Neuroprotective and antiangiogenic actions of PEDF in the eye: molecular targets and therapeutic potential

Cell therapy in the cornea: The emerging role of microenvironment

The cornea is an ideal testing field for cell therapies. Its highly ordered structure, where specific cell populations are sequestered in different layers, together with its accessibility, has allowed the development of the first stem cell-based therapy approved by the European Medicine Agency. Today, different techniques have been proposed for autologous and allogeneic limbal and non-limbal cell transplantation. Cell replacement has also been attempted in cases of endothelial cell decompensation as it occurs in Fuchs dystrophy: injection of cultivated allogeneic endothelial cells is now in advanced phases of clinical development. Recently, stromal substitutes have been developed with excellent integration capability and transparency. Finally, cell-derived products, such as exosomes obtained from different sources, have been investigated for the treatment of severe corneal diseases with encouraging results. Optimization of the success rate of cell therapies obviously requires high-quality cultured cells/products, but the role of the surrounding microenvironment is equally important to allow engraftment of transplanted cells, to preserve their functions and, ultimately, lead to restoration of tissue integrity and transparency of the cornea.

Circulating Blood Prognostic Biomarker Signatures for Hemorrhagic Cerebral Cavernous Malformations (CCMs)

Cerebral cavernous malformations (CCMs) are a neurological disorder characterized by enlarged intracranial capillaries in the brain, increasing the susceptibility to hemorrhagic strokes, a major cause of death and disability worldwide. The limited treatment options for CCMs underscore the importance of prognostic biomarkers to predict the likelihood of hemorrhagic events, aiding in treatment decisions and identifying potential pharmacological targets. This study aimed to identify blood biomarkers capable of diagnosing and predicting the risk of hemorrhage in CCM1 patients, establishing an initial set of circulating biomarker signatures. By analyzing proteomic profiles from both human and mouse CCM models and conducting pathway enrichment analyses, we compared groups to identify potential blood biomarkers with statistical significance. Specific candidate biomarkers primarily associated with metabolism and blood clotting pathways were identified. These biomarkers show promise as prognostic indicators for CCM1 deficiency and the risk of hemorrhagic stroke, strongly correlating with the likelihood of hemorrhagic cerebral cavernous malformations (CCMs). This lays the groundwork for further investigation into blood biomarkers to assess the risk of hemorrhagic CCMs.

PEDF protects retinal pigment epithelium from ferroptosis and ameliorates dry AMD-like pathology in a murine model

Age-related macular degeneration (AMD) is the leading cause of irreversible vision damage among elderly individuals. There is still no efficient treatment for dry AMD. Retinal pigment epithelial (RPE) degeneration has been confirmed to play an important role in dry AMD. Recent studies have reported that ferroptosis caused by iron overload and lipid peroxidation may be the primary causes of RPE degeneration. However, the upstream regulatory molecules of RPE ferroptosis remain largely unknown. Pigment epithelium-derived factor (PEDF) is an important endogenic protective factor for the RPE. Our results showed that in the murine dry AMD model induced by sodium iodate (SI), PEDF expression was downregulated. Moreover, dry AMD-like pathology was observed in PEDF-knockout mice. Therefore, the aim of this study was to reveal the effects and mechanism of PEDF on RPE ferroptosis and investigate potential therapeutic targets for dry AMD. The results of lipid peroxidation and transmission electron microscope showed that retinal ferroptosis was significantly activated in SI-treated mice and PEDF-knockout mice. Restoration of PEDF expression ameliorated SI-induced retinal dysfunction in mice, as assessed by electroretinography and optical coherence tomography. Mechanistically, western blotting and immunofluorescence analysis demonstrated that the overexpression of PEDF could upregulate the expression of glutathione peroxidase 4 (GPX4) and ferritin heavy chain-1 (FTH1), which proved to inhibit lipid peroxidation and RPE ferroptosis induced by SI. This study revealed the novel role of PEDF in ferroptosis inhibition and indicated that PEDF might be a potential therapeutic target for dry AMD.

The role of retinal glial cells and related factors in macular edema

Macular edema (ME) has emerged as a leading cause of visual impairment, representing a critical clinical manifestation and complication associated with many eye diseases. In the occurrence and development of ME, retinal glial cells like Müller cells and microglial cells play vital roles. Moreover, growth factor and cytokines associated with them, such as vascular endothelial growth factor (VEGF), pigment epithelium-derived factor (PEDF), hypoxia-inducible factor-1α (HIF-1α), angiopoietin-like protein 4 (ANGPTL4), interleukin-6(IL-6), interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1), prostaglandin, etc., also take part in the pathogenesis of ME. Changes in these cytokines can lead to retinal angiogenesis, increased vascular permeability, blood-retinal barrier (BRB) breakdown, and fluid leakage, further causing ME to occur or deteriorate. Research on the role of retinal glial cells and related cytokines in ME will provide new therapeutic directions and effective remedies. This article is a literature review on the role of Müller cells, microglial cells and related factors in ME pathogenesis.

The Various Roles of PEDF in Cancer

Pigment epithelium-derived factor (PEDF) is a natural immunomodulator, anti-inflammatory, anti-angiogenic, anti-tumour growth and anti-metastasis factor, which can enhance tumour response to PEDF but can also conversely have pro-cancerous effects. Inflammation is a major cause of cancer, and it has been proven that PEDF has anti-inflammatory properties. PEDF's functional activity can be investigated through measuring metastatic and metabolic biomarkers that will be discussed in this review.

Pigment epithelium-derived factor (PEDF) represses the glucose transporter 1 (GLUT1) mRNA expression and may be a potential therapeutic agent in psoriasis: a case-control and experimental study

We investigated the whole blood GLUT1 mRNA expression and serum pigment epithelium-derived factor (PEDF), interleukin-6 (IL-6), fetuin-A, and pentraxin-3 (PTX3) levels in psoriatic patients and tested their correlations with the severity of psoriasis using the psoriasis area and severity index (PASI) score. Also, we tested the GLUT1 mRNA expression after an in vitro treatment of human skin fibroblast (HSF) cell lines with PEDF. The case-control part of the study recruited 74 participants (44 psoriatic patients and 30 healthy volunteers). Whole blood GLUT1 mRNA fold changes were estimated by RT-PCR, and serum PEDF, IL-6, fetuin-A, and PTX3 levels were measured by ELISA kits. In the experimental part, the HSF cell lines were treated with different concentrations of PEDF for different times to test its effect on the GLUT1 mRNA expression. The whole blood GLUT 1 expression significantly increased in psoriatic patients and correlated positively with serum IL-6, fetuin-A, PTX3 levels and with the severity of psoriasis while negatively with serum PEDF levels. The PEDF-treated HSF cell lines showed a time- and dose-dependent decline in the GLUT 1 mRNA expression. The whole blood GLUT 1 mRNA is a non-invasive biomarker that is associated with the severity of psoriasis. PEDF represses GLUT 1 expression and may be a potential therapeutic agent in psoriasis.Trial registration: ClinicalTrials.gov Identifier: NCT04242082.

N-cadherin cleavage: A critical function that induces diabetic retinopathy fibrosis via regulation of β-catenin translocation

Retinal fibrosis is a severe pathological change in the late stage of diabetic retinopathy and is also the leading cause of blindness. We have previously revealed that N-cadherin was significantly increased in type 1 and type 2 diabetic mice retinas and the fibrovascular membranes from proliferative diabetic retinopathy (PDR) patients. However, whether N-cadherin directly induces retinal fibrosis in DR and the related mechanism is unknown. Here, we investigated the pathogenic role of N-cadherin in mediating retinal fibrosis and further explored the relevant therapeutic targets. We found that the level of N-cadherin was significantly increased in PDR patients and STZ-induced diabetic mice and positively correlated with the fibrotic molecules Connective Tissue Growth Factor (CTGF) and fibronectin (FN). Moreover, intravitreal injection of N-cadherin adenovirus significantly increased the expression of FN and CTGF in normal mice retinas. Mechanistically, overexpression of N-cadherin promotes N-cadherin cleavage, and N-cadherin cleavage can further induce translocation of non-p-β-catenin in the nucleus and upregulation of fibrotic molecules. Furthermore, we found a novel N-cadherin cleavage inhibitor, pigment epithelial-derived factor (PEDF), which ameliorated the N-cadherin cleavage and subsequent retinal fibrosis in diabetic mice. Thus, our findings provide novel evidence that elevated N-cadherin level not only acts as a classic EMT maker but also plays a causative role in diabetic retinal fibrosis, and targeting N-cadherin cleavage may provide a strategy to inhibit retinal fibrosis in DR patients.

Optic Nerve Regeneration in Diabetic Retinopathy: Potentials and Challenges Ahead

Diabetic retinopathy (DR), the most common microvascular compilation of diabetes, is the leading cause of vision loss and blindness worldwide. Recent studies indicate that retinal neuron impairment occurs before any noticeable vascular changes in DR, and retinal ganglion cell (RGC) degeneration is one of the earliest signs. Axons of RGCs have little capacity to regenerate after injury, clinically leading the visual functional defects to become irreversible. In the past two decades, tremendous progress has been achieved to enable RGC axon regeneration in animal models of optic nerve injury, which holds promise for neural repair and visual restoration in DR. This review summarizes these advances and discusses the potential and challenges for developing optic nerve regeneration strategies treating DR.

Proteomics profiling of vitreous humor reveals complement and coagulation components, adhesion factors, and neurodegeneration markers as discriminatory biomarkers of vitreoretinal eye diseases

Introduction

Diabetic retinopathy (DR) and age-related macular degeneration (AMD) are leading causes of visual impairment and blindness in people aged 50 years or older in middle-income and industrialized countries. Anti-VEGF therapies have improved the management of neovascular AMD (nAMD) and proliferative DR (PDR), no treatment options exist for the highly prevalent dry form of AMD.

Methods

To unravel the biological processes underlying these pathologies and to find new potential biomarkers, a label-free quantitative (LFQ) method was applied to analyze the vitreous proteome in PDR (n=4), AMD (n=4) compared to idiopathic epiretinal membranes (ERM) (n=4).

Results and discussion

Post-hoc tests revealed 96 proteins capable of differentiating among the different groups, whereas 118 proteins were found differentially regulated in PDR compared to ERM and 95 proteins in PDR compared to dry AMD. Pathway analysis indicates that mediators of complement, coagulation cascades and acute phase responses are enriched in PDR vitreous, whilst proteins highly correlated to the extracellular matrix (ECM) organization, platelet degranulation, lysosomal degradation, cell adhesion, and central nervous system development were found underexpressed. According to these results, 35 proteins were selected and monitored by MRM (multiple reaction monitoring) in a larger cohort of patients with ERM (n=21), DR/PDR (n=20), AMD (n=11), and retinal detachment (n=13). Of these, 26 proteins could differentiate between these vitreoretinal diseases. Based on Partial least squares discriminant and multivariate exploratory receiver operating characteristic (ROC) analyses, a panel of 15 discriminatory biomarkers was defined, which includes complement and coagulation components (complement C2 and prothrombin), acute-phase mediators (alpha-1-antichymotrypsin), adhesion molecules (e.g., myocilin, galectin-3-binding protein), ECM components (opticin), and neurodegeneration biomarkers (beta-amyloid, amyloid-like protein 2).

Sustained therapeutic effect of an anti-inflammatory peptide encapsulated in nanoparticles on ocular vascular leakage in diabetic retinopathy

Pigment epithelium-derived factor (PEDF), an endogenous Wnt signaling inhibitor in the serine proteinase inhibitors (SERPIN) super family, is present in multiple organs, including the vitreous. Significantly low levels of PEDF in the vitreous are found to associate with pathological retinal vascular leakage and inflammation in diabetic retinopathy (DR). Intravitreal delivery of PEDF represents a promising therapeutic approach for DR. However, PEDF has a short half-life after intravitreal injection, which represents a major hurdle for the long-term treatment. Here we report the prolonged therapeutic effects of a 34-mer peptide of the PEDF N-terminus, encapsulated in poly (lactic-co-glycolic acid) (PLGA) nanoparticles (PEDF34-NP), on DR. PEDF34-NP inhibited hypoxia-induced expression of vascular endothelial growth factor and reduced levels of intercellular adhesion molecule 1 (ICAM-1) in cultured retinal cells. In addition, PEDF34-NP significantly ameliorated ischemia-induced retinal neovascularization in the oxygen-induced retinopathy rat model, and significantly reduced retinal vascular leakage and inflammation in streptozotocin-induced diabetic rats up to 4 weeks after intravitreal injection, as compared to PLGA-NP control. Intravitreal injection of PEDF34-NP did not display any detectable toxicities to retinal structure and function. Our findings suggest that PEDF34-NP can confer sustained therapeutic effects on retinal inflammation and vascular leakage, having considerable potential to provide long-term treatment options for DR.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Poor eyesight reveals a new vision gene

Comparing the genomes of mammals which evolved to have poor vision identifies an important gene for eyesight.

Vision-related convergent gene losses reveal SERPINE3's unknown role in the eye

Despite decades of research, knowledge about the genes that are important for development and function of the mammalian eye and are involved in human eye disorders remains incomplete. During mammalian evolution, mammals that naturally exhibit poor vision or regressive eye phenotypes have independently lost many eye-related genes. This provides an opportunity to predict novel eye-related genes based on specific evolutionary gene loss signatures. Building on these observations, we performed a genome-wide screen across 49 mammals for functionally uncharacterized genes that are preferentially lost in species exhibiting lower visual acuity values. The screen uncovered several genes, including SERPINE3, a putative serine proteinase inhibitor. A detailed investigation of 381 additional mammals revealed that SERPINE3 is independently lost in 18 lineages that typically do not primarily rely on vision, predicting a vision-related function for this gene. To test this, we show that SERPINE3 has the highest expression in eyes of zebrafish and mouse. In the zebrafish retina, serpine3 is expressed in Müller glia cells, a cell type essential for survival and maintenance of the retina. A CRISPR-mediated knockout of serpine3 in zebrafish resulted in alterations in eye shape and defects in retinal layering. Furthermore, two human polymorphisms that are in linkage with SERPINE3 are associated with eye-related traits. Together, these results suggest that SERPINE3 has a role in vertebrate eyes. More generally, by integrating comparative genomics with experiments in model organisms, we show that screens for specific phenotype-associated gene signatures can predict functions of uncharacterized genes.

The Acute Effects of Furosemide on Na-K-Cl Cotransporter-1, Fetuin-A and Pigment Epithelium-Derived Factor in the Guinea Pig Cochlea

Background

Furosemide is a loop diuretic used to treat edema; however, it also targets the Na-K-Cl cotransporter-1 (NKCC1) in the inner ear. In very high doses, furosemide abolishes the endocochlear potential (EP). The aim of the study was to gain a deeper understanding of the temporal course of the acute effects of furosemide in the inner ear, including the protein localization of Fetuin-A and PEDF in guinea pig cochleae.

Material and Method

Adult guinea pigs were given an intravenous injection of furosemide in a dose of 100 mg per kg of body weight. The cochleae were studied using immunohistochemistry in controls and at four intervals: 3 min, 30 min, 60 min and 120 min. Also, cochleae of untreated guinea pigs were tested for Fetuin-A and PEDF mRNA using RNAscope^® technology.

Results

At 3 min, NKCC1 staining was abolished in the type II fibrocytes in the spiral ligament, followed by a recovery period of up to 120 min. In the stria vascularis, the lowest staining intensity of NKCC1 presented after 30 min. The spiral ganglion showed a stable staining intensity for the full 120 min. Fetuin-A protein and mRNA were detected in the spiral ganglion type I neurons, inner and outer hair cells, pillar cells, Deiters cells and the stria vascularis. Furosemide induced an increased staining intensity of Fetuin-A at 120 min. PEDF protein and mRNA were found in the spiral ganglia type I neurons, the stria vascularis, and in type I and type II fibrocytes of the spiral ligament. PEDF protein staining intensity was high in the pillar cells in the organ of Corti. Furosemide induced an increased staining intensity of PEDF in type I neurons and pillar cells after 120 min.

Conclusion

The results indicate rapid furosemide-induced changes of NKCC1 in the type II fibrocytes. This could be part of the mechanism that causes reduction of the EP within minutes after high dose furosemide injection. Fetuin-A and PEDF are present in many cells of the cochlea and probably increase after furosemide exposure, possibly as an otoprotective response.

Pigment Epithelium-Derived Factor-Loaded PEGylated Nanoparticles as a New Antiangiogenic Therapy for Neovascularization

BACKGROUND

Pathological neovascularization, which involves a disruption in the balance between angiogenic and antiangiogenic factors under pathological conditions, is the basis of many intraocular diseases. Pigment epithelium-derived factor (PEDF) is a potent natural, endogenous inhibitor of neovascularization because of its antiangiogenic and neuroprotective benefits. However, its application is restricted by its instability and short half-life. The present study is aimed at investigating the cytotoxicity and antiangiogenic effects of PEDF-loaded PEGylated nanoparticles (NP-PEG-PEDF) on high glucose-stimulated human umbilical vein endothelial cells (HUVECs).

METHODS

In this study, NP-PEG-PEDF were fabricated using the multiple emulsion method for the first time. HUVECs were cultured in a high concentration of glucose (30 mmol/L D-glucose), simulating diabetic conditions. The antiangiogenic effects of vascular endothelial growth factor (VEGF), pure PEDF, and NP-PEG-PEDF on proliferation, migration, and tube formation were evaluated. VEGF secretion in high glucose-stimulated HUVECs was further tested in vitro.

RESULTS

NP-PEG-PEDF exhibited low cytotoxicity in HUVECs. Our results indicated that in vitro, NP-PEG-PEDF attenuated diabetes-induced HUVEC proliferation, migration, and tube formation and suppressed VEGF secretion. The apoptosis of diabetes-induced HUVECs occurred in a dose-dependent manner, which showed a statistically significant difference compared with the PEDF treatment group.

CONCLUSION

Our study is the first to demonstrate that NP-PEG-PEDF exert antiangiogenic effects on high glucose-stimulated HUVECs and have the potential to alleviate microvascular dysfunction. These data suggest that the NP-PEG-PEDF delivery system may offer an innovative therapeutic strategy for preventing neovascularization of the fundus.

The role of inflammation and neurodegeneration in diabetic macular edema

The pathogenesis of diabetic macular edema (DME) is complex. Persistently high blood glucose activates multiple cellular pathways and induces inflammation, oxidation stress, and vascular dysfunction. Retinal ganglion cells, macroglial and microglial cells, endothelial cells, pericytes, and retinal pigment epithelium cells are involved. Neurodegeneration, characterized by dysfunction or apoptotic loss of retinal neurons, occurs early and independently from the vascular alterations. Despite the increasing knowledge on the pathways involved in DME, only limited therapeutic strategies are available. Besides antiangiogenic drugs and intravitreal corticosteroids, alternative therapeutic options tackling inflammation, oxidative stress, and neurodegeneration have been considered, but none of them has been currently approved.

Diabetic macular ischaemia- a new therapeutic target?

Diabetic macular ischaemia (DMI) is traditionally defined and graded based on the angiographic evidence of an enlarged and irregular foveal avascular zone. However, these anatomical changes are not surrogate markers for visual impairment. We postulate that there are vascular phenotypes of DMI based on the relative perfusion deficits of various retinal capillary plexuses and choriocapillaris. This review highlights several mechanistic pathways, including the role of hypoxia and the complex relation between neurons, glia, and microvasculature. The current animal models are reviewed, with shortcomings noted. Therefore, utilising the advancing technology of optical coherence tomography angiography (OCTA) to identify the reversible DMI phenotypes may be the key to successful therapeutic interventions for DMI. However, there is a need to standardise the nomenclature of OCTA perfusion status. Visual acuity is not an ideal endpoint for DMI clinical trials. New trial endpoints that represent disease progression need to be developed before irreversible vision loss in patients with DMI. Natural history studies are required to determine the course of each vascular and neuronal parameter to define the DMI phenotypes. These DMI phenotypes may also partly explain the development and recurrence of diabetic macular oedema. It is also currently unclear where and how DMI fits into the diabetic retinopathy severity scales, further highlighting the need to better define the progression of diabetic retinopathy and DMI based on both multimodal imaging and visual function. Finally, we discuss a complete set of proposed therapeutic pathways for DMI, including cell-based therapies that may provide restorative potential.

[The role of neurodegeneration biomarkers in the management of patients with diabetic retinopathy]

This literature review focuses on the role of disease biomarkers in the management of patients with diabetic retinopathy (DR) investigating in detail the problem of retinal neurodegeneration in such patients. Identification and assessment of the significance of qualitative and quantitative biomarkers of DR and neurodegeneration can complement screening examination, as well as help predict the course of the disease and the response to therapy. A comprehensive analysis of these factors allows for effective treatment and prevention of complications in patients with DR based on prognostic models and dynamic monitoring of these indicators.

Exploration of novel anti-angiogenic PEDF-derived peptides with improved activitives by inhibiting proliferation, suppressing migration, and inducing 67LR internalization

Diabetic retinopathy (DR) remains high incidence and accounts for severe impact on vision in diabetics, but its mechanism is still poorly understood. Abnormal migration and proliferation of endothelial cells (ECs) drive neovascular retinopathies, which has an important role in promoting the occurrence and development of DR. In this study, we designed and synthesized a series of PEDF-derived peptides as angiogenesis inhibitors. Especially, compound G24 significantly inhibited the cell proliferation in VEGF-activated human umbilical vein endothelial cells (HUVECs) with IC₅₀ values of 2.88 ± 0.19 μM. Further biological evaluation demonstrated that compound G24 exhibited strong inducing-effects on cell apoptosis and internalization of 67LR, and advanced inhibitory potency in cell migration and angiogenesis formed by HUVECs in vitro. In summary, the optimal compound G24 as a novel angiogenesis inhibitor showed the potentiality in the further research for the treatment for DR.

Neurovascular Unit: A New Target for Treating Early Stages of Diabetic Retinopathy

The concept of diabetic retinopathy as a microvascular disease has evolved and is now considered a more complex diabetic complication in which neurovascular unit impairment plays an essential role and, therefore, can be considered as a main therapeutic target in the early stages of the disease. However, neurodegeneration is not always the apparent primary event in the natural story of diabetic retinopathy, and a phenotyping characterization is recommendable to identify those patients in whom neuroprotective treatment might be of benefit. In recent years, a myriad of treatments based on neuroprotection have been tested in experimental models, but more interestingly, there are drugs with a dual activity (neuroprotective and vasculotropic). In this review, the recent evidence concerning the therapeutic approaches targeting neurovascular unit impairment will be presented, along with a critical review of the scientific gaps and problems which remain to be overcome before our knowledge can be transferred to clinical practice.

Neuroprotective effects of glial mediators in interactions between retinal neurons and Müller cells

Progressive retinal ganglion cell (RGC) loss underlies a number of retinal neurodegenerative disorders, which may lead to permanent vision loss. However, secreted neuroprotective factors, such as PEDF, VEGF and IL-6, which are produced by Müller cells, have been shown to promote RGC survival. Assuming that the communication of RGCs with Müller cells involves a release of glioactive substances we sought to determine whether retinal neurons are able to modulate expression levels of Müller cell-derived PEDF, VEGF and IL-6. We demonstrate elevated mRNA levels of these factors in Müller cells in co-cultures with RGCs or R28 cells when compared to homotypic Müller cell cultures. Furthermore, R28 cells were more protected from apoptosis when co-cultured with Müller cells. IL-6 and VEGF were upregulated in Müller cells under hypoxia. Both cytokines, as well as PEDF, induced an altered neuronal expression of members of the Bcl-2 family, which are central molecules in the regulation of apoptosis. These results suggest that in retinal ischemia, via own secreted mediators, RGCs can resist a potential demise by stimulating Müller cells to increase production of neuroprotective factors, which counteract RGC apoptosis.

Pigment epithelium-derived factor (PEDF) and derived peptides promote survival and differentiation of photoreceptors and induce neurite-outgrowth in amacrine neurons

Pigment epithelium-derived factor (PEDF) is a cytoprotective protein for the retina. We hypothesize that this protein acts on neuronal survival and differentiation of photoreceptor cells in culture. The purpose of the present study was to evaluate the neurotrophic effects of PEDF and its fragments in an in vitro model of cultured primary retinal neurons that die spontaneously in the absence of trophic factors. We used Wistar albino rats. Cell death was assayed by immunofluorescence and flow cytometry through TUNEL assay, propidium iodide, mitotracker, and annexin V. Immunofluorescence of cells for visualizing rhodopsin, CRX, and antisyntaxin under confocal microscopy was performed. Neurite outgrowth was also quantified. Results show that PEDF protected photoreceptor precursors from apoptosis, preserved mitochondrial function and promoted polarization of opsin enhancing their developmental process, as well as induced neurite outgrowth in amacrine neurons. These effects were abolished by an inhibitor of the PEDF receptor or receptor-derived peptides that block ligand/receptor interactions. While all the activities were specifically conferred by short peptide fragments (17 amino acid residues) derived from the PEDF neurotrophic domain, no effects were triggered by peptides from the PEDF antiangiogenic region. The observed effects on retinal neurons imply a specific activation of the PEDF receptor by a small neurotrophic region of PEDF. Our findings support the neurotrophic PEDF peptides as neuronal guardians for the retina, highlighting their potential as promoters of retinal differentiation, and inhibitors of retinal cell death and its blinding consequences.

Diabetic retinal neurodegeneration as a form of diabetic retinopathy

PURPOSE

To review the evidence supporting diabetic retinal neurodegeneration (DRN) as a form of diabetic retinopathy.

METHOD

Review of literature.

RESULTS

DRN is recognized to be a part of retinopathy in patients with diabetes mellitus (DM), in addition to the well-established diabetic retinal vasculopathy (DRV). DRN has been noted in the early stages of DM, before the onset of clinically evident diabetic retinopathy. The occurrence of DRN has been confirmed in animal models of DM, histopathological examination of donor's eyes from diabetic individuals and assessment of neural structure and function in humans. DRN involves alterations in retinal ganglion cells, photoreceptors, amacrine cells and bipolar cells, and is thought to be driven by glutamate, oxidative stress and dysregulation of neuroprotective factors in the retina. Potential therapeutic options for DRN are under evaluation.

CONCLUSIONS

Literature is divided on the temporal relation between DRN and DRV, with evidence of both precedence and simultaneous occurrence. The relationship between DRN and multi-system neuropathy in DM is yet to be evaluated critically.

Photobiomodulation therapy in age-related macular degeneration

PURPOSE OF REVIEW

To review the available data supporting the use of photobiomodulation therapy (PBT) in the treatment of age-related macular degeneration (AMD).

RECENT FINDINGS

PBT might be used in treating nonexudative AMD. Limited evidence suggests that exudative AMD may also benefit from PBT.

SUMMARY

The optimal device would deliver doses of 60 J/cm2 or more with a multiwavelength composition through the pupil over short treatment intervals. Safe upper limits have not been established. More studies are needed to evaluate the efficacy of PBT in treating exudative and nonexudative AMD.

Neurovascular unit in diabetic retinopathy: pathophysiological roles and potential therapeutical targets

Diabetic retinopathy (DR), one of the common complications of diabetes, is the leading cause of visual loss in working-age individuals in many industrialized countries. It has been traditionally regarded as a purely microvascular disease in the retina. However, an increasing number of studies have shown that DR is a complex neurovascular disorder that affects not only vascular structure but also neural tissue of the retina. Deterioration of neural retina could precede microvascular abnormalities in the DR, leading to microvascular changes. Furthermore, disruption of interactions among neurons, vascular cells, glia and local immune cells, which collectively form the neurovascular unit, is considered to be associated with the progression of DR early on in the disease. Therefore, it makes sense to develop new therapeutic strategies to prevent or reverse retinal neurodegeneration, neuroinflammation and impaired cell-cell interactions of the neurovascular unit in early stage DR. Here, we present current perspectives on the pathophysiology of DR as a neurovascular disease, especially at the early stage. Potential novel treatments for preventing or reversing neurovascular injuries in DR are discussed as well.

The retinal pigment epithelium: Development, injury responses, and regenerative potential in mammalian and non-mammalian systems

Diseases that result in retinal pigment epithelium (RPE) degeneration, such as age-related macular degeneration (AMD), are among the leading causes of blindness worldwide. Atrophic (dry) AMD is the most prevalent form of AMD and there are currently no effective therapies to prevent RPE cell death or restore RPE cells lost from AMD. An intriguing approach to treat AMD and other RPE degenerative diseases is to develop therapies focused on stimulating endogenous RPE regeneration. For this to become feasible, a deeper understanding of the mechanisms underlying RPE development, injury responses and regenerative potential is needed. In mammals, RPE regeneration is extremely limited; small lesions can be repaired by the expansion of adjacent RPE cells, but large lesions cannot be repaired as remaining RPE cells are unable to functionally replace lost RPE tissue. In some injury paradigms, RPE cells proliferate but do not regenerate a morphologically normal monolayer, while in others, proliferation is pathogenic and results in further disruption to the retina. This is in contrast to non-mammalian vertebrates, which possess tremendous RPE regenerative potential. Here, we discuss what is known about RPE formation during development in mammalian and non-mammalian vertebrates, we detail the processes by which RPE cells respond to injury, and we describe examples of RPE-to-retina and RPE-to-RPE regeneration in non-mammalian vertebrates. Finally, we outline barriers to RPE-dependent regeneration in mammals that could potentially be overcome to stimulate a regenerative response from the RPE.

Retinal Vascular Endothelial Cell Dysfunction and Neuroretinal Degeneration in Diabetic Patients

Diabetes mellitus (DM) has become a vital societal problem as epidemiological studies demonstrate the increasing incidence of type 1 and type 2 diabetes. Lesions observed in the retina in the course of diabetes, referred to as diabetic retinopathy (DR), are caused by vascular abnormalities and are ischemic in nature. Vascular lesions in diabetes pertain to small vessels (microangiopathy) and involve precapillary arterioles, capillaries and small veins. Pericyte loss, thickening of the basement membrane, and damage and proliferation of endothelial cells are observed. Endothelial cells (monolayer squamous epithelium) form the smooth internal vascular lining indispensable for normal blood flow. Breaking its continuity initiates blood coagulation at that site. The endothelium controls the process of exchange of chemical substances (nutritional, regulatory, waste products) between blood and the retina, and blood cell passing through the vascular wall. Endothelial cells produce biologically active substances involved in blood coagulation, regulating vascular wall tension and stimulating neoangiogenesis. On the other hand, recent studies have demonstrated that diabetic retinopathy may be not only a microvascular disease, but is a result of neuroretinal degeneration. Neuroretinal degeneration appears structurally, as neural apoptosis of amacrine and Muller cells, reactive gliosis, ganglion cell layer/inner plexiform (GCL) thickness, retinal thickness, and retinal nerve fiber layer thickness, and a reduction of the neuroretinal rim in minimum rim width (MRW) and functionally as an abnormal electroretinogram (ERG), dark adaptation, contrast sensitivity, color vision, and microperimetric test. The findings in early stages of diabetic retinopathy may precede microvascular changes of this disease. Furthermore, the article's objective is to characterize the factors and mechanisms conducive to microvascular changes and neuroretinal apoptosis in diabetic retinopathy. Only when all the measures preventing vascular dysfunction are determined will the risk of complications in the course of diabetes be minimized.

The Expression Levels and Cellular Localization of Pigment Epithelium Derived Factor (PEDF) in Mouse Testis: Its Possible Involvement in the Differentiation of Spermatogonial Cells

Pigment epithelium derived factor (PEDF) is a multifunctional secretory soluble glycoprotein that belongs to the serine protease inhibitor (serpin) family. It was reported to have neurotrophic, anti-angiogenic and anti-tumorigenic activity. Recently, PEDF was found in testicular peritubular cells and it was assumed to be involved in the avascular nature of seminiferous tubules. The aim of this study was to determine the cellular origin, expression levels and target cells of PEDF in testicular tissue of immature and adult mice under physiological conditions, and to explore its possible role in the process of spermatogenesis in vitro. Using immunofluorescence staining, we showed that PEDF was localized in spermatogenic cells at different stages of development as well as in the somatic cells of the testis. Its protein levels in testicular homogenates and Sertoli cells supernatant showed a significant decrease with age. PEDF receptor (PEDF-R) was localized within the seminiferous tubule cells and in the interstitial cells compartment. Its RNA expression levels showed an increase with age until 8 weeks followed by a decrease. RNA levels of PEDF-R showed the opposite trend of the protein. Addition of PEDF to cultures of isolated cells from the seminiferous tubules did not changed their proliferation rate, however, a significant increase was observed in number of meiotic/post meiotic cells at 1000 ng/mL of PEDF; indicating an in vitro differentiation effect. This study may suggest a role for PEDF in the process of spermatogenesis.

A Safe GDNF and GDNF/BDNF Controlled Delivery System Improves Migration in Human Retinal Pigment Epithelial Cells and Survival in Retinal Ganglion Cells: Potential Usefulness in Degenerative Retinal Pathologies

We assessed the sustained delivery effect of poly (lactic-co-glycolic) acid (PLGA)/vitamin E (VitE) microspheres (MSs) loaded with glial cell-derived neurotrophic factor (GDNF) alone (GDNF-MSs) or combined with brain-derived neurotrophic factor (BDNF; GDNF/BDNF-MSs) on migration of the human adult retinal pigment epithelial cell-line-19 (ARPE-19) cells, primate choroidal endothelial (RF/6A) cells, and the survival of isolated mouse retinal ganglion cells (RGCs). The morphology of the MSs, particle size, and encapsulation efficiencies of the active substances were evaluated. In vitro release, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell viability, terminal deoxynucleotidyl transferase (TdT) deoxyuridine dUTP nick-end labelling (TUNEL) apoptosis, functional wound healing migration (ARPE-19; migration), and (RF/6A; angiogenesis) assays were conducted. The safety of MS intravitreal injection was assessed using hematoxylin and eosin, neuronal nuclei (NeuN) immunolabeling, and TUNEL assays, and RGC in vitro survival was analyzed. MSs delivered GDNF and co-delivered GDNF/BDNF in a sustained manner over 77 days. The BDNF/GDNF combination increased RPE cell migration, whereas no effect was observed on RF/6A. MSs did not alter cell viability, apoptosis was absent in vitro, and RGCs survived in vitro for seven weeks. In mice, retinal toxicity and apoptosis was absent in histologic sections. This delivery strategy could be useful as a potential co-therapy in retinal degenerations and glaucoma, in line with future personalized long-term intravitreal treatment as different amounts (doses) of microparticles can be administered according to patients’ needs.

The innate immune system in diabetic retinopathy

The prevalence of diabetes has been rising steadily in the past half-century, along with the burden of its associated complications, including diabetic retinopathy (DR). DR is currently the most common cause of vision loss in working-age adults in the United States. Historically, DR has been diagnosed and classified clinically based on what is visible by fundoscopy; that is vasculature alterations. However, recent technological advances have confirmed pathology of the neuroretina prior to any detectable vascular changes. These, coupled with molecular studies, and the positive impact of anti-inflammatory therapeutics in DR patients have highlighted the central involvement of the innate immune system. Reminiscent of the systemic impact of diabetes, immune dysregulation has become increasingly identified as a key element of the pathophysiology of DR by interfering with normal homeostatic systems. This review uses the growing body of literature across various model systems to demonstrate the clear involvement of all three pillars of the immune system: immune-competent cells, mediators, and the complement system. It also demonstrates how the relative contribution of each of these requires more extensive analysis, including in human tissues over the continuum of disease progression. Finally, although this review demonstrates how the complex interactions of the immune system pose many more questions than answers, the intimately connected nature of the three pillars of the immune system may also point to possible new targets to reverse or even halt reverse retinopathy.

Pigment Epithelium-Derived Factor (PEDF) Receptors Are Involved in Survival of Retinal Neurons

The demise of retinal ganglion cells (RGCs) is characteristic of diseases of the retina such as glaucoma and diabetic or ischemic retinopathies. Pigment epithelium-derived factor (PEDF) is a multifunctional secreted protein that mediates neuroprotection and inhibition of angiogenesis in the retina. We have studied expression and regulation of two of several receptors for PEDF, patatin-like phospholipase 2 gene product/PEDF-R and laminin receptor (LR), in serum-starved RGC under normoxia and hypoxia and investigated their involvement in the survival of retinal neuronal cells. We show that PEDF-R and LR are co-expressed in RGC and R28 retinal precursor cells. Expression of both receptors was enhanced in the presence of complex secretions from retinal glial (Müller) cells and upregulated by VEGF and under hypoxic conditions. PEDF-R- and LR-knocked-down cells demonstrated a markedly attenuated expression of anti-apoptotic Bcl-2 family members (Bcl-2, Bcl-xL) and neuroprotective mediators (PEDF, VEGF, BDNF) suggesting that both PEDF-R and LR mediate pro-survival effects of PEDF on RGC. While this study does not provide evidence for a differential survival-promoting influence of either PEDF-R or LR, it nevertheless highlights the importance of both PEDF receptors for the viability of retinal neurons.

Improving cell survival and engraftment in vivo via layer-by-layer nanocoating of hESC-derived RPE cells

BACKGROUND

Human embryonic stem cell-derived retinal pigment epithelial (hESC-RPE) cell transplants have served as a cell therapy for treating retinal degenerative diseases. However, how to optimize the survival and engraftment of hESC-RPE cells is a great challenge.

METHODS

Here, we report hESC-RPE cells that are embedded with polyelectrolytes gelatin and alginate by layer-by-layer (LbL) self-assembly technique, based on the opposite charge of alternate layers. Cells were assessed for cell survival, immunogenicity, and function in vitro and in vivo.

RESULTS

This strategy obviously decreased the immunogenicity of hESC-RPE cells without affecting its activity. LbL-RPE cell transplants into the subretinal space of Royal College of Surgeons (RCS) rats optimized cell engraftment and decreased immunogenicity compared to untreated RPE cell transplants (immunosuppression was not used during the 21-week study). Visual-functional assay with electroretinogram recordings (ERGs) also showed higher B wave amplitudes in RCS rats with LbL-RPE cell transplants.

CONCLUSIONS

We demonstrate that transplanted LbL-RPE cells have better viability and grafting efficiency, optimized immunogenicity, and visual function. Therefore, LbL engineering is a promising method to increase the efficacy of hESC-RPE cell transplantation.

Functional assessment of cryopreserved clinical grade hESC-RPE cells as a qualified cell source for stem cell therapy of retinal degenerative diseases

The biosafety and efficiency of transplanting retinal pigment epithelial (RPE) cells derived from both human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have been evaluated in phase I and phase II clinical trials. For further large-scale application, cryopreserved RPE cells must be used; thus, it is highly important to investigate the influence of cryopreservation and thawing on the biological characteristics of hESC-RPE cells and their post-transplantation vision-restoring function. Here, via immunofluorescence, qPCR, transmission electron microscopy, transepithelial electrical resistance, and enzyme-linked immunosorbent assays (ELISAs), we showed that cryopreserved hESC-RPE cells retained the specific gene expression profile, morphology, ultrastructure, and maturity-related functions of induced RPE cells. Additionally, cryopreserved hESC-RPE cells exhibited a polarized monolayer, tight junction, and gap junction structure and an in vitro nanoparticle phagocytosis capability similar to those of induced hESC-RPE cells. However, the level of pigment epithelium-derived factor (PEDF) secretion was significantly decreased in cryopreserved hESC-RPE cells. Royal College of Surgeons rats with cryopreserved hESC-RPE cells engrafted into the subretinal space exhibited a significant decrease in the b-wave amplitude compared with rats engrafted with induced hESC-RPE cells at 4 weeks post transplantation. However, the difference disappeared at 8 weeks and 12 weeks post operation. No significant difference in the outer nuclear layer (ONL) thickness was observed between the two groups. Our data showed that even after cryopreservation and thawing, cryopreserved hESC-RPE cells are still qualified as a donor cell source for cell-based therapy of retinal degenerative diseases.

Epigallocatechin-3-Gallate and PEDF 335 Peptide, 67LR Activators, Attenuate Vasogenic Edema, and Astroglial Degeneration Following Status Epilepticus

Non-integrin 67-kDa laminin receptor (67LR) is involved in cell adherence to the basement membrane, and it regulates the interactions between laminin and other receptors. The dysfunction of 67LR leads to serum extravasation via blood-brain barrier (BBB) disruption. Polyphenol (–)-epigallocatechin-3-O-gallate (EGCG) and pigment epithelium-derived factor (PEDF) bind to 67LR and inhibit neovascularization. Therefore, in the present study, we investigated the effects of EGCG and NU335, a PEDF-derive peptide, on BBB integrity and their possible underlying mechanisms against vasogenic edema formation induced by status epilepticus (SE, a prolonged seizure activity). Following SE, both EGCG and NU335 attenuated serum extravasation and astroglial degeneration in the rat piriform cortex (PC). Both EGCG and NU335 reversely regulated phosphatidylinositol 3 kinase (PI3K)/AKT–eNOS (endothelial nitric oxide synthase) mediated BBB permeability and aquaporin 4 (AQP4) expression in endothelial cells and astrocytes through the p38 mitogen-activated protein kinase (p38 MAPK) and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathways, respectively. Furthermore, EGCG and NU335 decreased p47Phox (a nicotinamide adenine dinucleotide phosphate oxidase subunit) expression in astrocytes under physiological and post-SE conditions. Therefore, we suggest that EGCG and PEDF derivatives may activate 67LR and its downstream effectors, and they may be considerable anti-vasogenic edema agents.

In Vitro Maturation of Retinal Pigment Epithelium Is Essential for Maintaining High Expression of Key Functional Genes

Age-related macular degeneration (AMD) is the leading cause of blindness in the industrialized world. AMD is associated with dysfunction and atrophy of the retinal pigment epithelium (RPE), which provides critical support for photoreceptor survival and function. RPE transplantation is a promising avenue towards a potentially curative treatment for early stage AMD patients, with encouraging reports from animal trials supporting recent progression toward clinical treatments. Mature RPE cells have been reported to be superior, but a detailed investigation of the specific changes in the expression pattern of key RPE genes during maturation is lacking. To understand the effect of maturity on RPE, we investigated transcript levels of 19 key RPE genes using ARPE-19 cell line and human embryonic stem cell-derived RPE cultures. Mature RPE cultures upregulated PEDF, IGF-1, CNTF and BDNF—genes that code for trophic factors known to enhance the survival and function of photoreceptors. Moreover, the mRNA levels of these genes are maximized after 42 days of maturation in culture and lost upon dissociation to single cells. Our findings will help to inform future animal and human RPE transplantation efforts.

Pigment epithelium-derived factor (PEDF) and PEDF-receptor in the adult mouse brain: Differential spatial/temporal localization pattern

Pigment epithelium-derived factor (PEDF) is a multifunctional protein which was initially described in the retina, although it is also present in other tissues. It functions as an antioxidant agent promoting neuronal survival. Recently, a PEDF receptor has shown an elevated binding affinity for PEDF. There are no relevant data regarding the distribution of both proteins in the brain, therefore the main goal of this work was to investigate the spatiotemporal presence of PEDF and PEDFR in the adult mouse brain, and to determine the PEDF blood level in mouse and human. The localization of both proteins was analyzed by different experimental methods such as immunohistochemistry, western-blotting, and also by enzyme-linked immunosorbent assay. Differential expression was found in some telencephalic structures and positive signals for both proteins were detected in the cerebellum. The magnitude of the PEDFR labeling pattern was higher than PEDF and included some cortical and subventricular areas. Age-dependent changes in intensity of both protein immunoreactions were found in the cortical and hippocampal areas with greater reactivity between 4 and 8 months of age, whilst others, like the subventricular zones, these differences were more evident for PEDFR. Although ubiquitous presence was not found in the brain for these two proteins, their relevant functions must not be underestimated. It has been described that PEDF plays an important role in neuroprotection and data provided in the present work represents the first extensive study to understand the relevance of these two proteins in specific brain areas.

[The state of ocular neurosensory apparatus in diabetes mellitus]

Diabetes mellitus is one of the most common chronic diseases in the world. Among its late complications that can lead to disability is diabetic neuropathy (DN). Patients diagnosed with DN often also have diabetic retinopathy (DR). According to available data, DR prevalence ranges from 30 to 60%. For a long time, DR has been considered a microvascular disease. However, more data has been emerging recently that indicates the presence of other components in the pathophysiology of DR. Neurodegenerative changes in the retina can be detected in patients with diabetes mellitus before clinical signs of diabetic retinopathy appear. Accumulation of extracellular glutamate, oxidative stress, reduction of neuroprotective factors synthesized by the retina are all believed to lead to neuronal apoptosis and glial cell dysfunction. This cascade of reactions subsequently causes disruption of the hematoretinal barrier and damage to neurovascular apparatus of the retina. This results in the defeat of capillaries of the retinal vascular system, which is consistently characterized by the loss of pericytes and the formation of unperfusable capillaries. The concept of neurodegeneration being an early component of DR provides an opportunity to explore alternative therapies to prevent the onset of vision loss in diabetes mellitus.

Pigment Epithelium-derived Factor Protects Retinal Pigment Epithelial Cells Against Cytotoxicity "In Vitro"

Oxidative stress has been implicated in neurodegenerative diseases, such as age-related macular degeneration. Hydrogen peroxide and sodium iodate can mediate oxidative injury. Sodium iodate induces a selective retinal degeneration targeting the RPE. We describe a method of chronic sodium iodate-mediated injury on RPE cells that may serve to evaluate protective factors against oxidative stress. Cytotoxicity and cell viability curves of ARPE-19 cells with sodium iodate were generated. The antioxidant pigment epithelium-derived factor decreased sodium iodate-mediated cytotoxicity without affecting ARPE-19 cell viability. A cell culture system to evaluate protection against oxidative stress injury with PEDF is discussed.

Human vitreous in proliferative diabetic retinopathy: Characterization and translational implications

Diabetic retinopathy (DR) is one of the leading causes of visual impairment in the working-age population. DR is a progressive eye disease caused by long-term accumulation of hyperglycaemia-mediated pathological alterations in the retina of diabetic patients. DR begins with asymptomatic retinal abnormalities and may progress to advanced-stage proliferative diabetic retinopathy (PDR), characterized by neovascularization or preretinal/vitreous haemorrhages. The vitreous, a transparent gel that fills the posterior cavity of the eye, plays a vital role in maintaining ocular function. Structural and molecular alterations of the vitreous, observed during DR progression, are consequences of metabolic and functional modifications of the retinal tissue. Thus, vitreal alterations reflect the pathological events occurring at the vitreoretinal interface. These events are caused by hypoxic, oxidative, inflammatory, neurodegenerative, and leukostatic conditions that occur during diabetes. Conversely, PDR vitreous can exert pathological effects on the diabetic retina, resulting in activation of a vicious cycle that contributes to disease progression. In this review, we recapitulate the major pathological features of DR/PDR, and focus on the structural and molecular changes that characterize the vitreal structure and composition during DR and progression to PDR. In PDR, vitreous represents a reservoir of pathological signalling molecules. Therefore, in this review we discuss how studying the biological activity of the vitreous in different in vitro, ex vivo, and in vivo experimental models can provide insights into the pathogenesis of PDR. In addition, the vitreous from PDR patients can represent a novel tool to obtain preclinical experimental evidences for the development and characterization of new therapeutic drug candidates for PDR therapy.

Retinal Neurodegeneration as an Early Manifestation of Diabetic Eye Disease and Potential Neuroprotective Therapies

PURPOSE OF REVIEW

Diabetic retinopathy (DR) is a major cause of visual impairment and blindness throughout the world. Microvascular changes have long been regarded central to disease pathogenesis. In recent years, however, retinal neurodegeneration is increasingly being hypothesized to occur prior to the vascular changes classically associated with DR and contribute to disease pathogenesis.

RECENT FINDINGS

There is growing structural and functional evidence from human and animal studies that suggests retinal neurodegeneration to be an early component of DR. Identification of new therapeutic targets is an ongoing area of research with several different molecules undergoing testing in animal models for their neuroprotective properties and for possible use in humans. Retinal neurodegeneration may play a central role in DR pathogenesis. As new therapies are developed, it will be important to develop criteria for clinically defining retinal neurodegeneration. A standardization of the methods for monitoring neurodegeneration along with more sensitive means of detecting preclinical damage is also needed.

Pigment epithelium-derived factor mediates retinal ganglion cell neuroprotection by suppression of caspase-2

Retinal ganglion cells (RGCs) undergo rapid cell death by apoptosis after injury but can be rescued by suppression of caspase-2 (CASP2) using an siRNA to CASP2 (siCASP2). Pigment epithelium-derived factor (PEDF), has neuroprotective and anti-angiogenic functions and protects RGC from death. The purpose of this study was to investigate if suppression of CASP2 is a possible mechanism of neuroprotection by PEDF in RGC. Adult rat retinal cells were treated in vitro with sub-optimal and optimal concentrations of siCASP2 and PEDF and levels of CASP2 mRNA and RGC survival were then quantified. Optic nerve crush (ONC) injury followed by intravitreal injections of siCASP2 or PEDF and eye drops of PEDF-34 were also used to determine CASP2 mRNA and protein reduction. Results showed that PEDF and PEDF-34 significantly suppressed CASP2 mRNA in culture, by 1.85- and 3.04-fold, respectively, and increased RGC survival by 63.2 ± 3.8% and 81.9 ± 6.6%, respectively compared to cells grown in Neurobasal-A alone. RGC survival was significantly reduced in glial proliferation inhibited and purified RGC cultures suggesting that some of the effects of PEDF were glia-mediated. In addition, intravitreal injection of PEDF and eye drops of PEDF-34 after ONC also suppressed CASP2 mRNA levels by 1.82- and 3.89-fold and cleaved caspase-2 (C-CASP2) protein levels by 4.98- and 8.93-fold compared to ONC + PBS vehicle groups, respectively, without affecting other executioner caspases. Treatment of retinal cultures with PEDF and PEDF-34 promoted the secretion of neurotrophic factors (NTF) into the culture media, of which brain-derived neurotrophic factor (BDNF) caused the greatest reduction in CASP2 mRNA and C-CASP2 protein. The neuroprotective effects of PEDF were blocked by a polyclonal antibody and PEDF suppressed key elements in the apoptotic pathway. In conclusion, this study shows that some of the RGC neuroprotective effects of PEDF is regulated through suppression of CASP2 and downstream apoptotic signalling molecules.

Photoreceptor degeneration in microphthalmia (Mitf) mice: partial rescue by pigment epithelium-derived factor

Dysfunction and loss of the retinal pigment epithelium (RPE) are hallmarks of retinal degeneration, but the underlying pathogenetic processes are only partially understood. Using mice with a null mutation in the transcription factor gene Mitf, in which RPE deficiencies are associated with retinal degeneration, we evaluated the role of trophic factors secreted by the RPE in retinal homeostasis. In such mice, the thickness of the outer nuclear layer (ONL) is as in wild type up to postnatal day 10, but then is progressively reduced, associated with a marked increase in the number of apoptotic cells and a decline in staining for rhodopsin. We show that retinal degeneration and decrease in rhodopsin staining can be prevented partially in three different ways: first, by recombining mutant-derived postnatal retina with postnatal wild-type RPE in tissue explant cultures; second, by adding to cultured mutant retina the trophic factor pigment epithelium-derived factor (PEDF; also known as SERPINF1), which is normally produced in RPE under the control of Mitf; and third, by treating the eyes of Mitf mutant mice in vivo with drops containing a bioactive PEDF 17-mer peptide. This latter treatment also led to marked increases in a number of rod and cone genes. The results indicate that RPE-derived trophic factors, in particular PEDF, are instrumental in retinal homeostasis, and suggest that PEDF or its bioactive fragments may have therapeutic potential in RPE deficiency-associated retinal degeneration.

Diabetic Retinopathy-An Underdiagnosed and Undertreated Inflammatory, Neuro-Vascular Complication of Diabetes

Diabetes mellitus is a world-wide epidemic and diabetic retinopathy, a devastating, vision-threatening condition, is one of the most common diabetes-specific complications. Diabetic retinopathy is now recognized to be an inflammatory, neuro-vascular complication with neuronal injury/dysfunction preceding clinical microvascular damage. Importantly, the same pathophysiologic mechanisms that damage the pancreatic β-cell (e.g., inflammation, epigenetic changes, insulin resistance, fuel excess, and abnormal metabolic environment), also lead to cell and tissue damage causing organ dysfunction, elevating the risk of all complications, including diabetic retinopathy. Viewing diabetic retinopathy within the context whereby diabetes and all its complications arise from common pathophysiologic factors allows for the consideration of a wider array of potential ocular as well as systemic treatments for this common and devastating complication. Moreover, it also raises the importance of the need for methods that will provide more timely detection and prediction of the course in order to address early damage to the neurovascular unit prior to the clinical observation of microangiopathy. Currently, treatment success is limited as it is often initiated far too late and after significant neurodegeneration has occurred. This forward-thinking approach of earlier detection and treatment with a wider array of possible therapies broadens the physician's armamentarium and increases the opportunity for prevention and early treatment of diabetic retinopathy with preservation of good vision, as well the prevention of similar destructive processes occurring among other organs.

Pluripotent Stem Cells for Retinal Tissue Engineering: Current Status and Future Prospects

The retina is a very fine and layered neural tissue, which vitally depends on the preservation of cells, structure, connectivity and vasculature to maintain vision. There is an urgent need to find technical and biological solutions to major challenges associated with functional replacement of retinal cells. The major unmet challenges include generating sufficient numbers of specific cell types, achieving functional integration of transplanted cells, especially photoreceptors, and surgical delivery of retinal cells or tissue without triggering immune responses, inflammation and/or remodeling. The advances of regenerative medicine enabled generation of three-dimensional tissues (organoids), partially recreating the anatomical structure, biological complexity and physiology of several tissues, which are important targets for stem cell replacement therapies. Derivation of retinal tissue in a dish creates new opportunities for cell replacement therapies of blindness and addresses the need to preserve retinal architecture to restore vision. Retinal cell therapies aimed at preserving and improving vision have achieved many improvements in the past ten years. Retinal organoid technologies provide a number of solutions to technical and biological challenges associated with functional replacement of retinal cells to achieve long-term vision restoration. Our review summarizes the progress in cell therapies of retina, with focus on human pluripotent stem cell-derived retinal tissue, and critically evaluates the potential of retinal organoid approaches to solve a major unmet clinical need—retinal repair and vision restoration in conditions caused by retinal degeneration and traumatic ocular injuries. We also analyze obstacles in commercialization of retinal organoid technology for clinical application.