes to glial scarring which has protective effects on retinal degeneration and adverse effects on retinal regeneration. Pharmacological modulation of purinergic receptors, e.g., inhibition of P2X and activation of adenosine receptors, may have clinical importance for the prevention of photoreceptor, neuronal, and microvascular cell death in diabetic retinopathy, retinitis pigmentosa, age-related macular degeneration, and glaucoma, respectively, for the clearance of retinal edema, and the inhibition of dysregulated cell proliferation in proliferative retinopathies. This article is part of a Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

The effect of melatonin on retinal ganglion cell survival in ischemic retina

Our objective was to determine whether melatonin increases retinal ganglion cell (RGC) survival in ischemic mouse retina. Transient retinal ischemia was induced by an acute elevation of intraocular pressure in C57BL/6 mice. To evaluate the effect of melatonin on retinal ischemia, an equal amount of either melatonin or vehicle was intraperitoneally injected into the mice 1 hour before ischemia, at the time of ischemia, and 1 hour after ischemia. Hypoxia inducible factor 1α (HIF-1α) and glial fibrillary acidic protein (GFAP) expression were assessed 6, 12, and 24 hours after ischemia-reperfusion by Western blot. RGC survival was measured 2 weeks after ischemia-reperfusion. The expression of HIF-1α and GFAP peaked 24 hours after ischemia-reperfusion in ischemic retina. The treatment of ischemic retina with melatonin resulted in the inhibition of increased expression of HIF-1α and GFAP. RGC survival was greater in retinas treated with melatonin than in retinas treated with vehicle 2 weeks after ischemia-reperfusion. On the basis of our results, we suggest that melatonin treatment increased RGC survival in ischemic mouse retina. The neuroprotective effect of melatonin is mediated by the inhibition of HIF-1α stabilization and reduced activity of glial cells in ischemic mouse retina.

Neuronal and glial alterations in complex long-term rhegmatogenous retinal detachment

PURPOSE

To explore neuronal and glial alterations in eyes with complex long-term rhegmatogenous retinal detachment (RRD).

METHODS

Morphological analysis was performed on eight retinal specimens derived from patients treated with peripheral retinectomy for RRD complicated by retinal shortening or retinal thinning. All eyes had undergone previous surgeries including silicone oil tamponade, and had a median total detachment time of 2.5 months (range 2-12). Specimens were examined with hematoxylin and eosin staining and immunohistochemistry directed against activated Müller cells, ganglion cells, rod bipolar cells, and photoreceptors.

RESULTS

Retinal specimens displayed severe loss of photoreceptor and rod bipolar cells. Remaining neuronal cells exhibited disorganized perikarya and neurites with disruption of the normal retinal lamination. Müller cell activation was evident in all specimens with subretinal and epiretinal hypertrophy present in tissue derived from shortened retinal detachments.

CONCLUSION

Long-term RRD leads to retinal remodeling characterized by loss of first and second order retinal neurons, disruption of the entire retinal lamination and gliosis. The severity of histopathological changes indicates that anatomical as well as functional recovery of the involved retina is precarious. The findings may be important when devising surgical strategies to avoid permanent retinal detachment.

An Assessment of the Ocular Safety of Excipient Maleic Acid Following Intravitreal Injection in Rabbits

Maleic acid was formulated in 0.7% saline and injected intravitreally in rabbits in order to evaluate ocular safety and tolerability. Maleic acid was formulated within a narrow pH range (2–3), administered in a fixed volume (100 µl), and concentrations ranged from 0.00 to 2.00 mg/eye (0.00 to 12.30 mM vitreous). Ocular evaluations were conducted at 2, 4, and 8 days post injection. Ocular irritation responses were observed at doses from 0.50 mg/eye (3.07 mM vitreous) to 2.00 mg/eye (12.30 mM vitreous) and included conjunctival redness and scleral swelling. Chemosis was observed at 2.00 mg/eye (12.30 mM vitreous). Funduscopic evaluations revealed enlarged retinal blood vessels and optic disk swelling at doses ≥1.50 mg/eye (9.22 mM vitreous), retinal folds and retinal discoloration at 2.00 mg/eye (12.30 mM vitreous). Histopathologic evaluations on days 4 and 8 post injection revealed retinal degeneration at doses ≥1.0 mg/eye (6.15 mM vitreous), conjunctival inflammation at doses ≥1.5 mg/eye (9.22 mM vitreous), and retinal pigment epithelial hypertrophy, optic nerve demyelination, anterior chamber fluid, and conjunctival fibrosis at 2.00 mg/eye (12.30 mM vitreous) maleic acid. The data suggest that maleic acid formulations at ≥1.00 mg/eye (6.15 mM vitreous) were not suitable for intraocular indications.

Anatomical and functional macular changes after rhegmatogenous retinal detachment with macula off

PURPOSE

To evaluate the correlation between morphologic changes in the outer retina and visual function after successful repair of rhegmatogenous retinal detachment with macula off.

DESIGN

Observational case series.

METHODS

SETTINGS

Dijon University Hospital.

PATIENTS

Thirty patients (30 eyes) with successful repair of rhegmatogenous retinal detachment after macula off and a minimum 6-month follow-up after surgery.

MAIN OUTCOME MEASURES

Spectral-domain optical coherence tomography (SD-OCT) of the outer retina, fundus autofluorescence (FAF), and microperimetry.

RESULTS

Twenty of 30 eyes presented microstructural changes within the photoreceptor layer (66.7%). Of these, half of the patients (50%) had more than 1 lesion. Disrupted inner segment/outer segment (IS/OS) junction was noted in 16 out of 30 eyes (53.3%), irregular hyporeflectivity in the photoreceptor outer segments (PROS) was observed in 17 eyes (56.7%), external limiting membrane was discontinued in 10 eyes (33.3%), and hyperreflective spots in the outer nuclear layer were observed in 5 eyes (16.7%). FAF changes were detected in only 5 eyes (16.7%). Abnormalities in the IS/OS junction were significantly associated with lower foveal and macular sensitivity, thinner PROS, and global photoreceptor changes (P = .014, P = .003, P = .006, P < .0001, respectively). Patients with a normal foveal profile showed similar findings.

CONCLUSIONS

SD-OCT and microperimetry seem to be appropriate tools to determine the visual and the anatomic recovery of the macula after surgery.

The role of purinergic receptors in retinal function and disease

Extracellular ATP acts as a neurotransmitter in the central and peripheral nervous systems. In this review, the role of purinergic receptors in neuronal signaling and bi-directional glial-neuronal communication in the retina will be considered. There is growing evidence that a range of P2X and P2Y receptors are expressed on most classes of retinal neurons and that activation of P2 receptors modulates retinal function. Furthermore, neuronal control of glial function is achieved through neuronal release of ATP and activation of P2Y receptors expressed by Müller cells. Altered purinergic signaling in Müller cells has been implicated in gliotic changes in the diseased retina and furthermore, elevations in extracellular ATP may lead to apoptosis of retinal neurons.

Reaction of Müller cells in an experimental rat model of increased intraocular pressure following timolol, latanoprost and brimonidine

The aim of this study was to evaluate the reaction of Müller cells in an experimental rat model of intraocular pressure (IOP) and their response to treatment with ocular hypotensive drugs. Episcleral vein cauterization in unilateral eyes of Wistar rats was performed to produce elevated IOP. The animals were divided into five groups: control, experimental, and experimental treated with timolol, latanoprost or brimonidine. Histological sections of retina were studied by immunochemistry with antibodies to glial fibrillary acidic protein (GFAP), and the percentage of labeled area was measured to evaluate the degree of reactive gliosis. In the experimental group, the Müller cells showed hypertrophy and a significant increase in GFAP (4.39+/-0.32%) in relation to retinas of the control group (2.05+/-0.14%). Gliosis was detected in all three treated groups, with a varying increase in GFAP intensity. The timolol-treated group showed the most intense and persistent glial reactivity after 3 months of treatment (13.89+/-0.63%). Treatment with brimonidine, however, resulted in a decrease in the level of GFAP immunoreactivity (8.37+/-0.4%). The group treated with latanoprost showed the lowest glial reactivity (4.8+/-0.36%). Given that all three drugs are effective hypotensive agents, their neuroprotective effect could be related with other factors, such as gliosis, which, over long periods may have noxious effects on the neurons. Thus, hypotensives like brimonidine, and specially latanoprost, may afford greater neuroprotection to the ganglion cells by attenuating the retinal glial reaction.

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

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

Synaptic pathology in retinoschisis knockout (Rs1-/y) mouse retina and modification by rAAV-Rs1 gene delivery

PURPOSE

At an early age, the retinoschisin knockout (Rs1-KO) mouse retina has progressive photoreceptor degeneration with severe disruption of the outer plexiform layer (OPL) that decreases at older ages. The electroretinogram (ERG) undergoes parallel changes. The b-wave amplitude from bipolar cells is reduced disproportionately to the photoreceptor a-wave at young but not at older ages. The protein expression and morphology of the OPL in Rs1-KO mice was investigated at different ages, to explore the role of the synaptic layer in these ERG changes.

METHODS

Retinas of wild-type (Wt) and Rs1-KO mice from postnatal day (P)7 to 12 months were evaluated by light and electron microscopy (EM) and biochemistry. PSD95 (postsynaptic density protein), mGluR6 (metabotropic glutamate receptor subtype 6), retinoschisin (Rs1), the Müller cell proteins glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS), the bipolar cell marker protein kinase C alpha (PKCalpha), and the horizontal cell marker calbindin were localized by immunofluorescence and immuno-EM. Levels of PSD95 and mGluR6 were determined by quantitative Western blot. Rs1-KO mice treated by intravitreous injection of rAAV(2/2)-CMV-Rs1 in one eye at P14 were evaluated at 8 months by full-field scotopic ERG responses and retinal immunohistochemistry.

RESULTS

Rs1 was associated with the outer surface of synaptic membranes in wild-type (Wt) retinas. PSD95 and mGluR6 were juxtaposed in the OPL of the Rs1-KO retinas by P14, implying that synaptic structures are formed. Light microscopic retinal morphology was similar in Wt and Rs1-KO at P14, but by P21, the OPL was disrupted in Rs1-KO, and some PSD95 and mGluR6 was mislocalized in the outer nuclear layer (ONL). GFAP expression spanned all retinal layers. EM showed synaptic structures adjacent to photoreceptor nuclei. PSD95 and mGluR6 levels were normal at 1 month on Western blot but declined to 59% (P < 0.001) and 55% (P < 0.05) of Wt, respectively, by 4 months. Levels thereafter showed no further reduction out to 12 months. Eyes injected with AAV-Rs1 were studied at 8 months by immunohistochemistry and had higher expression of PSD95 and mGluR6 and less GFAP expression compared with fellow untreated eyes.

CONCLUSIONS

In the Rs1-KO mouse, retinal layer formation and synaptic protein expression in the OPL is normal up to P14, implying normal development of synaptic connections. Aberrant localization of synaptic proteins by P21 indicates that displacement of developing and/or mature synapses contributes to the b-wave reduction at young ages, when photoreceptor numbers and synaptic protein levels are normal. The subsequent decline in PSD95 and mGluR6 between 1 and 12 months in Rs1-KO retina mirrors the course of b-wave change and provides evidence of causal relationship between the ERG and OPL changes. These findings and the improved structural integrity of the OPL and b-wave amplitude after Rs1 gene transfer therapy provide a cellular and molecular basis for interpreting the changes in retinal signaling in this model.

The role of alkylphosphocholines in retinal Müller glial cell proliferation

PURPOSE

Alkylphosphocholines (APCs) are investigated for their effect on Müller glial cell proliferation and F-actin stress fiber distribution in vitro.

MATERIALS AND METHODS

Müller cells were incubated with APCs (C18:1-PC and C22:1-PC) +/- fetal calf serum. Proliferation was assessed by BrdU labeling and with the tetrazolium dye-reduction assay. Toxicity was measured using the trypan blue exclusion assay. The distribution of F-actin stress fibers was determined using FITC-phalloidin staining.

RESULTS

APCs are effective inhibitors of human and rat Müller glial cell proliferation and hypoxia-induced up-regulation of F-actin stress fibers in vitro in non-toxic concentrations.

CONCLUSIONS

APCs might prevent intraretinal changes as a result of serum stimulation and hypoxia following retinal detachment.

Long-term cellular and regional specificity of the photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina

This study investigated the anatomical consequences of a photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina. Retinae were examined 2 weeks, 1, 3, and 6 months after systemic IAA injection. The retinae were processed using standard histological methods to assess the gross morphology and topographical distribution of damage, and by immunohistochemistry to examine specific cell populations in the retina. Degeneration was restricted to the photoreceptors and was most common in the ventral retina and visual streak. In damaged regions, the outer nuclear layer was reduced in thickness or eliminated entirely, with a concomitant loss of immunoreactivity for rhodopsin. However, the magnitude of the effect varied between animals with the same IAA dose and survival time, suggesting individual differences in the bioavailability of the toxin. In all eyes, the inner retina remained intact, as judged by the thickness of the inner nuclear layer, and by the pattern of immunoreactivity for protein kinase C-alpha (rod bipolar cells) and calbindin D-28 (horizontal cells). Müller cell stalks became immunoreactive for glial fibrillary acidic protein (GFAP) even in IAA-treated retinae that had no signs of cell loss, indicating a response of the retina to the toxin. However, no marked hypertrophy or proliferation of Müller cells was observed with either GFAP or vimentin immunohistochemistry. Thus the selective, long lasting damage to the photoreceptors produced by this toxin did not lead to a reorganization of the surviving cells, at least with survival as long as 6 months, in contrast to the remodeling of the inner retina that is observed in inherited retinal degenerations such as retinitis pigmentosa and retinal injuries such as retinal detachment.

Complex rectification of Müller cell Kir currents

Although Kir4.1 channels are the major inwardly rectifying channels in glial cells and are widely accepted to support K+- and glutamate-uptake in the nervous system, the properties of Kir4.1 channels during vital changes of K+ and polyamines remain poorly understood. Therefore, the present study examined the voltage-dependence of K+ conductance with varying physiological and pathophysiological external [K+] and intrapipette spermine ([SP]) concentrations in Müller glial cells and in tsA201 cells expressing recombinant Kir4.1 channels. Two different types of [SP] block were characterized: "fast" and "slow." Fast block was steeply voltage-dependent, with only a low sensitivity to spermine and strong dependence on extracellular potassium concentration, [K+]o. Slow block had a strong voltage sensitivity that begins closer to resting membrane potential and was essentially [K+]o-independent, but with a higher spermine- and [K+]i-sensitivity. Using a modified Woodhull model and fitting i/V curves from whole cell recordings, we have calculated free [SP](in) in Müller glial cells as 0.81 +/- 0.24 mM. This is much higher than has been estimated previously in neurons. Biphasic block properties underlie a significantly varying extent of rectification with [K+] and [SP]. While confirming similar properties of glial Kir and recombinant Kir4.1, the results also suggest mechanisms underlying K+ buffering in glial cells: When [K+]o is rapidly increased, as would occur during neuronal excitation, "fast block" would be relieved, promoting potassium influx to glial cells. Increase in [K+]in would then lead to relief of "slow block," further promoting K+-influx.

[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.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.