P2Y(2) receptor agonist INS37217 enhances functional recovery after detachment caused by subretinal injection in normal and rds mice

Downregulation of rhodopsin is an effective therapeutic strategy in ameliorating peripherin-2-associated inherited retinal disorders

Given the absence of approved treatments for pathogenic variants in Peripherin-2 (PRPH2), it is imperative to identify a universally effective therapeutic target for PRPH2 pathogenic variants. To test the hypothesis that formation of the elongated discs in presence of PRPH2 pathogenic variants is due to the presence of the full complement of rhodopsin in absence of the required amounts of functional PRPH2. Here we demonstrate the therapeutic potential of reducing rhodopsin levels in ameliorating disease phenotype in knockin models for p.Lys154del (c.458-460del) and p.Tyr141Cys (c.422 A > G) in PRPH2. Reducing rhodopsin levels improves physiological function, mitigates the severity of disc abnormalities, and decreases retinal gliosis. Additionally, intravitreal injections of a rhodopsin-specific antisense oligonucleotide successfully enhance the physiological function of photoreceptors and improves the ultrastructure of discs in mutant mice. Presented findings shows that reducing rhodopsin levels is an effective therapeutic strategy for the treatment of inherited retinal degeneration associated with PRPH2 pathogenic variants.

Experimental models and examination methods of retinal detachment

Retinal detachment refers to the separation of the retinal neuroepithelium and pigment epithelium, usually involving the death of photoreceptor cells. Severe detachment may lead to permanent visual impairment if not treated properly and promptly. According to the underlying causes, retinal detachment falls into one of three categories: exudative retinal detachment, traction detachment, and rhegmatogenous retinal detachment. Like many other diseases, it is difficult to study the pathophysiology of retinal detachment directly in humans, because the human retinal tissues are precious, scarce and non-regenerative; thus, establishing experimental models that better mimic the disease is necessary. In this review, we summarize the existing models of the three categories of retinal detachment both in vivo and in vitro, along with an overview of their examination methods and the major strengths and weaknesses of each model.

Microglia inhibit photoreceptor cell death and regulate immune cell infiltration in response to retinal detachment

Photoreceptor cell death resulting from retinal detachment (RD) causes significant visual loss. While the immune system is activated during RD, its role is still unclear. Microglia are resident immune cells in the retina and are thought to be either protective or deleterious in response to neuronal injury, suggesting context-dependent effects. Here, we demonstrate that microglia limit retinal damage during acute injury, since microglial ablation led to increased photoreceptor death. Microglial morphological–activation changes triggered their migration into injured tissue where they formed intimate connections with infiltrating immune cells and phagocytized injured photoreceptors. These findings provide insight into the microglial response and function during RD, indicating microglia promote photoreceptor survival during acute phase injury by removing potentially damaging cell debris.

Retinal detachment (RD) is a sight-threatening complication common in many highly prevalent retinal disorders. RD rapidly leads to photoreceptor cell death beginning within 12 h following detachment. In patients with sustained RD, progressive visual decline due to photoreceptor cell death is common, leading to significant and permanent loss of vision. Microglia are the resident immune cells of the central nervous system, including the retina, and function in the homeostatic maintenance of the neuro-retinal microenvironment. It is known that microglia become activated and change their morphology in retinal diseases. However, the function of activated microglia in RD is incompletely understood, in part because of the lack of microglia-specific markers. Here, using the newly identified microglia marker P2ry12 and microglial depletion strategies, we demonstrate that retinal microglia are rapidly activated in response to RD and migrate into the injured area within 24 h post-RD, where they closely associate with infiltrating macrophages, a population distinct from microglia. Once in the injured photoreceptor layer, activated microglia can be observed to contain autofluorescence within their cell bodies, suggesting they function to phagocytose injured or dying photoreceptors. Depletion of retinal microglia results in increased disease severity and inhibition of macrophage infiltration, suggesting that microglia are involved in regulating neuroinflammation in the retina. Our work identifies that microglia mediate photoreceptor survival in RD and suggests that this effect may be due to microglial regulation of immune cells and photoreceptor phagocytosis.

Increased Ap4A levels and ecto-nucleotidase activity in glaucomatous mice retina

The pathogenesis of glaucoma involves numerous intracellular mechanisms including the purinergic system contribution. Furthermore, the presence and release of nucleotides and dinucleotides during the glaucomatous damage and the maintenance of degradation machinery through ecto-nucleotidase activity are participating in the modulation of the suitable extracellular complex balance. The aim of this study was to investigate the levels of diadenosine tetraphosphate (Ap₄A) and the pattern of ecto-nucleotidase activity expression in glaucomatous retinas during the progress the pathology. Ap₄A levels were analyzed by HPLC in glaucomatous retinas from the DBA/2J mice at 3, 9, 15, and 23 months of age. For that, retinas were dissected as flattened whole-mounts and stimulated in Ringer buffer with or without 59 mM KCl. NPP1 expression was analyzed by RT-PCR and western blot and its distribution was assessed by immunohistochemistry studies examined under confocal microscopy. Glaucomatous mice exhibited Ap₄A values, which changed in stimulated retinas as long as the pathology progressed varying from 0.73 ± 0.04 (3 months) to 0.170 ± 0.05 pmol/mg retina (23 months). Concomitantly, NPP1 expression was significantly increased (82.15%) in the DBA/2J mice at 15 months. Furthermore, immunohistochemical studies showed that NPP1 labeling was stronger in OPL and IPL labeling tangentially in the vitreal part of the retina and was upregulated at 15 months of age. Our findings demonstrate that Ap₄A decreased levels may be related with exacerbated activity of NPP1 protein in glaucomatous degeneration and in this way contributing to elucidate different mechanisms involved in retinal impairment in glaucomatous degeneration.

Subretinal delivery of erythropoietin alleviates the N-methyl-N-nitrosourea-induced photoreceptor degeneration and visual functional impairments: an in vivo and ex vivo study

Retinitis pigmentosa (RP) is a heterogeneous group hereditary retinal disease that is characterized by photoreceptor degeneration. The present study sought to explore the therapeutic effects of erythropoietin (EPO) on the N-methyl-N-nitrosourea (MNU)-induced photoreceptor degeneration. The MNU-administered mouse or normal control received a subretinal injection of EPO (at the dose of 10U). Twenty-four hours after EPO injection, the retinal EPO levels of experimental animals were quantified. Subsequently, the experimental animals were subjected to optokinetic tests, ERG examination, SD-OCT examination, histology assessment, and immunohistochemistry evaluation. The retinal superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, and expression levels of several apoptotic factors were also quantified. The subretinal injection of EPO up-regulated the retinal EPO level in the retinas of MNU-administered mice. The optokinetic tests and ERG examination suggested the visual functional impairments in MNU-administered mice were ameliorated after EPO treatment. The SD-OCT and histological examination suggested the morphological devastations in MNU-administered mice were alleviated after EPO treatment. The cone photoreceptors in MNU-administered mice were protected from the MNU-induced detrimental effects. Moreover, the EPO treatment rectified the apoptotic abnormalities in MNU-administered mice, and enhanced the expression level of Foxo3, a critical mediator of autophagy. The EPO treatment also mitigated the MDA concentration and enhanced the retinal SOD activity, thereby counteracting the retinal oxidative stress in MNU administered mice. In ophthalmological practice, the subretinal delivery of EPO is a feasible therapeutic strategy to alleviate photoreceptor degeneration. These findings would enrich our pharmacological knowledge about EPO and shed light on the development of an effective therapy against RP.

Understanding the Presence and Roles of Ap4A (Diadenosine Tetraphosphate) in the Eye

Diadenosine tetraphosphate abbreviated Ap₄A is a naturally occurring dinucleotide, which is present in most of the ocular fluids. Due to its intrinsic resistance to enzyme degradation compared to mononucleotides, this molecule can exhibit profound actions on ocular tissues, including the ocular surface, ciliary body, trabecular meshwork, and probably the retina. The actions of Ap₄A are mostly carried out by P2Y₂ receptors, but the participation of P2X2 and P2Y₆ in processes such as the regulation of intraocular pressure (IOP), together with the P2Y₂, is pivotal. Beyond the physiological role, this dinucleotide can present on the ocular surface keeping a right production of tear secretion or regulating IOP. It is important to note that exogenous application of Ap₄A to cells or animal models can significantly modify pathophysiological conditions and thus is an attractive therapeutic molecule. The ocular location where Ap₄A actions have not been fully elucidated is in the retina. Although some analogues show interesting actions on pathological situations such as retinal detachment, little is known about the real effect of this dinucleotide, this being one of the challenges that require pursuing in the near future.

The role of dinucleoside polyphosphates on the ocular surface and other eye structures

Dinucleoside polyphosphates comprises a group of dinucleotides formed by two nucleosides linked by a variable number of phosphates, abbreviated NpnN (where n represents the number of phosphates). These compounds are naturally occurring substances present in tears, aqueous humour and in the retina. As the consequence of their presence, these dinucleotides contribute to many ocular physiological processes. On the ocular surface, dinucleoside polyphosphates can stimulate tear secretion, mucin release from goblet cells and they help epithelial wound healing by accelerating cell migration rate. These dinucleotides can also stimulate the presence of proteins known to protect the ocular surface against microorganisms, such as lysozyme and lactoferrin. One of the latest discoveries is the ability of some dinucleotides to facilitate the paracellular way on the cornea, therefore allowing the delivery of compounds, such as antiglaucomatous ones, more easily within the eye. The compound Ap₄A has been described being abnormally elevated in patient's tears suffering of dry eye, Sjogren syndrome, congenital aniridia, or after refractive surgery, suggesting this molecule as biomarker for dry eye condition. At the intraocular level, some diadenosine polyphosphates are abnormally elevated in glaucoma patients, and this can be related to the stimulation of a P2Y₂ receptor that increases the chloride efflux and water movement in the ciliary epithelium. In the retina, the dinucleotide dCp₄U, has been proven to be useful to help in the recovery of retinal detachments. Altogether, dinucleoside polyphosphates are a group of compounds which present relevant physiological actions but which also can perform promising therapeutic benefits.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Trans-Corneal Subretinal Injection in Mice and Its Effect on the Function and Morphology of the Retina

Purpose

To introduce a practical method of subretinal injection in mice and evaluate injection-induced retinal detachment (RD) and damage using a dynamic imaging system, electrophysiology, and histology.

Methods

After full dilation of a 2-month-old C57BL/6J mouse pupil, the cornea near the limbus was punctured with a 30 ½-gague disposable beveled needle. A 33 ½-gauge blunt needle was inserted through the corneal perforation into the anterior chamber, avoiding the lens before going deeper into the vitreous cavity, and penetrating the inner retina to reach the subretinal space. The mice were divided into four groups: in group 1, about 80–100% of the retina was filled with subretinally injected solution; in group 2, approximately 50–70% of the retina was filled with injected solution; in group 3, the procedures were stopped before solution injection; and non-injected eyes were used as the negative control in group 4. An optical coherence tomography (OCT) imaging system was used to monitor retinal reattachment during the first three days following the injections. Histological and functional changes were examined by light microscopy and electroretinography (ERG) at five weeks post-injection.

Results

After a short-term training, a 70% success rate with 50% or more coverage (i.e., retinal blebs occupied 50% or more retinal area and filled with the injected solution) with minimal injection-related damages can be achieved. Bleb formation was associated with retinal detachment (RD) between the neuroretina and the retinal pigment epithelium (RPE) layer. Partial RD could be observed at post-injection day 1, and by day 2 most of the retina had reattached. At 5 weeks post-injection, compared to uninjected control group 4, the b-wave amplitudes of ERG decreased 22% in group 1, 16% in group 2, and 7% in group 3; the b-wave amplitudes were statistically different between the uninjected group and the groups with either 50–70% or 80–100% coverage. The subretinal injection-induced RD reattached and became stable at five weeks post-injection, although some photoreceptor damage could still be observed in and around the injection sites, especially in 80–100% coverage group.

Conclusions

Trans-corneal subretinal injection is effective and practical, although subretinal injection-related damages can cause some morphological and functional loss.

Fibulin 2, a tyrosine O-sulfated protein, is up-regulated following retinal detachment

Retinal detachment is the physical separation of the retina from the retinal pigment epithelium. It occurs during aging, trauma, or during a variety of retinal disorders such as age-related macular degeneration, diabetic retinopathy, retinopathy of prematurity, or as a complication following cataract surgery. This report investigates the role of fibulin 2, an extracellular component, in retinal detachment. A major mechanism for detachment resolution is enhancement of cellular adhesion between the retina and the retinal pigment epithelium and prevention of its cellular migration. This report shows that fibulin 2 is mainly present in the retinal pigment epithelium, Bruch membrane, choriocapillary, and to a lesser degree in the retina. In vitro studies revealed the presence of two isoforms for fibulin 2. The small isoform is located inside the cell, and the large isoform is present inside and outside the cells. Furthermore, fibulin 2 is post-translationally modified by tyrosine sulfation, and the sulfated isoform is present outside the cell, whereas the unsulfated pool is internally located. Interestingly, sulfated fibulin 2 significantly reduced the rate of cellular growth and migration. Finally, levels of fibulin 2 dramatically increased in the retinal pigment epithelium following retinal detachment, suggesting a direct role for fibulin 2 in the re-attachment of the retina to the retinal pigment epithelium. Understanding the role of fibulin 2 in enhancing retinal attachment is likely to help improve the current therapies or allow the development of new strategies for the treatment of this sight-threatening condition.

Necrostatin-1 protects photoreceptors from cell death and improves functional outcome after experimental retinal detachment

Necroptosis is a recently discovered programmed necrosis. Evidence demonstrated the importance of necroptosis in neuronal cell death. Necrostatin-1 is a specific inhibitor of necroptosis. In this study, we investigated the role of necrostatin-1 on photoreceptor survival and functional protection after experimental retinal detachment (RD) in rats. Necrostatin-1/inactive analogue of necrostatin-1 was introduced into the subretinal space at RD induction and 6 hours afterward, respectively. We found that necrostatin-1 attenuated retinal histopathological damage and reduced plasma membrane breakdown (a morphological hallmark of necroptosis) in outer retinal layers. Transmission electron microscopy showed that necrostatin-1 directly protected neurons by inhibiting necroptotic, not apoptotic, cell death. Treatment with necrostatin-1 inhibited the induction of receptor-interacting protein kinase phosphorylation after RD (a biomarker of necroptosis). Finally, electroretinographic recording proved that necrostatin-1 contributed to objective functional improvement after RD. These findings indicate that necrostatin-1 is a promising therapeutic agent that protects photoreceptors from necroptosis and improves functional outcome.

Intravitreal injection of soluble erythropoietin receptor exacerbates photoreceptor cell apoptosis in a rat model of retinal detachment

PURPOSE

To evaluate the effects of intravitreal injection of soluble erythropoietin (EPO) receptor (sEPOR) on photoreceptor cell apoptosis in an animal model of retinal detachment (RD).

METHODS

Various dosages of sEPOR (2, 20, or 200 ng) were injected into the vitreous cavities of normal rats. Three days after injection, retinal function was measured by flash electroretinography (ERG). On day 7, histopathology and retinal morphology were examined by light and transmission electron microscopy (TEM), respectively. Rat models of RD were successfully established by injection of 1.4% sodium hyaluronate into the subretinal space, followed by immediate injection of phosphate-buffered saline (PBS) or sEPOR into the vitreous cavity. On day 3, photoreceptor cell apoptosis was evaluated using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and caspase-3 activity assayed by Western blotting and immunofluorescence. Light microscopic examination of retinal histopathology was used to determine the thickness of the outer nuclear layer (ONL) 14 days after establishment of RD.

RESULTS

There were no significant differences in the latency and amplitude of maximal a, b and oscillatory potential (OP) wave responses by flash ERG before or 3 days after sEPOR injection (p > 0.05). Retinal tissues showed no obvious pathological changes by either light or transmission electron microscopy. Both Western blotting and immunofluorescence indicated consistent sEPOR enhanced caspase-3 activation aggravated apoptosis of photoreceptor cells in RD rat retinas. On day 14, RD ONLs were thinner, according to increasing dosages of sEPOR.

CONCLUSION

Intravitreal injection of sEPOR exacerbates photoreceptor cell apoptosis in RD models via activation of caspase-3.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.

Dextran and protamine-based solid lipid nanoparticles as potential vectors for the treatment of X-linked juvenile retinoschisis

The goal of the present study was to analyze the potential application of nonviral vectors based on solid lipid nanoparticles (SLN) for the treatment of ocular diseases by gene therapy, specifically X-linked juvenile retinoschisis (XLRS). Vectors were prepared with SLN, dextran, protamine, and a plasmid (pCMS-EGFP or pCEP4-RS1). Formulations were characterized and the in vitro transfection capacity as well as the cellular uptake and the intracellular trafficking were studied in ARPE-19 cells. Formulations were also tested in vivo in Wistar rat eyes, and the efficacy was studied by monitoring the expression of enhanced green fluorescent protein (EGFP) after intravitreal, subretinal, and topical administration. The presence of dextran and protamine in the SLN improved greatly the expression of retinoschisin and EGFP in ARPE-19 cells. The nuclear localization signals of protamine, its ability to protect the DNA, and a shift in the entry mechanism from caveola-mediated to clathrin-mediated endocytosis promoted by the dextran, justify the increase in transfection. After ocular administration of the dextran-protamine-DNA-SLN complex to rat eyes, we detected the expression of EGFP in various types of cells depending on the administration route. Our vectors were also able to transfect corneal cells after topical application. We have demonstrated the potential usefulness of our nonviral vectors loaded with XLRS1 plasmid and provided evidence for their potential application for the management or treatment of degenerative retinal disorders as well as ocular surface diseases.

Direct gene transfer with compacted DNA nanoparticles in retinal pigment epithelial cells: expression, repeat delivery and lack of toxicity

AIM

To evaluate the safety of compacted DNA nanoparticles (NPs) in retinal pigment epithelial (RPE) cells.

MATERIALS & METHODS

Enhanced GFP expression cassettes controlled by the RPE-specific vitelloform macular dystrophy promoter were constructed with and without a bacterial backbone and compacted into NPs formulated with polyethylene glycol-substituted lysine 30-mers. Single or double subretinal injections were administered in adult BALB/c mice. Expression levels of enhanced GFP, proinflammatory cytokines and neutrophil/macrophage mediators, and retinal function by electroretinogram were evaluated at different time-points postinjection.

RESULTS

Immunohistochemistry and real-time PCR demonstrated that NPs specifically transfect RPE cells at a higher efficiency than naked DNA and similar results were observed after the second injection. At 6 h postinjections, a transient inflammatory response was observed in all cohorts, including saline, indicating an adverse effect to the injection procedure. Subsequently, no inflammation was detected in all experimental groups.

CONCLUSION

This study demonstrates the safety and efficacy of NP-mediated RPE gene transfer therapy following multiple subretinal administrations.

Retinoid receptors trigger neuritogenesis in retinal degenerations

Anomalous neuritogenesis is a hallmark of neurodegenerative disorders, including retinal degenerations, epilepsy, and Alzheimer's disease. The neuritogenesis processes result in a partial reinnervation, new circuitry, and functional changes within the deafferented retina and brain regions. Using the light-induced retinal degeneration (LIRD) mouse model, which provides a unique platform for exploring the mechanisms underlying neuritogenesis, we found that retinoid X receptors (RXRs) control neuritogenesis. LIRD rapidly triggered retinal neuron neuritogenesis and up-regulated several key elements of retinoic acid (RA) signaling, including retinoid X receptors (RXRs). Exogenous RA initiated neuritogenesis in normal adult retinas and primary retinal cultures and exacerbated it in LIRD retinas. However, LIRD-induced neuritogenesis was partly attenuated in retinol dehydrogenase knockout (Rdh12(-/-)) mice and by aldehyde dehydrogenase inhibitors. We further found that LIRD rapidly increased the expression of glutamate receptor 2 and β Ca(2+)/calmodulin-dependent protein kinase II (βCaMKII). Pulldown assays demonstrated interaction between βCaMKII and RXRs, suggesting that CaMKII pathway regulates the activities of RXRs. RXR antagonists completely prevented and RXR agonists were more effective than RA in inducing neuritogenesis. Thus, RXRs are in the final common path and may be therapeutic targets to attenuate retinal remodeling and facilitate global intervention methods in blinding diseases and other neurodegenerative disorders.

DPOFA, a Cl⁻/HCO₃⁻ exchanger antagonist, stimulates fluid absorption across basolateral surface of the retinal pigment epithelium

Background

Retinal detachment is a disorder of the eye in which sensory retina separates from the retinal pigment epithelium (RPE) due to accumulation of fluid in subretinal space. Pharmacological stimulation of fluid reabsorption from subretinal space to choroid across the RPE has been suggested as a treatment strategy for retinal detachment. DPOFA, (R)-(+)-(5,6-dichloro 2,3,9,9a-tetrahydro 3-oxo-9a-propyl-1H-fluoren-7-yl)oxy]acetic acid, is an abandoned drug capable of inhibiting Cl^-/HCO₃^- exchanger activity. We hypothesized that DPOFA may increase fluid absorption across basolateral surface of the RPE.

Methods

Reverse transcription polymerase chain reaction (RT-PCR) analysis of mRNA for six different transporters that may act as Cl^-/HCO₃^- exchangers was conducted in bovine and human RPE to confirm that RPE from two species expresses the same repertoire of Cl^-/HCO₃^- exchanger isoforms. The degree of amino acid homology between orthologous human and bovine RPE-specific isoforms was calculated after performing protein alignments. Transport of fluid across bovine RPE-choroid explants mounted in the Ussing chamber was used to assess the ability of DPOFA to modulate fluid absorption across the RPE.

Results

Using RT-PCR we showed that three isoforms (SLC4A2, SLC4A3, and SLC26A6) are strongly expressed in human and bovine RPE preparations. Amino acid comparisons conducted for RPE-specific isoforms support the use of bovine RPE-choroid explants as an adequate experimental system for assessing fluid absorption activity for DPOFA. Our data is consistent with the fact that DPOFA stimulates fluid absorption across the RPE in bovine RPE-choroid explants.

Conclusions

DPOFA seems to stimulate transport of water across the RPE in bovine RPE-choroid explants. Additional experiments are required to establish dose-dependent effect of DPOFA on fluid absorption in the bovine RPE-choroid experimental system.

Nanoparticle-mediated gene transfer specific to retinal pigment epithelial cells

Previously, we demonstrated that CK30PEG10k-compacted DNA nanoparticles (NPs) efficiently target photoreceptor cells and improve visual function in a retinitis pigmentosa model. Here, we test the ability of these NPs in driving transgene expression in the retinal pigment epithelium (RPE), using an RPE-specific reporter vector (VMD2-eGFP). NPs, uncompacted plasmid, or saline were subretinally delivered to adult BALB/c mice. NP-based expression was specific to RPE cells and caused no deleterious effects on retinal structure and function. eGFP expression levels in NP-injected eyes peaked at post-injection day 2 (PI-2), stabilized at levels ~3-fold higher than in naked DNA-injected eyes, and remained elevated at the latest time-point examined (PI-30). Unlike naked DNA, which only transfected cells at the site of injection, NPs were able to transfect cells throughout the RPE. Subretinal injections of rhodamine labeled NPs and naked DNA showed comparable initial uptake into RPE cells. However, at PI-7 and -30 days significantly more fluorescence was detected inside the RPE of NP-injected eyes compared to naked DNA, suggesting NPs are stable inside the cell which could possibly lead to higher and sustained expression. Overall, our results demonstrate that NPs can efficiently deliver genes to the RPE and hold great potential for the treatment of RPE-associated diseases.

The effect of subretinal viscoelastics on the porcine retinal function

The functional consequence of long-term retinal detachment in the porcine model is examined by multifocal electroretinography (mfERG). Retinal detachment (RD) in humans leaves permanent visual impairment, despite anatomical successful reattachment surgery. To improve treatment, adjuvant pharmaceutical therapy is needed, and can only be tested in a suitable animal model. The porcine model is promising and the mfERG is well validated in this model. RD was induced in 18 pigs by vitrectomy and healon injection of various concentrations. Preoperatively and 6 weeks postoperatively eight animals were examined by mfERG. The major component P1 was analyzed statistically. Indirect ophthalmoscopy and bilateral color fundus photography (FP) were performed. Selected animals underwent high-resolution optical coherence tomography (OCT). Examination by ophthalmoscopy and FP showed that the RDs remained detached for the 6 weeks of follow-up. The P1 amplitude of the mfERG did not differ significantly between the detached areas, the surrounding attached areas, and the healthy eye (p = 0.25). Similarly, P1 implicit time did not differ between the areas (p = 0.85). The lack of functional consequences of long-term RD makes the porcine model unsuitable for examining adjuvant pharmaceutical RD treatment. Future studies should focus on foveated primates.

Functional Expression of P2Y Receptors in WERI-Rb1 Retinoblastoma Cells

P2Y receptors are metabotropic G-protein-coupled receptors, which are involved in many important biologic functions in the central nervous system including retina. Subtypes of P2Y receptors in retinal tissue vary according to the species and the cell types. We examined the molecular and pharmacologic profiles of P2Y purinoceptors in retinoblastoma cell, which has not been identified yet. To achieve this goal, we used Ca(2+) imaging technique and western blot analysis in WERI-Rb-1 cell, a human retinoblastoma cell line. ATP (10 µM) elicited strong but transient [Ca(2+)](i) increase in a concentration-dependent manner from more than 80% of the WERI-Rb-1 cells (n=46). Orders of potency of P2Y agonists in evoking [Ca(2+)](i) transients were 2MeS-ATP>ATP>>UTP=αβ-MeATP, which was compatible with the subclass of P2Y(1) receptor. The [Ca(2+)](i) transients evoked by applications of 2MeS-ATP and/or ATP were also profoundly suppressed in the presence of P2Y(1) selective blocker (MRS 2179; 30 µM). P2Y(1) receptor expression in WERI-Rb-1 cells was also identified by using western blot. Taken together, P2Y(1) receptor is mainly expressed in a retinoblastoma cell, which elicits Ca(2+) release from internal Ca(2+) storage sites via the phospholipase C-mediated pathway. P2Y(1) receptor activation in retinoblastoma cell could be a useful model to investigate the role of purinergic [Ca(2+)](i) signaling in neural tissue as well as to find a novel therapeutic target to this lethal cancer.

Nanoparticles for retinal gene therapy

Ocular gene therapy is becoming a well-established field. Viral gene therapies for the treatment of Leber's congentinal amaurosis (LCA) are in clinical trials, and many other gene therapy approaches are being rapidly developed for application to diverse ophthalmic pathologies. Of late, development of non-viral gene therapies has been an area of intense focus and one technology, polymer-compacted DNA nanoparticles, is especially promising. However, development of pharmaceutically and clinically viable therapeutics depends not only on having an effective and safe vector but also on a practical treatment strategy. Inherited retinal pathologies are caused by mutations in over 220 genes, some of which contain over 200 individual disease-causing mutations, which are individually very rare. This review will focus on both the progress and future of nanoparticles and also on what will be required to make them relevant ocular pharmaceutics.

Gene delivery to mitotic and postmitotic photoreceptors via compacted DNA nanoparticles results in improved phenotype in a mouse model of retinitis pigmentosa

The purpose of the present study was to test the therapeutic efficiency and safety of compacted-DNA nanoparticle-mediated gene delivery into the subretinal space of a juvenile mouse model of retinitis pigmentosa. Nanoparticles containing the mouse opsin promoter and wild-type mouse Rds gene were injected subretinally into mice carrying a haploinsufficiency mutation in the retinal degeneration slow (rds(+ or -)) gene at postnatal day (P)5 and 22. Control mice were either injected with saline, injected with uncompacted naked plasmid DNA carrying the Rds gene, or remained untreated. Rds mRNA levels peaked at postinjection day 2 to 7 (PI-2 to PI-7) for P5 injections, stabilized at levels 2-fold higher than in uninjected controls for both P5 and P22 injections, and remained elevated at the latest time point examined (PI-120). Rod function (measured by electroretinography) showed modest but statistically significant improvement compared with controls after both P5 and P22 injections. Cone function in nanoparticle-injected eyes reached wild-type levels for both ages of injections, indicating full prevention of cone degeneration. Ultrastructural examination at PI-120 revealed significant improvement in outer segment structures in P5 nanoparticle-injected eyes, while P22 injection had a modest structural improvement. There was no evidence of macrophage activation or induction of IL-6 or TNF-alpha mRNA in P5 or P22 nanoparticle-dosed eyes at either PI-2 or PI-30. Thus, compacted-DNA nanoparticles can efficiently and safely drive gene expression in both mitotic and postmitotic photoreceptors and retard degeneration in this model. These findings, using a clinically relevant treatment paradigm, illustrate the potential for application of nanoparticle-based gene replacement therapy for treatment of human retinal degenerations.-Cai, X., Conley, S. M., Nash, Z., Fliesler, S. J., Cooper, M. J., Naash, M. I. Gene delivery to mitotic and postmitotic photoreceptors via compacted DNA nanoparticles results in improved phenotype in a mouse model of retinitis pigmentosa.

Ocular delivery of compacted DNA-nanoparticles does not elicit toxicity in the mouse retina

Subretinal delivery of polyethylene glycol-substituted lysine peptide (CK30PEG)-compacted DNA nanoparticles results in efficient gene expression in retinal cells. This work evaluates the ocular safety of compacted DNA nanoparticles. CK30PEG-compacted nanoparticles containing an EGFP expression plasmid were subretinally injected in adult mice (1 µl at 0.3, 1.0 and 3.0 µg/µl). Retinas were examined for signs of inflammation at 1, 2, 4 and 7 days post-injection. Neither infiltration of polymorphonuclear neutrophils or lymphocytes was detected in retinas. In addition, elevation of macrophage marker F4/80 or myeloid marker myeloperoxidase was not detected in the injected eyes. The chemokine KC mRNA increased 3–4 fold in eyes injected with either nanoparticles or saline at 1 day post-injection, but returned to control levels at 2 days post-injection. No elevation of KC protein was observed in these mice. The monocyte chemotactic protein-1, increased 3–4 fold at 1 day post-injection for both nanoparticle and saline injected eyes, but also returned to control levels at 2 days. No elevations of tumor necrosis factor alpha mRNA or protein were detected. These investigations show no signs of local inflammatory responses associated with subretinal injection of compacted DNA nanoparticles, indicating that the retina may be a suitable target for clinical nanoparticle-based interventions.

Involvement of Müller glial cells in epiretinal membrane formation

BACKGROUND

Proliferative retinopathies are considered to represent maladapted retinal wound repair processes driven by growth factor- and cytokine-induced overstimulation of proliferation, migration, extracellular matrix production and contraction of retinal cells. The formation of neovascular membranes represents an attempt to reoxygenize non-perfused retinal areas. Müller glial cells play a crucial role in the pathogenesis of proliferative retinopathies. This review summarizes the present knowledge regarding the role of Müller cells in periretinal membrane formation, especially in the early steps of epiretinal membrane formation, which involve an interaction of inflammatory and glial cells, and gives a survey of the factors which are suggested to be implicated in the induction of Müller cell gliosis and proliferation.

CONCLUSIONS

Alterations in the membrane conductance of Müller cells suggest that Müller cells may alter their phenotype into progenitor-like cells in the course of proliferative retinopathies; transdifferentiated Müller cells may have great impact for the development of new cell-based therapies.

A partial structural and functional rescue of a retinitis pigmentosa model with compacted DNA nanoparticles

Previously we have shown that compacted DNA nanoparticles can drive high levels of transgene expression after subretinal injection in the mouse eye. Here we delivered compacted DNA nanoparticles containing a therapeutic gene to the retinas of a mouse model of retinitis pigmentosa. Nanoparticles containing the wild-type retinal degeneration slow (Rds) gene were injected into the subretinal space of rds^+/− mice on postnatal day 5. Gene expression was sustained for up to four months at levels up to four times higher than in controls injected with saline or naked DNA. The nanoparticles were taken up into virtually all photoreceptors and mediated significant structural and biochemical rescue of the disease without histological or functional evidence of toxicity. Electroretinogram recordings showed that nanoparticle-mediated gene transfer restored cone function to a near-normal level in contrast to transfer of naked plasmid DNA. Rod function was also improved. These findings demonstrate that compacted DNA nanoparticles represent a viable option for development of gene-based interventions for ocular diseases and obviate major barriers commonly encountered with non-viral based therapies.

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.

Subsets of retinal neurons and glia express P2Y1 receptors

Recent evidence suggests that extracellular ATP modulates retinal processing and could play a role in modulating glial cells during retinal diseases. Here, we evaluated the localization of P2Y(1) receptors in the rat retina using indirect immunofluorescence immunocytochemistry. We observed labeling within defined populations of inner retinal neurons and Müller cell processes and end feet. Double labeling of P2Y(1) receptor with choline acetyltransferase revealed extensive colocalization indicating the expression of this receptor by cholinergic amacrine cells. Ganglion cell labeling for P2Y(1) receptors was also observed. Having established the normal pattern of immunolabeling within the retina, we next examined whether immunolabeling was altered by retinal disease. P2Y(1) receptor immunolabeling of Müller cells was of greater intensity following light-induced retinal degeneration, suggesting that Müller cell gliosis is accompanied by changes in P2Y(1) receptor expression. Overall, these data provide further evidence for a role of extracellular ATP in retinal signaling within subsets of retinal neurons as well as glia.

Functions of aquaporins in the eye

The aquaporins (AQPs) are integral membrane proteins whose main function is to transport water across cell membranes in response to osmotic gradients. At the ocular surface, AQP1 is expressed in corneal endothelium, AQP3 and AQP5 in corneal epithelium, and AQP3 in conjunctival epithelium. AQPs are also expressed in lens fiber cells (AQP0), lens epithelium (AQP1), ciliary epithelium (AQP1, AQP4) and retinal Müller cells (AQP4). Mutations in AQP0 produce congenital cataracts in humans. Analysis of knockout mice lacking individual AQPs suggests their involvement in maintenance of corneal and lens transparency, corneal epithelial repair, intraocular pressure (IOP) regulation, retinal signal transduction and retinal swelling following injury. The mouse phenotype findings implicate AQPs as potential drug targets for therapy of elevated IOP and ocular disorders involving the cornea, lens and retina. However, much research remains in defining cell-level mechanisms for the ocular AQP functions, in establishing the relevance to human eye disease of conclusions from knockout mice, and in developing AQP-modulating drugs.

Expression profiling after retinal detachment and reattachment: a possible role for aquaporin-0

PURPOSE

Retinal detachment (RD) is associated with acute visual loss caused by anatomic displacement of the photoreceptors and with chronic visual loss/disturbance caused by retinal remodeling and photoreceptor cell death, which may occur even after successful reattachment. The P2Y(2) receptor agonist INS37217 improves the rate of retinal reattachment in animal models of induced RD, and has been shown to also significantly enhance the rate of ERG recovery in a mouse model of RD. The identification of genes modulated by INS37217 may allow further drug discovery for treating RD and edema.

METHODS

To identify genes involved in RD and subsequent reattachment, a retinal microarray screen was performed using a mouse model of RD in the presence or absence of INS37217.

RESULTS

Ninety-two genes were identified as differentially expressed across three time points, most of which were upregulated in the presence of this agonist. Furthermore, it was shown that RD alters the expression of aquaporin-0 (AQP-0), and this modulation is prevented by treatment with INS37217. The presence of AQP-0 in retinal bipolar cells was also demonstrated, whereas it was previously thought to be specific to the lens. Mice lacking functional alleles of AQP-0 had a phototransduction deficit as assessed by electroretinography; however, their photoreceptor structure was normal, indicative of a problem with signal transmission between neurons.

CONCLUSIONS

This study establishes the genes involved in RD and reattachment, and also demonstrates for the first time a physiologically significant role for AQP-0 in retinal function.

Late-onset cone photoreceptor degeneration induced by R172W mutation in Rds and partial rescue by gene supplementation

PURPOSE

R172W is a common mutation in the human retinal degeneration slow (RDS) gene, associated with a late-onset dominant macular dystrophy. In this study, the authors characterized a mouse model that closely mimics the human phenotype and tested the feasibility of gene supplementation as a disease treatment strategy.

METHODS

Transgenic mouse lines carrying the R172W mutation were generated. The retinal phenotype associated with this mutation in a low-expresser line (L-R172W) was examined, both structurally (histology with correlative immunohistochemistry) and functionally (electroretinography). By examining animals over time and with various rds genetic backgrounds, the authors evaluated the dominance of the defect. To assess the efficacy of gene transfer therapy as a treatment for this defect, a previously characterized transgenic line expressing the normal mouse peripherin/Rds (NMP) was crossed with a higher-expresser Rds line harboring the R172W mutation (H-R172W). Functional, structural, and biochemical analyses were used to assess rescue of the retinal disease phenotype.

RESULTS

In the wild-type (WT) background, L-R172W mice exhibited late-onset (12-month) dominant cone degeneration without any apparent effect on rods. The degeneration was slightly accelerated (9 months) in the rds(+/-) background. L-R172W retinas did not form outer segments in the absence of endogenous Rds. With use of the H-R172W line on an rds(+/-) background for proof-of-principle genetic supplementation studies, the NMP transgene product rescued rod and cone functional defects and supported outer segment integrity up to 3 months of age, but the rescue effect did not persist in older (11-month) animals.

CONCLUSIONS

The R172W mutation leads to dominant cone degeneration in the mouse model, regardless of the expression level of the transgene. In contrast, effects of the mutation on rods are dose dependent, underscoring the usefulness of the L-R172W line as a faithful model of the human phenotype. This model may prove helpful in future studies on the mechanisms of cone degeneration and for elucidating the different roles of Rds in rods and cones. This study provides evidence that Rds genetic supplementation can be used to partially rescue visual function. Although this strategy is capable of rescuing haploinsufficiency, it does not rescue the long-term degeneration associated with a gain-of-function mutation.

Purinergic signalling in the subretinal space: a role in the communication between the retina and the RPE

The retinal pigment epithelium (RPE) is separated from the photoreceptor outer segments by the subretinal space. While the actual volume of this space is minimal, the communication that occurs across this microenvironment is important to the visual process, and accumulating evidence suggests the purines ATP and adenosine contribute to this communication. P1 and P2 receptors are localized to membranes on both the photoreceptor outer segments and on the apical membrane of the RPE which border subretinal space. ATP is released across the apical membrane of the RPE into this space in response to various triggers including glutamate and chemical ischemia. This ATP is dephosphorylated into adenosine by a series of ectoenzymes on the RPE apical membrane. Regulation of release and ectoenzyme activity in response to light-sensitive signals can alter the balance of purines in subretinal space, and thus coordinate communication across subretinal space with the visual process.

Efficient non-viral ocular gene transfer with compacted DNA nanoparticles

Background

The eye is an excellent candidate for gene therapy as it is immune privileged and much of the disease-causing genetics are well understood. Towards this goal, we evaluated the efficiency of compacted DNA nanoparticles as a system for non-viral gene transfer to ocular tissues. The compacted DNA nanoparticles examined here have been shown to be safe and effective in a human clinical trial, have no theoretical limitation on plasmid size, do not provoke immune responses, and can be highly concentrated.

Methods and Findings

Here we show that these nanoparticles can be targeted to different tissues within the eye by varying the site of injection. Almost all cell types of the eye were capable of transfection by the nanoparticle and produced robust levels of gene expression that were dose-dependent. Most impressively, subretinal delivery of these nanoparticles transfected nearly all of the photoreceptor population and produced expression levels almost equal to that of rod opsin, the highest expressed gene in the retina.

Conclusions

As no deleterious effects on retinal function were observed, this treatment strategy appears to be clinically viable and provides a highly efficient non-viral technology to safely deliver and express nucleic acids in the retina and other ocular tissues.

Agonist-induced phosphorylation and desensitization of the P2Y2 nucleotide receptor

Purification of HA-tagged P2Y2 receptors from transfected human 1321N1 astrocytoma cells yielded a protein with a molecular size determined by SDS-PAGE to be in the range of 57-76 kDa, which is typical of membrane glycoproteins with heterogeneous complex glycosylation. The protein phosphatase inhibitor, okadaic acid, attenuated the recovery of receptor activity from the agonist-induced desensitized state, suggesting a role for P2Y2 receptor phosphorylation in desensitization. Isolation of HA-tagged P2Y2 nucleotide receptors from metabolically [32P]-labelled cells indicated a (3.8 +/- 0.2)-fold increase in the [32P]-content of the receptor after 15 min of treatment with 100 microM UTP, as compared to immunoprecipitated receptors from untreated control cells. Receptor sequestration studies indicated that approximately 40% of the surface receptors were internalized after a 15-min stimulation with 100 microM UTP. Point mutation of three potential GRK and PKC phosphorylation sites in the third intracellular loop and C-terminal tail of the P2Y2 receptor (namely, S243A, T344A, and S356A) extinguished agonist-induced receptor phosphorylation, caused a marked reduction in the efficacy of UTP to desensitize P2Y2 receptor signalling to intracellular calcium mobilization, and impaired agonist-induced receptor internalization. Activation of PKC isoforms with phorbol 12-myristate 13-acetate that caused heterologous receptor desensitization did not increase the level of P2Y2 receptor phosphorylation. Our results indicate a role for receptor phosphorylation by phorbol-insensitive protein kinases in agonist-induced desensitization of the P2Y2 nucleotide receptor.

The genomic response to retinal disease and injury: evidence for endothelin signaling from photoreceptors to glia

Regardless of proximal cause, photoreceptor injury or disease almost invariably leads to the activation of Muller cells, the principal glial cells in the retina. This observation implies the existence of signaling systems that inform Muller cells of the health status of photoreceptors. It further suggests that diverse types of photoreceptor damage elicit a limited range of biochemical responses. Using the mouse retina, we show by microarray, RNA blot, and in situ hybridization that the genomic responses to both light damage and inherited photoreceptor degeneration involve a relatively small number of genes and that the genes activated by these two insults overlap substantially with one another and with the genes activated by retinal detachment. Among the induced transcripts, those coding for endothelin2 (Edn2) are unusual in that they are localized to photoreceptors and are also highly induced in all of the tested models of photoreceptor disease or injury. Acute light damage also leads to a >10-fold increase in endothelin receptor B (Ednrb) in Muller cells 24 h after injury. These observations suggest that photoreceptor-derived EDN2 functions as a general stress signal, that EDN2 signals to Muller cells by binding to EDNRB, and that Muller cells can increase their sensitivity to EDN2 as part of the injury response.

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.