Novel engineered, membrane-localized variants of vascular endothelial growth factor (VEGF) protect retinal ganglion cells: a proof-of-concept study

Trolox aids coenzyme Q10 in neuroprotection against NMDA induced damage via upregulation of VEGF in rat model of glutamate excitotoxicity

Glutamate induced damage to retinal ganglion cells (RGCs) requires tight physiological regulation of the N-methyl-D-aspartate (NMDA) receptors. Previously, studies have demonstrated the neuroprotective abilities of antioxidants like coenzyme Q10 (CoQ10) and vitamin E analogs like α-tocopherol against neuropathies resulting from NMDA insult, but have failed to shed light on the effect of CoQ10 and trolox, a hydrophilic analog of vitamin E, on glaucomatous neurodegeneration. In the current study, we wanted to investigate whether the combined effect of trolox with CoQ10 could alleviate NMDA-induced death of retinal cells while also trying to elucidate the underlying mechanism in relation to the yet unexplained role of vascular endothelial growth factor (VEGF) in NMDA-mediated excitotoxicity. After successful NMDA-induced degeneration, we followed it up with the treatment of combination of Trolox and CoQ10. The structural damage by NMDA was repaired significantly and retina retained structural integrity comparable to levels of control in the treatment group of Trolox and CoQ10. Detection of ROS generation after NMDA insult showed that together, Trolox and CoQ10 could significantly bring down the high levels of free radicals while also rescuing mitochondrial membrane potential (MMP). A significant increase in NMDA receptor Grin2A by CoQ10 alone as well as by CoQ10 and trolox was accompanied by a lowered Grin2B receptor expression, suggesting neuroprotective action of Trolox and CoQ10. Subsequently, lowered VEGFR1 and VEGFR2 receptor expression by NMDA treatment also recovered when subjected to combined treatment of Trolox and CoQ10. Western blot analyses also indicated the same whereby Trolox and CoQ10 could increase the diminished levels of phosphorylated VEGFR2. Immunofluorescence studies also indicated a positive correlation between recovered VEGFR2 and NMDAR2A levels and diminished levels of NMDAR2D, confirming the results obtained by RT-PCR analysis. This is the first report in our knowledge that demonstrates the efficacy of trolox in combination with CoQ10 highlighting the importance of maintaining VEGF levels that are lowered in ocular diseases due to NMDA-related toxicities.

The concept of gene therapy for glaucoma: the dream that has not come true yet

Gene therapies, despite of being a relatively new therapeutic approach, have a potential to become an important alternative to current treatment strategies in glaucoma. Since glaucoma is not considered a single gene disease, the identified goals of gene therapy would be rather to provide neuroprotection of retinal ganglion cells, especially, in intraocular-pressure-independent manner. The most commonly reported type of vector for gene delivery in glaucoma studies is adeno-associated virus serotype 2 that has a high tropism to retinal ganglion cells, resulting in long-term expression and low immunogenic profile. The gene therapy studies recruit inducible and genetic animal models of optic neuropathy, like DBA/2J mice model of high-tension glaucoma and the optic nerve crush-model. Reported gene therapy-based neuroprotection of retinal ganglion cells is targeting specific genes translating to growth factors (i.e., brain derived neurotrophic factor, and its receptor TrkB), regulation of apoptosis and neurodegeneration (i.e., Bcl-xl, Xiap, FAS system, nicotinamide mononucleotide adenylyl transferase 2, Digit3 and Sarm1), immunomodulation (i.e., Crry, C3 complement), modulation of neuroinflammation (i.e., erythropoietin), reduction of excitotoxicity (i.e., CamKIIα) and transcription regulation (i.e., Max, Nrf2). On the other hand, some of gene therapy studies focus on lowering intraocular pressure, by impacting genes involved in both, decreasing aqueous humor production (i.e., aquaporin 1), and increasing outflow facility (i.e., COX2, prostaglandin F2α receptor, RhoA/RhoA kinase signaling pathway, MMP1, Myocilin). The goal of this review is to summarize the current state-of-art and the direction of development of gene therapy strategies for glaucomatous neuropathy.

In vitro protective effect of recombinant prominin-1 combined with microRNA-29b on N-methyl-D-aspartate-induced excitotoxicity in retinal ganglion cells

AIM

To determine the in vitro protective effect of recombinant prominin-1 (Prominin-1)+microRNA-29b (P1M29) on N-methyl-D-aspartate (NMDA)-induced excitotoxicity in retinal ganglion cells (RGCs).

METHODS

RGC-5 cells were cultured, and NMDA-induced excitotoxicity at the range of 100-800 µmol/L was assessed using the MTT assay. NMDA (800 µmol/L) was selected as the appropriate concentration for preparing the cell model. To evaluate the protective effect of P1M29 on the cell model, Prominin-1 was added at the concentration of 1-6 ng/mL for 48h, and the cell survival was investigated with/without microRNA-29b. After obtaining the appropriate concentration and time of P1M29 at 48h, real-time polymerase chain reaction (PCR) was utilized to detect the relative mRNA expression of vascular endothelial growth factor (VEGF) and transforming growth factor (TGF)-β2. Western blot detection was applied to measure the phosphorylation levels of protein kinase B (AKT) and extracellular regulated protein kinases (ERK) in RGC-5 cells after treatment with Prominin-1. Apoptosis study of the cell model was conducted by flow cytometry for estimating the anti-apoptotic effect of P1M29. Immunofluorescence analysis was used to analyze the expression levels of VEGF and TGF-β2.

RESULTS

MTT cytotoxicity assays demonstrated that P1M29 group had significantly higher cell survival rate than Prominin-1 group (P<0.05). Real-time PCR data indicated that the expression levels of VEGF were significantly increased in both Prominin-1 and P1M29 groups compared NMDA and microRNA-29b group (P<0.05), while TGF-β2 were significantly decreased in both microRNA-29b and P1M29 groups compared NMDA and Prominin-1 group (P<0.05). Western blot results showed that both Prominin-1 and P1M29 groups significantly increased the phosphorylation levels of AKT and ERK compared to NMDA and microRNA-29b groups (P<0.05). Flow cytometry analysis revealed that P1M29 could prevent RGC-5 cell apoptosis in the early stage of apoptosis, while immunofluorescence results showed that P1M29 group had higher expression of VEGF and lower expression of TGF-β2 with a stronger green fluorescence than NMDA group.

CONCLUSION

Prominin-1 combined with microRNA-29b can provide a suitable therapeutic option for ameliorating NMDA-induced excitotoxicity in RGC-5 cells.

Pathophysiology of Retinopathy of Prematurity

Retinopathy of prematurity (ROP) is a complex disease involving development of the neural retina, ocular circulations, and other organ systems of the premature infant. The external stresses of the ex utero environment also influence the pathophysiology of ROP through interactions among retinal neural, vascular, and glial cells. There is variability among individual infants and presentations of the disease throughout the world, making ROP challenging to study. The methods used include representative animal models, cell culture, and clinical studies. This article describes the impact of maternal-fetal interactions; stresses that the preterm infant experiences; and biologic pathways of interest, including growth factor effects and cell-cell interactions, on the complex pathophysiology of ROP phenotypes in developed and emerging countries.

NGF Prevents Loss of TrkA/VEGFR2 Cells, and VEGF Isoform Dysregulation in the Retina of Adult Diabetic Rats

Among the factors involved in diabetic retinopathy (DR), nerve growth factor (NGF) and vascular endothelial growth factor A (VEGFA) have been shown to affect both neuronal survival and vascular function, suggesting that their crosstalk might influence DR outcomes. To address this question, the administration of eye drops containing NGF (ed-NGF) to adult Sprague Dawley rats receiving streptozotocin (STZ) intraperitoneal injection was used as an experimental paradigm to investigate NGF modulation of VEGFA and its receptor VEGFR2 expression. We show that ed-NGF treatment prevents the histological and vascular alterations in STZ retina, VEGFR2 expression decreased in GCL and INL, and preserved the co-expression of VEGFR2 and NGF-tropomyosin-related kinase A (TrkA) receptor in retinal ganglion cells (RGCs). The WB analysis confirmed the NGF effect on VEGFR2 expression and activation, and showed a recovery of VEGF isoform dysregulation by suppressing STZ-induced VEGFA₁₂₁ expression. Reduction in inflammatory and pro-apoptotic intracellular signals were also found in STZ+NGF retina. These findings suggest that ed-NGF administration might favor neuroretina protection, and in turn counteract the vascular impairment by regulating VEGFR2 and/or VEGFA isoform expression during the early stages of the disease. The possibility that an increase in the NGF availability might contribute to the switch from the proangiogenic/apoptotic to the neuroprotective action of VEGF is discussed.

New strategies for neuro protection in glaucoma

Glaucoma is a progressive, irreversible loss of retinal ganglion cells (RGCs) and axons that results in characteristic optic atrophy and corresponding progressive visual field defect. The exact mechanisms underlying glaucomatous neuron loss are not clear. The main risk factor for glaucoma onset and development is high intraocular pressure (IOP), however traditional IOP-lowering therapies are often not sufficient to prevent degeneration of RGCs and the vision loss may progress, indicating the need for complementary neuroprotective therapy. This review summarizes the progress for neuro protection in glaucoma in recent 5 years, including modulation of neuroinflammation, gene and cell therapy, dietary supplementation, and sustained-release system.

Novel engineered, membrane-tethered VEGF-A variants promote formation of filopodia, proliferation, survival, and cord or tube formation by endothelial cells via persistent VEGFR2/ERK signaling and activation of CDC42/ROCK pathways

Therapeutic angiogenesis would be clinically valuable in situations such as peripheral vascular disease in diabetic patients and tissue reperfusion following ischemia or injury, but approaches using traditional isoforms of vascular endothelial growth factor-A (VEGF) have had little success. The isoform VEGF165 is both soluble and matrix-associated, but can cause pathologic vascular changes. Freely diffusible VEGF121 is not associated with pathologic angiogenesis, but its failure to remain in the vicinity of the targeted area presents therapeutic challenges. In this study, we evaluate the cellular effects of engineered VEGF variants that tether extracellular VEGF121 to the cell membrane with the goal of activating VEGF receptor 2 (VEGFR2) in a sustained, autologous fashion in endothelial cells. When expressed by primary human retinal endothelial cells (hRECs), the engineered, membrane-tethered variants eVEGF-38 and eVEGF-53 provide a lasting VEGF signal that induces cell proliferation and survival, increases endothelial permeability, promotes the formation of a cord/tube network, and stimulates the formation of elongated filopodia on the endothelial cells. The engineered VEGF variants activate VEGFR2, MAPK/ERK, and the Rho GTPase mediators CDC42 and ROCK, activities that are required for the formation of the elongated filopodia. The sustained, pro-angiogenic activities induced by eVEGF-38 and eVEGF-53 support the potential of engineered VEGF variants-overexpressing endothelial cells as a novel combination of gene and cell-based therapeutic strategy for stimulating endothelial cell-autologous therapeutic angiogenesis.

Protection of retinal ganglion cells in glaucoma: Current status and future

Glaucoma is a neuropathic disease that causes optic nerve damage, loss of retinal ganglion cells (RGCs), and visual field defects. Most glaucoma patients have no early signs or symptoms. Conventional pharmacological glaucoma medications and surgeries that focus on lowering intraocular pressure are not sufficient; RGCs continue to die, and the patient's vision continues to decline. Recent evidence has demonstrated that neuroprotective approaches could be a promising strategy for protecting against glaucoma. In the case of glaucoma, neuroprotection aims to prevent or slow down disease progression by mitigating RGCs death and optic nerve degeneration. Notably, new pharmacologic medications such as antiglaucomatous agents, antibiotics, dietary supplementation, novel neuroprotective molecules, neurotrophic factors, translational methods such as gene therapy and cell therapy, and electrical stimulation-based physiotherapy are emerging to attenuate the death of RGCs, or to make RGCs resilient to attacks. Understanding the roles of these interventions in RGC protection may offer benefits over traditional pharmacological medications and surgeries. In this review, we summarize the recent neuroprotective strategy for glaucoma, both in clinical trials and in laboratory research.

Neuroprotective Factors of the Retina and Their Role in Promoting Survival of Retinal Ganglion Cells: A Review

Retinal ganglion cells (RGCs) play a crucial role in the visual pathway. As their axons form the optic nerve, apoptosis of these cells causes neurodegenerative vision loss. RGC death could be triggered by increased intraocular pressure, advanced glycation end products, or mitochondrial dysfunction. In this review, we summarize the role of some neuroprotective factors in RGC injury: ciliary neurotrophic factor (CNTF), nerve growth factor (NGF), brain-derived neurotrophic factor, vascular endothelial growth factor, pigment epithelium-derived factor, glial cell line-derived neurotrophic factor, and Norrin. Each, in their own unique way, prevents RGC damage caused by glaucoma, ocular hypertension, ischemic neuropathy, and even oxygen-induced retinopathy. These factors are produced mainly by neurons, leukocytes, glial cells, and epithelial cells. Neuroprotective factors act via various signaling pathways, including JAK/STAT, MAPK, TrkA, and TrkB, which promotes RGC survival. Many attempts have been made to develop therapeutic strategies using these factors. There are ongoing clinical trials with CNTF and NGF, but they have not yet been accepted for clinical use.

Tissue engineering of the retina: from organoids to microfluidic chips

Despite advancements in tissue engineering, challenges remain for fabricating functional tissues that incorporate essential features including vasculature and complex cellular organisation. Monitoring of engineered tissues also raises difficulties, particularly when cell population maturity is inherent to function. Microfluidic, or lab-on-a-chip, platforms address the complexity issues of conventional 3D models regarding cell numbers and functional connectivity. Regulation of biochemical/biomechanical conditions can create dynamic structures, providing microenvironments that permit tissue formation while quantifying biological processes at a single cell level. Retinal organoids provide relevant cell numbers to mimic in vivo spatiotemporal development, where conventional culture approaches fail. Modern bio-fabrication techniques allow for retinal organoids to be combined with microfluidic devices to create anato-physiologically accurate structures or 'retina-on-a-chip' devices that could revolution ocular sciences. Here we present a focussed review of retinal tissue engineering, examining the challenges and how some of these have been overcome using organoids, microfluidics, and bioprinting technologies.

Fibrotic Changes and Endothelial-to-Mesenchymal Transition Promoted by VEGFR2 Antagonism Alter the Therapeutic Effects of VEGFA Pathway Blockage in a Mouse Model of Choroidal Neovascularization

Many patients with wet age-related macular degeneration do not respond well to anti- vascular endothelial growth factor A (VEGFA) therapy for choroidal neovascularization (CNV), and the efficacy of anti-VEGFA decreases over time. We investigated the hypothesis that fibrotic changes, in particular via endothelial-to-mesenchymal transition (EndoMT), play a role in CNV and alter the therapeutic effects of VEGFA pathway blockage. Induction of EndoMT of primary human retinal endothelial cells led to a significantly reduced response to VEGFA at the level of gene expression, cellular proliferation, migration, and tube formation. Suppression of EndoMT restored cell responsiveness to VEGFA. In a mouse model of spontaneous CNV, fibrotic changes and EndoMT persisted as the CNV lesions became more established over time. VEGFA receptor-2 (VEGFR2) antagonism further induced fibrosis and EndoMT in the CNV. The combination of VEGFR2 antagonism and fibrosis/EndoMT inhibition was more effective than either individual treatment in reducing CNV. Our data indicate that fibrosis and EndoMT are involved in the progression of CNV, are exacerbated by VEGFR2 inhibition, and could provide an explanation for the reduced efficacy of anti-VEGFA treatment over time.

Relief of Cystoid Macular Edema-Induced Focal Axonal Compression with Anti-Vascular Endothelial Growth Factor Treatment

Purpose

To evaluate the mechanical compression of retinal nerve fiber layer (RNFL) by intraretinal cysts in macular edema and its relief with anti-vascular endothelial growth factor (anti-VEGF) treatment.

Methods

Optical coherence tomography scans were used to measure RNFL thickness and reflectance at seven preselected points at and around the peak of the edema before and after anti-VEGF treatment in 10 patients (11 eyes) with branch retina vein occlusion (BRVO) and diabetic macular edema (DME). Scans through nonedematous retina and from the fellow eyes were taken as controls. Correlations were sought between the changes in retinal and RNFL thickness, RNFL reflectance, and the size of the intraretinal cysts.

Results

Postinjection RNFL thickness decreased significantly only at peak point of the edema (18.1 ± 2.7 vs. 13.8 ± 1.2 µm; P = 0.038), at its nasal edge (20.1 ± 2.7 vs. 15.5 ± 1.4 µm; P = 0.026), and 500 µm away from its nasal border (35.7 ± 6.0 vs. 20.1 ± 2.7 µm; P = 0.006) suggesting focal stagnation of the axoplasmic flow owing to compression at its peak point. Significant postinjection decreases in RNFL reflectivity were also noted at peak point of the cyst (164.9 ± 10.3 vs. 141.5 ± 12.6 arbitrary units [AU]; P = 0.037), at its nasal edge (166.8 ± 7.8 vs. 135.1 ± 10.2 AU; P = 0.02), and 1500 µm away from temporal edge (160.2 ± 6.2 vs. 141.1 ± 6.4 AU; P = 0.022). Cyst proximity to RNFL (D₅₀ = 50 µm) was the only determinant significantly affecting the magnitude of the RNFL thickness change after anti-VEGF treatment (P = 0.001).

Conclusions

Intraretinal cysts due to BRVO and DME locally compress overlying axons and induce anatomic changes suggestive of axoplasmic stagnation. This compression can be relieved with anti-VEGF treatment.

Translational Relevance

Focal compression of RFNL by retinal cysts may indicate a need for early treatment of macular edema to prevent axonal loss, especially in patients with low axonal reserve