Protective effects of TGF-β inhibitors in a rat model of NMDA-induced retinal degeneration

Association of Primary Open-Angle Glaucoma with Diabetic Retinopathy among Patients with Type 1 and Type 2 Diabetes: A Large Global Database Study

PURPOSE

To assess the correlation between primary open-angle glaucoma (POAG) and the risk of developing diabetic retinopathy (DR) in patients with type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM).

DESIGN

A retrospective cohort study leveraging the global patient database of TriNetX Research Network.

PARTICIPANTS

The study included 44 359 patients with diabetes mellitus (DM) with POAG and 4 393 300 patients with DM without any glaucoma ≥ 18 years of age. Propensity score matching harmonized the cohorts to 39 680 patients each, covering diagnoses from January 1, 2005, to January 1, 2023.

METHODS

We analyzed data using specific International Classification of Diseases, 10th Revision (ICD-10) codes for DM and glaucoma. We matched the cohorts using propensity score matching, adjusting for age, sex, race/ethnicity, blood markers, relevant medical history, and ophthalmic service use.

MAIN OUTCOME MEASURES

The primary outcome was the first-time occurrence of DR, including nonproliferative DR (NPDR) and proliferative DR (PDR), in patients with DM with and without glaucoma at 1-, 5-, and 10-year intervals from their individual index dates.

RESULTS

At 10 years, patients with T1DM with POAG exhibited a heightened risk for any DR (adjusted risk ratios [RRs], 4.12; 95% confidence interval [CI], 3.05-5.57, P < 0.0001) and PDR (RR, 7.02; 95% CI, 3.62-13.61, P < 0.0001). Patients with T2DM and POAG also faced an increased 10-year risk for any DR (RR, 2.47; 95% CI, 2.28-2.68, P < 0.0001) and PDR (RR, 3.82; 95% CI, 3.09-4.70, P < 0.0001). The combined association of POAG on DR risk in those with T1DM and T2DM at 10 years was found to be significantly higher among patients with POAG (5.45%) compared with those without glaucoma (2.12%) (adjusted hazard ratio [aHR], 2.33; 95% CI, 2.14-2.53). The cumulative incidence of DR was significantly higher in the POAG group compared with nonglaucoma counterparts after a decade (log-rank P < 0.001).

CONCLUSIONS

Our findings underscore a substantial association between POAG and DR development in both T1DM and T2DM patients, emphasizing the need for vigilant screening and comprehensive management in glaucomatous patients with DM to mitigate the risk of DR. Future research should delve into elucidating the causal mechanisms driving these observed associations.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Tumor necrosis factor-α and matrix metalloproteinase-9 cooperatively exacerbate neurovascular degeneration in the neonatal rat retina

Matrix metalloproteinases (MMPs) and tumor necrosis factor (TNF)-α contribute to the pathogenesis of several ocular diseases. Previous studies have shown that MMP-9 activation plays an important role in capillary degeneration in injured retinas. In this study, we aimed to determine the roles of TNF-α in capillary degeneration and MMP-9 activation in the injured retina. In rats, retinal injury was induced by intravitreal injection of N-methyl-D-aspartic acid (NMDA, 200 nmol) at postnatal day 7. We examined (1) the effects of blocking MMP-9 and TNF-α signaling pathway on capillary degeneration, (2) changes in protein levels and distribution of MMP-9 and TNF-α, and (3) the interaction between MMP-9 and TNF-α in regulating the expression level of each protein in retinas of NMDA-injected eyes. Intravitreal injection of GM6001, an MMP inhibitor, or TNF-α neutralizing antibody (anti-TNF-α Ab) attenuated capillary degeneration in retinas of NMDA-injected eyes. Protein levels of TNF-α increased 2 h after NMDA injection, whereas those of MMP-9 increased 4 h after the injection. Anti-TNF-α Ab suppressed activation of MMP-9 in retinas of NMDA-injected eyes, whereas GM6001 diminished the TNF-α protein expression. Incubation of recombinant TNF-α with supernatants of homogenized retina increased protein levels and activity of MMP-9. These results suggest that TNF-α and MMP-9 collaboratively increase their expression levels in the retina following neurodegeneration, thus leading to retinal capillary degeneration. The cooperative interaction between MMP-9 and TNF-α could be involved in the exacerbation of retinal neurovascular degeneration.

Comparison of Glaucoma-Relevant Transcriptomic Datasets Identifies Novel Drug Targets for Retinal Ganglion Cell Neuroprotection

Glaucoma is a leading cause of blindness and is characterized by the progressive dysfunction and irreversible death of retinal ganglion cells. We aimed to identify shared differentially expressed genes (DE genes) between different glaucoma relevant models of retinal ganglion cell injury using existing RNA-sequencing data, thereby discovering targets for neuroprotective therapies. A comparison of DE genes from publicly available transcriptomic datasets identified 12 shared DE genes. The Comparative Toxicogenomics Database (CTD) was screened for compounds targeting a significant proportion of the identified DE genes. Forty compounds were identified in the CTD that interact with >50% of these shared DE genes. We next validated this approach by testing select compounds for an effect on retinal ganglion cell survival using a mouse retinal explant model. Folic acid, genistein, SB-431542, valproic acid, and WY-14643 (pirinixic acid) were tested. Folic acid, valproic acid, and WY-14643 demonstrated significant protection against retinal ganglion cell death in this model. The increasing prevalence of open access-omics data presents a resource to discover targets for future therapeutic investigation.

VEGFD Protects Retinal Ganglion Cells and, consequently, Capillaries against Excitotoxic Injury

In the central nervous system, neurons and the vasculature influence each other. While it is well described that a functional vascular system is trophic to neurons and that vascular damage contributes to neurodegeneration, the opposite scenario in which neural damage might impact the microvasculature is less defined. In this study, using an in vivo excitotoxic approach in adult mice as a tool to cause specific damage to retinal ganglion cells, we detected subsequent damage to endothelial cells in retinal capillaries. Furthermore, we detected decreased expression of vascular endothelial growth factor D (VEGFD) in retinal ganglion cells. In vivo VEGFD supplementation via neuronal-specific viral-mediated expression or acute intravitreal delivery of the mature protein preserved the structural and functional integrity of retinal ganglion cells against excitotoxicity and, additionally, spared endothelial cells from degeneration. Viral-mediated suppression of expression of the VEGFD-binding receptor VEGFR3 in retinal ganglion cells revealed that VEGFD exerts its protective capacity directly on retinal ganglion cells, while protection of endothelial cells is the result of upheld neuronal integrity. These findings suggest that VEGFD supplementation might be a novel, clinically applicable approach for neuronal and vascular protection.

Cellular Mechanisms of Angiogenesis in Neonatal Rat Models of Retinal Neurodegeneration

Νeuronal and glial cells play an important role in the development of vasculature in the retina. In this study, we investigated whether re-vascularization occurs in retinal neurodegenerative injury models. To induce retinal injury, N-methyl-D-aspartic acid (NMDA, 200 nmol) or kainic acid (KA, 20 nmol) was injected into the vitreous chamber of the eye on postnatal day (P)7. Morphological changes in retinal neurons and vasculature were assessed on P14, P21, and P35. Prevention of vascular growth and regression of some capillaries were observed on P14 in retinas of NMDA- and KA-treated eyes. However, vascular growth and re-vascularization started on P21, and the retinal vascular network was established by P35 in retinas with neurodegenerative injuries. The re-vascularization was suppressed by a two-day treatment with KRN633, an inhibitor of VEGF receptor tyrosine kinase, on P21 and P22. Astrocytes and Müller cells expressed vascular endothelial growth factor (VEGF), and the distribution pattern of VEGF was almost the same between the control and the NMDA-induced retinal neurodegenerative injury model, except for the difference in the thickness of the inner retinal layer. During re-vascularization, angiogenic sprouts from pre-existing blood vessels were present along the network of fibronectins formed by astrocytes. These results suggest that glial cells contribute to angiogenesis in neonatal rat models of retinal neurodegeneration.

Role of TGF-Beta1/SMAD2/3 Pathway in Retinal Outer Deep Vascular Plexus and Photoreceptor Damage in Rat 50/10 Oxygen-Induced Retinopathy

In retinopathy of prematurity (ROP), outer deep vascular plexus (oDVP) was the emerging field, and the mechanisms of photoreceptor dysfunction remained to be explored. ODVP and photoreceptors were related, with oDVP being part of the supplier of oxygen and nutrients to photoreceptors, while their possible relationship in ROP was not clear. TGF-beta1 has been reported indispensable in oDVP development and altered in ROP patients and animal models. We hypothesized that the TGF-beta1 alteration in rat 50/10 oxygen-induced retinopathy (OIR) model contributed to oDVP malformation and exerted consequent effects on photoreceptor development. We first explored the profile of oDVP development in rat after birth and compared the expression of TGF-beta1 and pSMAD2/3 in Normoxia and OIR groups. Afterwards, the inhibitor of the pathway, LY364947, was used to establish the OIR, OIR+LY364947, Normoxia, and Normoxia+LY364947 groups. The oDVP and photoreceptor were examined by Isolectin B4 staining, western-blot of CD31 and Rho, and electron microscopy. ODVP sprouted at postnatal day 10 (D10) and reached the edge of retina at D14. The TGF-beta1/SMAD2/3 pathway was compromised during the critical period of oDVP development. The inhibitor simulated the oDVP retardation, pericyte, and photoreceptor malformation in the Normoxia+LY364947 group and might further compromise the development of oDVP and photoreceptor in the OIR+LY364947 group. The inhibition of the TGF-beta1/SMAD2/3 pathway indicated its critical role in oDVP malformation and photoreceptor damage, suggesting a possible therapeutic target of ROP treatment.

Involvement of matrix metalloproteinases in capillary degeneration following NMDA-induced neurotoxicity in the neonatal rat retina

Interactions between neuronal cells and vascular cells in the retina are critical for maintaining retinal tissue homeostasis. Impairment of cellular interactions contributes to development and progression of retinal diseases. Previous studies demonstrated that neuronal cell damage leads to capillary degeneration in an N-methyl-D-aspartic acid (NMDA)-induced retinal degeneration model. However, the mechanisms underlying this phenomenon are not fully understood. In this study, we examined the possible role of matrix metalloproteinase (MMP)-9 in neuronal cell loss and capillary degeneration in NMDA-treated retinas of neonatal rats. Intravitreal injection of NMDA (50 or 200 nmol) was performed on postnatal day (P) 7 and morphological changes in retinal neurons and vasculature were examined on P14. The MMP inhibitor CP101537 (100 nmol) or vehicle (dimethyl sulfoxide) was intravitreally injected simultaneously with, or 2 days after, NMDA injection. CP101537 protected against neurovascular degeneration in a time-dependent manner as follows: 1) simultaneous injection of CP101537 with NMDA prevented morphological changes in retinal neurons induced by NMDA (50 nmol); and 2) reduction in capillary density and number of vertical sprouts induced by NMDA (200 nmol) was prevented when CP101537 was injected 2 days after NMDA injection. Gelatin zymography and western blot analyses indicated that activity and protein levels of MMP-9 were enhanced from 4 h to 2 days after NMDA injection. Increased activity and protein levels of MMP-9 were suppressed by MMP inhibitors (CP101537 and GM6001). In situ zymography revealed that MMP activity was enhanced throughout the retinal vasculature in NMDA-treated retinas. These results indicate that MMP-9 plays an important role in neurovascular degeneration in the injured retina. Inhibition of MMP-9 may be an effective strategy for preventing and reducing neurovascular degeneration.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

Extract of the Blood Circulation-Promoting Recipe-84 Can Protect Rat Retinas by Inhibiting the β-Catenin Signaling Pathway

Extract of the Blood Circulation-Promoting Recipe (EBR-84) from the Chinese Herbal medicine “Blood Circulation Promoting Recipe” could retard retinopathy development. This study investigated whether EBR-84 protects retinas by inhibiting the β-catenin pathway using a rat model of retinopathy and a retinal ganglion cell 5 (RGC-5) cell death model. RGC death was induced by either N-methyl-d-aspartic acid (NMDA) or TWS119 (an activator of the β-catenin pathway). After the corresponding treatment with EBR-84, RGC death and the protein expression levels of β-catenin, cyclooxygenase-2 (COX-2), and vascular endothelial growth factor (VEGF) in rat retinas were examined. β-Catenin accumulated in the retinal ganglion cell layer (GCL) of NMDA-treated rats. EBR-84 (3.9, 7.8, and 15.6 g/kg) significantly attenuated the NMDA-induced RGC loss accompanying the reduction of β-catenin expression. Moreover, the expression levels of COX-2 and VEGF were decreased by EBR-84 in a dose-dependent manner. For the TWS119-treated rats, EBR-84 also ameliorated RGC loss and lowered the expression levels of β-catenin, COX-2, and VEGF. In vitro, EBR-84 increased the viability of NMDA-treated RGC-5 while decreased β-catenin expression. In conclusion, EBR-84 retarded ratretinopathy, and the β-catenin signaling pathway played an important role during this protective process.

PI3K/Akt Pathway: A Role in δ-Opioid Receptor-Mediated RGC Neuroprotection

Purpose

This study examines the role of PI3K/Akt pathway in δ-opioid receptor agonist (SNC-121)-induced RGC neuroprotection in a chronic glaucoma rat model.

Methods

Injecting hypertonic saline into the limbal veins of Brown Norway rats elevated IOP. Rats were treated either with 1 mg/kg SNC-121 or 3 mg/kg 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY-294002; PI3K/Akt inhibitor) plus SNC-121 once daily for 7 days. Pattern ERGs were recorded in response to contrast reversal of patterned visual stimuli. Retinal ganglion cells (RGC) were visualized by Fluorogold retrograde labeling. Optic nerve head (ONH) astrocytes were pretreated with PI3K/Akt inhibitors for 30 minutes followed by 1-μM SNC-121 treatment. Changes in matrix metalloproteinases (MMP-1, -2, and -3) production and PI3K/Akt activation in optic nerve and TNF-α treated ONH astrocytes were measured by immunohistochemistry and Western blotting.

Results

SNC-121 activates the PI3K/Akt pathway in ONH astrocytes and the retina. In ONH astrocytes, SNC-121–induced Akt activation was fully inhibited by PI3K/Akt inhibitors. A sustained decline (7–42 days post injury) in Akt activation was seen in the ocular-hypertensive retina and optic nerve. This decline is reversed to normal levels by 1-mg/kg intraperitoneally (i.p.) SNC-121 treatment. Both pattern ERG amplitudes and RGC numbers were reduced in ocular hypertensive eyes, which were significantly increased in SNC-121–treated animals. Interestingly, SNC-121–induced increase in pattern-ERG amplitudes and RGC numbers were inhibited in LY-294002 pretreated animals. Additionally, SNC-121 treatment inhibited MMP-1, -2, and -3 production from the optic nerve of ocular hypertensive rats and TNF-α–treated ONH astrocytes.

Conclusions

PI3K/Akt pathway plays a crucial role in SNC-121–mediated RGC neuroprotection against glaucomatous injury.

Inhibition of the transforming growth factor-β/SMAD cascade mitigates the anti-neurogenic effects of the carbamate pesticide carbofuran

The widely used carbamate pesticide carbofuran causes neurophysiological and neurobehavioral deficits in rodents and humans and therefore poses serious health hazards around the world. Previously, we reported that gestational carbofuran exposure has detrimental effects on hippocampal neurogenesis, the generation of new neurons from neural stem cells (NSC), in offspring. However, the underlying cellular and molecular mechanisms for carbofuran-impaired neurogenesis remain unknown. Herein, we observed that chronic carbofuran exposure from gestational day 7 to postnatal day 21 altered expression of genes and transcription factors and levels of proteins involved in neurogenesis and the TGF-β pathway (i.e. TGF-β; SMAD-2, -3, and -7; and SMURF-2) in the rat hippocampus. We found that carbofuran increases TGF-β signaling (i.e. increased phosphorylated SMAD-2/3 and reduced SMAD-7 expression) in the hippocampus, which reduced NSC proliferation because of increased p21 levels and reduced cyclin D1 levels. Moreover, the carbofuran-altered TGF-β signaling impaired neuronal differentiation (BrdU/DCX⁺ and BrdU/NeuN⁺ cells) and increased apoptosis and neurodegeneration in the hippocampus. Blockade of the TGF-β pathway with the specific inhibitor SB431542 and via SMAD-3 siRNA prevented carbofuran-mediated inhibition of neurogenesis in both hippocampal NSC cultures and the hippocampus, suggesting the specific involvement of this pathway. Of note, both in vitro and in vivo studies indicated that TGF-β pathway attenuation reverses carbofuran's inhibitory effects on neurogenesis and associated learning and memory deficits. These results suggest that carbofuran inhibits NSC proliferation and neuronal differentiation by altering TGF-β signaling. Therefore, we conclude that TGF-β may represent a potential therapeutic target against carbofuran-mediated neurotoxicity and neurogenesis disruption.

RORα modulates semaphorin 3E transcription and neurovascular interaction in pathological retinal angiogenesis

Pathological proliferation of retinal blood vessels commonly causes vision impairment in proliferative retinopathies, including retinopathy of prematurity. Dysregulated crosstalk between the vasculature and retinal neurons is increasingly recognized as a major factor contributing to the pathogenesis of vascular diseases. Class 3 semaphorins (SEMA3s), a group of neuron-secreted axonal and vascular guidance factors, suppress pathological vascular growth in retinopathy. However, the upstream transcriptional regulators that mediate the function of SEMA3s in vascular growth are poorly understood. Here we showed that retinoic acid receptor-related orphan receptor α (RORα), a nuclear receptor and transcription factor, is a novel transcriptional regulator of SEMA3E-mediated neurovascular coupling in a mouse model of oxygen-induced proliferative retinopathy. We found that genetic deficiency of RORα substantially induced Sema3e expression in retinopathy. Both RORα and SEMA3E were expressed in retinal ganglion cells. RORα directly bound to a specific ROR response element on the promoter of Sema3e and negatively regulated Sema3e promoter-driven luciferase expression. Suppression of Sema3e using adeno-associated virus 2 carrying short hairpin RNA targeting Sema3e promoted disoriented pathological neovascularization and partially abolished the inhibitory vascular effects of RORα deficiency in retinopathy. Our findings suggest that RORα is a novel transcriptional regulator of SEMA3E-mediated neurovascular coupling in pathological retinal angiogenesis.-Sun, Y., Liu, C.-H., Wang, Z., Meng, S. S., Burnim, S. B., SanGiovanni, J. P., Kamenecka, T. M., Solt, L. A., Chen, J. RORα modulates semaphorin 3E transcription and neurovascular interaction in pathological retinal angiogenesis.

Protective Effects of Everolimus against N-Methyl-D-aspartic Acid-Induced Retinal Damage in Rats

We previously demonstrated that rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), protects against N-methyl-D-aspartic acid (NMDA)-induced retinal neurotoxicity, but the mechanism underlying this protection is not fully understood. The present study aimed to examine the effects of everolimus, another inhibitor of mTOR, on neuronal cell loss and inflammation in a rat model of NMDA-induced retinal neurotoxicity, and to determine whether the extracellular signal-regulated kinase (ERK) pathway contributes to the protective effect of everolimus. Intravitreal injection of NMDA (200 nmol) resulted in (1) cell loss in the ganglion cell layer, (2) increase in the numbers of CD45-positive leukocytes and Iba1-positive microglia, and (3) phosphorylation of ribosomal protein S6 (pS6), a downstream indicator of mTOR activity. Simultaneous injection of everolimus with NMDA significantly attenuated these NMDA-induced responses. The neuroprotective effect of everolimus was almost completely prevented by the mitogen-activated protein kinase/ERK kinase inhibitor U0126 (1 nmol). NMDA increased the level of phosphorylated ERK (pERK) in Müller cells, and increase in pERK levels was also observed after co-injection of NMDA and everolimus. These results suggest that everolimus has a neuroprotective effect against NMDA-induced retinal neurotoxicity, an effect that seems to be mediated partly by activation of the ERK pathway in Müller cells.

Protective effects of PF-4708671 against N-methyl-d-aspartic acid-induced retinal damage in rats

We previously demonstrated that rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), protects against N-methyl-d-aspartic acid (NMDA)-induced retinal damage in rats. Rapamycin inhibits mTOR activity, thereby preventing the phosphorylation of ribosomal protein S6, which is a downstream target of S6 kinase. Therefore, we aimed to determine whether PF-4708671, an inhibitor of S6 kinase, protects against NMDA-induced retinal injury. Intravitreal injection of NMDA (200 nmol/eye) caused cell loss in the ganglion cell layer and neuroinflammatory responses, such as an increase in the number of CD45-positive leukocytes and Iba1-positive microglia. Surprisingly, simultaneous injection of PF-4708671 (50 nmol/eye) with NMDA significantly attenuated these responses without affecting phosphorylated S6 levels. These results suggest that PF-4708671 and rapamycin likely protect against NMDA-induced retinal damage via distinct pathways. The neuroprotective effect of PF-4708671 is unlikely to be associated with inhibition of the S6 kinase, even though PF-4708671 is reported to be a S6 kinase inhibitor.

Retinal Cell Degeneration in Animal Models

The aim of this review is to provide an overview of various retinal cell degeneration models in animal induced by chemicals (N-methyl-d-aspartate- and CoCl₂-induced), autoimmune (experimental autoimmune encephalomyelitis), mechanical stress (optic nerve crush-induced, light-induced) and ischemia (transient retinal ischemia-induced). The target regions, pathology and proposed mechanism of each model are described in a comparative fashion. Animal models of retinal cell degeneration provide insight into the underlying mechanisms of the disease, and will facilitate the development of novel effective therapeutic drugs to treat retinal cell damage.

Preventive effects of rapamycin on inflammation and capillary degeneration in a rat model of NMDA-induced retinal injury

Inhibitors of the mammalian target of rapamycin (mTOR) have been shown to protect against neuronal injury, but the mechanisms underlying this effect are not fully understood. The present study aimed to examine the effects of rapamycin, an inhibitor of the mTOR pathway, on inflammation and capillary degeneration in a rat model of N-methyl-D-aspartate (NMDA)-induced retinal neurotoxicity. Inflammation and capillary degeneration were evaluated by counting the numbers of CD45-positive leukocytes and Iba1-positive microglia, and by measuring the length of empty basement membrane sleeves, respectively. Marked increases in the numbers of leukocytes and microglia were observed 1 d after intravitreal injection of NMDA (200 nmol), and significant capillary degeneration was observed after 7 d. These NMDA-induced changes were significantly reduced by the simultaneous injection of rapamycin (20 nmol) with NMDA. These results suggest that rapamycin has preventive effects on inflammation and capillary degeneration during retinal injury.

Present and New Treatment Strategies in the Management of Glaucoma

Glaucoma is a neurodegenerative disease characterized by retinal ganglion cell (RGC) death and axonal loss. It remains a major cause of blindness worldwide. All current modalities of treatment are focused on lowering intraocular pressure (IOP), and it is evident that increased IOP is an important risk factor for progression of the disease. However, it is clear that a significant number of glaucoma patients show disease progression despite of pressure lowering treatments. Much attention has been given to the development of neuroprotective treatment strategies, but the identification of such has been hampered by lack of understanding of the etiology of glaucoma. Hence, in spite of many attempts no neuroprotective drug has yet been clinically approved. Even though neuroprotection is without doubt an important treatment strategy, many glaucoma subjects are diagnosed after substantial loss of RGCs. In this matter, recent approaches aim to rescue RGCs and regenerate axons in order to restore visual function in glaucoma. The present review seeks to provide an overview of the present and new treatment strategies in the management of glaucoma. The treatment strategies are divided into current available glaucoma medications, new pressure lowering targets, prospective neuroprotective interventions, and finally possible neuroregenrative strategies.

Structural and functional changes in retinal vasculature induced by retinal ischemia-reperfusion in rats

Recent studies have shown retinal blood vessel damage in experimental models of retinal degeneration. The present study aimed to provide a detailed description of the structural and functional changes in retinal vasculature induced by retinal ischemia-reperfusion (I/R) in rats. Retinal ischemia was induced for 60 min by raising the intraocular pressure to 130 mmHg. Morphological changes in vascular components (endothelial cells, pericytes, and basement membranes), the patency and perfusion of blood vessels, and expression of vascular endothelial growth factor (VEGF) were assessed in the retinas at 2, 7, and 14 days after I/R. Significant reductions in vascular densities were observed at 7 and 14 days after I/R. Pericyte loss occurred after the appearance of endothelial cell degeneration, whereas the vascular basement membranes remained unchanged. Some vessels showed no perfusion in damaged retina. A decrease in the immunoreactivity of VEGF in the region extending from the ganglion cell layer to the outer plexiform layer was evident 2 days after I/R. In retinal I/R model, retinal ganglion cells are rapidly (<2 day) damaged following reperfusion, therefore, the current results suggest that neuronal cell damage precedes capillary degeneration, and neuronal cells may play an important role in maintaining vascular structure and function through the production and release of endothelial cell survival factors, including VEGF. Neuronal cell damage could be an additional cause of progression of ischemic retinal damage by reducing blood supply to the retinal neurons due to the destruction of the blood vessel network.

Modulation of the NMDA Receptor Through Secreted Soluble Factors

Synaptic activity is a critical determinant in the formation and development of excitatory synapses in the central nervous system (CNS). The excitatory current is produced and regulated by several ionotropic receptors, including those that respond to glutamate. These channels are in turn regulated through several secreted factors that function as synaptic organizers. Specifically, Wnt, brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF), and transforming growth factor (TGF) particularly regulate the N-methyl-D-aspartate receptor (NMDAR) glutamatergic channel. These factors likely regulate early embryonic development and directly control key proteins in the function of important glutamatergic channels. Here, we review the secreted molecules that participate in synaptic organization and discuss the cell signaling behind of this fine regulation. Additionally, we discuss how these factors are dysregulated in some neuropathologies associated with glutamatergic synaptic transmission in the CNS.

Regression of retinal capillaries following N-methyl-D-aspartate-induced neurotoxicity in the neonatal rat retina

Degeneration of retinal capillaries occurs following N-methyl-D-aspartate (NMDA)-induced retinal neurotoxicity, and the degree of capillary degeneration decreases in an age-dependent manner. To determine the role of vascular endothelial growth factor (VEGF) in the high susceptibility of capillaries to neuronal damage during the early postnatal stage, this study compares the vascular regression patterns between NMDA-treated retinas and retinas treated with N-[2-chloro-4-{(6,7-dimethoxy-4-quinazolinyl)oxy}phenyl]-N'-propylurea (KRN633), a VEGF receptor tyrosine kinase inhibitor, in neonatal rats. Two days after a single intravitreal injection of NMDA (200 nmol/eye) on postnatal day (P) 7, substantial retinal neuron loss and delayed expansion of the retinal vascular bed were observed. The reduction in the capillary density in the central retina reached statistical significance 4 days after NMDA treatment. In retinas of rats injected subcutaneously with KRN633 (10 mg/kg) on P7 and P8, simplified vasculature attributable to capillary regression and prevention of endothelial cell growth were seen on P9, whereas no visible changes in the morphology of the retinal layers were observed. The degree of capillary degeneration in NMDA-treated retinas was less than that in KRN633-treated retinas. No apparent changes in immunoreactivities for VEGF were found 2 days after NMDA treatment. These results indicate that neuronal cell loss in the retina precedes retinal capillary degeneration following NMDA treatment, and VEGF-dependent immature capillaries might be more susceptible to NMDA-induced neuronal damage.

Age-dependent changes in the severity of capillary degeneration in rat retina following N-methyl-D-aspartate-induced neurotoxicity

PURPOSE

Previous studies have shown that injury to the retinal vasculature, including capillary degeneration, occurs following N-methyl-d-aspartate (NMDA)-induced neuronal cell loss, but it is unclear whether there are age-dependent differences in the severity of vascular damage. The purpose of the present study was to determine age-related changes in retinal capillary degeneration in NMDA-induced retinal damage rat model.

MATERIALS AND METHODS

A single intravitreal injection of NMDA (200 nmol/eye) was performed in 1-, 2-, 3-, 7- and 15-week-old male Sprague-Dawley rats. Seven days after NMDA injection, damage to retinal neurons and blood vessels were assessed by measuring the number of cells in the ganglion cell layer and the length of empty basement membrane sleeves, left behind by regressing endothelial cells, respectively.

RESULTS

At all ages examined, the NMDA-induced neurotoxicity exhibited similar levels of ganglion cell death. However, the degeneration of capillaries in NMDA-treated retina was severe during the early stages of retinal vascular development, and the degree of capillary degeneration decreased with age.

CONCLUSION

The degree of retinal capillary degeneration in NMDA-induced retinal damage rat model decreases in an age-dependent manner. Retinal capillaries may be resistant to retinal neuronal damage in young adults.

Differential effects of LY294002 and wortmannin on neurons and vascular endothelial cells in the rat retina

BACKGROUND

Neuronal damage leads to capillary degeneration in an N-methyl-D-aspartate (NMDA)-induced retinal degeneration model; however, the mechanisms underlying this phenomenon are not fully understood. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway has been recognized as an intracellular pro-survival signaling system. Therefore, we used the PI3K inhibitors LY294002 and wortmannin to investigate the role of this pathway in neuronal and blood vessel injury in the rat retina treated with NMDA.

METHODS

Male Sprague-Dawley rats weighing 220-240 g were used in this study. NMDA combined with LY294002, wortmannin, or vehicle was administered intravitreally, and histological evaluation was performed at 2 and 7 days after injection. The effects of LY294002 or wortmannin alone were also evaluated.

RESULTS

The number of cells in the ganglion cell layer (GCL) was significantly reduced at 2 and 7 days after intravitreal injection of NMDA, whereas enhanced capillary degeneration was observed at 7 days. Simultaneous injection of LY294002 with NMDA significantly attenuated NMDA-induced retinal cell loss and capillary degeneration at 7 days. However, simultaneous injection of wortmannin with NMDA did not affect cell loss, but enhanced capillary degeneration. Treatment with LY294002 alone showed no effect on neuronal or vascular cells, whereas wortmannin induced capillary degeneration without significantly affecting the cell number in the GCL.

CONCLUSIONS

Although both LY294002 and wortmannin are known as PI3K inhibitors, they exhibit differential effects on neurons and vascular endothelial cells in the rat retina. Therefore, the results obtained using these inhibitors should be carefully interpreted. However, our finding that LY294002 was protective against NMDA-induced retinal damage suggests that this compound may be an effective candidate for preventing the development of retinal diseases associated with glutamate-induced excitotoxicity.

A TGF-β receptor 1 inhibitor for prevention of proliferative vitreoretinopathy

This study evaluates the use of the TGF-β receptor 1 inhibitor LY-364947 (LY) to prevent proliferative vitreoretinopathy (PVR). For the in vitro experiments Human Tenon's Fibroblasts (HTFs) and retinal pigment epithelial (RPE) cells were treated with different concentrations of LY to determine HTF proliferation and RPE transdifferentiation. For in vivo testing 30 rabbits underwent a PVR trauma model. The animals received different concentrations of intravitreally injected LY, with or without vitrectomy. LY treatment reduced HTF proliferation and RPE transdifferentiation in vitro. In vivo intravitreal injection of LY prevented PVR development significantly. This positive effect was also present when LY injection was combined with vitrectomy. Intravitreal injection of LY prevented tractional retinal detachment in 14 out of 15 animals. In conclusion, treatment with the TGF-β receptor 1 inhibitor LY reduces HTF proliferation and RPE transdifferentiation in vitro and prevents proliferative vitreoretinopathy and subsequent tractional retinal detachment in vivo.

Rapamycin prevents N-methyl-D-aspartate-induced retinal damage through an ERK-dependent mechanism in rats

Recent studies have demonstrated that inhibition of the mammalian target of rapamycin (mTOR) protects against neuronal injury, but the mechanisms underlying this protection are not fully understood. The present study investigates whether rapamycin, an inhibitor of the mTOR pathway, protects against N-methyl-D-aspartate (NMDA)-induced retinal neurotoxicity and whether the extracellular signal-regulated kinase (ERK) pathway contributes to this protective effect in rats. Significant cell loss in the ganglion cell layer and a reduction in thickness of the inner plexiform layer were observed 7 days after a single intravitreal injection of NMDA (200 nmol/eye). These NMDA-induced morphological changes were significantly reduced by rapamycin (20 nmol/eye). The number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive apoptotic cells had increased 6 hr after NMDA injection, an effect that was significantly attenuated by rapamycin. The ERK inhibitor U0126 (1 nmol/eye) almost completely abolished rapamycin's inhibition of NMDA-induced apoptosis. Immunohistochemical studies showed that NMDA caused a time-dependent increase in levels of the phosphorylated form of the ribosomal protein S6 (pS6), a downstream indicator of mTOR activity. The increased pS6 levels were markedly decreased by rapamycin. Both NMDA and rapamycin increased the level of phosphorylated ERK (pERK) in Müller cells, and coinjection of both agents further increased pERK levels. These results suggest that rapamycin has a neuroprotective effect against NMDA-induced retinal neurotoxicity and that this effect could be patially mediated by activation of the ERK pathway in retinal Müller cells.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.

ISO-1, a macrophage migration inhibitory factor antagonist, prevents N-methyl-D-aspartate-induced retinal damage

Macrophage migration inhibitory factor (MIF) has been shown to play an important role in a variety of inflammatory and immune-mediated diseases. The inflammatory responses contribute to retinal neuronal degeneration. However, the role of MIF in the progression of retinal degeneration has not yet been elucidated. In this study, we determined whether pharmacological inhibition of MIF protects against the retinal damage induced by N-methyl-D-aspartate (NMDA) in rats. Intravitreal injection of NMDA (200 nmol) resulted in (1) cell loss in the ganglion cell layer and reduction in the thickness of the inner plexiform layer, (2) an increase in apoptotic cells, (3) a decrease in parvalbumin-positive amacrine cells, (4) accumulation of leukocytes, and (5) microglia activation. Injection of (S,R)-3-(4-hydroxyphenyl)-4,5-dihydro-5-isoxazole acetic acid methyl ester (ISO-1, 100 nmol), a MIF antagonist, significantly attenuated these NMDA-induced responses. These findings suggest that ISO-1 exerts protective effects against retinal injuries and that MIF may be a target for neuroprotective intervention in retinal diseases associated with glutamate-induced excitotoxicity.