The Role of Transforming Growth Factor-Beta in Retinal Ganglion Cells with Hyperglycemia and Oxidative Stress

Role of myeloid cells in ischemic retinopathies: recent advances and unanswered questions

Myeloid cells including microglia and macrophages play crucial roles in retinal homeostasis by clearing cellular debris and regulating inflammation. These cells are activated in several blinding ischemic retinal diseases including diabetic retinopathy, where they may exert both beneficial and detrimental effects on neurovascular function and angiogenesis. Myeloid cells impact the progression of retinal pathologies and recent studies suggest that targeting myeloid cells is a promising therapeutic strategy to mitigate diabetic retinopathy and other ischemic retinal diseases. This review summarizes the recent advances in our understanding of the role of microglia and macrophages in retinal diseases and focuses on the effects of myeloid cells on neurovascular injury and angiogenesis in ischemic retinopathies. We highlight gaps in knowledge and advocate for a more detailed understanding of the role of myeloid cells in retinal ischemic injury to fully unlock the potential of targeting myeloid cells as a therapeutic strategy for retinal ischemia.

TGF-β Signaling Pathways in the Development of Diabetic Retinopathy

Diabetic retinopathy (DR), a prevalent complication of diabetes mellitus affecting a significant portion of the global population, has long been viewed primarily as a microvascular disorder. However, emerging evidence suggests that it should be redefined as a neurovascular disease with multifaceted pathogenesis rooted in oxidative stress and advanced glycation end products. The transforming growth factor-β (TGF-β) signaling family has emerged as a major contributor to DR pathogenesis due to its pivotal role in retinal vascular homeostasis, endothelial cell barrier function, and pericyte differentiation. However, the precise roles of TGF-β signaling in DR remain incompletely understood, with conflicting reports on its impact in different stages of the disease. Additionally, the BMP subfamily within the TGF-β superfamily introduces further complexity, with BMPs exhibiting both pro- and anti-angiogenic properties. Furthermore, TGF-β signaling extends beyond the vascular realm, encompassing immune regulation, neuronal survival, and maintenance. The intricate interactions between TGF-β and reactive oxygen species (ROS), non-coding RNAs, and inflammatory mediators have been implicated in the pathogenesis of DR. This review delves into the complex web of signaling pathways orchestrated by the TGF-β superfamily and their involvement in DR. A comprehensive understanding of these pathways may hold the key to developing targeted therapies to halt or mitigate the progression of DR and its devastating consequences.

Melatonin antagonizes oxidative stress-induced apoptosis in retinal ganglion cells through activating the thioredoxin-1 pathway

This study aimed to explore the role of melatonin in oxidative stress-induced injury on retinal ganglion cells and the underlying mechanisms. The immortalized RGC-5 cells were treated with H2O2 to induce oxidative injury. Cell viability was measured by Cell Counting Kit-8, and apoptosis was determined by flow cytometry and western blot assays. Reactive oxygen species (ROS), lactate dehydrogenase (LDH), and malondialdehyde (MDA) levels were examined to evaluate oxidative stress levels. In addition, Thioredoxin-1 (Trx1) was silenced in RGC-5 cells using small interfering RNA followed by signaling pathway examination to explore the underlying mechanisms of melatonin in alleviating oxidative injury. Melatonin pre-treatment significantly alleviated H2O2-induced apoptosis in RGC-5 cells. Melatonin also markedly reversed the upregulation of cleaved-caspase 3, cleaved-caspase 9, and Bax expression and downregulation of Bcl-2 expression induced by H2O2. Further analyses presented that melatonin significantly attenuated the increase of ROS, LDH, and MDA levels in RGC-5 cells after H2O2 treatment. Melatonin also abolished the downregulated expression of Superoxide dismutase type 1, Trx1, and Thioredoxin reductase 1, and the reduced activity of thioredoxin reductase in RGC-5 cells after H2O2 treatment. Notably, Trx1 knockdown significantly mitigated the protective effect of melatonin in alleviating H2O2-induced apoptosis and oxidative stress, while administration of compound C, a common inhibitor of c-Jun N-terminal kinase (JNK) signaling, partially reversed the effect of Trx1 silencing, thereby ameliorating the apoptosis and oxidative injury induced by H2O2 in RGC-5 cells. Melatonin could significantly alleviate oxidative stress-induced injury of retinal ganglion cells via modulating Trx1-mediated JNK signaling pathway.

Recent Insights into the Etiopathogenesis of Diabetic Retinopathy and Its Management

Purpose: Diabetic retinopathy (DR) is a microvascular retinal disease associated with chronic diabetes mellitus, characterized by the damage of blood vessels in the eye. It is projected to become the leading cause of blindness, given the increasing burden of the diabetic population worldwide. The diagnosis and management of DR pose significant challenges for physicians because of the involvement of multiple biochemical pathways and the complexity of ocular tissues. This review aims to provide a comprehensive understanding of the molecular pathways implicated in the pathogenesis of DR, including the polyo pathway, hexosamine pathway, protein kinase C (PKC), JAK/STAT signaling pathways, and the renin-angiotensin system (RAS). Methods: Academic databases such as PubMed, Scopus, Google Scholar and Web of Science was systematically searched using a carefully constructed search strategy incorporating keywords like "Diabetic Retinopathy," "Molecular Pathways," "Pharmacological Treatments," and "Clinical Trials" to identify relevant literature for the comprehensive review. Results: In addition to activating other inflammatory cascades, these pathways contribute to the generation of oxidative stress within the retina. Furthermore, it aims to explore the existing pharmacotherapy options available for the treatment of DR. In addition to conventional pharmacological therapies such as corticosteroids, antivascular endothelial growth factors, and nonsteroidal anti-inflammatory drugs (NSAIDs), this review highlights the potential of repurposed drugs, phyto-pharmaceuticals, and novel pipeline drugs currently undergoing various stages of clinical trials. Conclusion: Overall, this review serves as a technical exploration of the complex nature of DR, highlighting both established and emerging molecular pathways implicated in its pathogenesis. Furthermore, it delves into the available pharmacological treatments, as well as the promising repurposed drugs, phyto-pharmaceuticals, and novel drugs currently being evaluated in clinical trials, with a focus on their specific mechanisms of action.

Alleviate oxidative stress in diabetic retinopathy: antioxidant therapeutic strategies

OBJECTIVES

This review outlines the function of oxidative stress in DR and discusses therapeutic strategies to treat DR with antioxidants.

METHODS

Published papers on oxidative stress in DR and therapeutic strategies to treat DR with antioxidants were collected and reviewed via database searching on PubMed.

RESULTS

The abnormal development of DR is a complicated process. The pathogenesis of DR has been reported to involve oxidative stress, despite the fact that the mechanisms underlying this are still not fully understood. Excessive reactive oxygen species (ROS) accumulation can damage retina, eventually leading to DR. Increasing evidence have demonstrated that antioxidant therapy can alleviate the degeneration of retinal capillaries in DR.

CONCLUSION

Oxidative stress can play an important contributor in the pathogenesis of DR. Furthermore, animal experiments have shown that antioxidants are a beneficial therapy for treating DR, but more clinical trial data is needed.

Sestrin2 in diabetes and diabetic complications

Diabetes is a global health problem which is accompanied with multi-systemic complications. It is of great significance to elucidate the pathogenesis and to identify novel therapies of diabetes and diabetic complications. Sestrin2, a stress-inducible protein, is primarily involved in cellular responses to various stresses. It plays critical roles in regulating a series of cellular events, such as oxidative stress, mitochondrial function and endoplasmic reticulum stress. Researches investigating the correlations between Sestrin2, diabetes and diabetic complications are increasing in recent years. This review incorporates recent findings, demonstrates the diverse functions and regulating mechanisms of Sestrin2, and discusses the potential roles of Sestrin2 in the pathogenesis of diabetes and diabetic complications, hoping to highlight a promising therapeutic direction.

Aldehyde Dehydrogenase and Aldo-Keto Reductase Enzymes: Basic Concepts and Emerging Roles in Diabetic Retinopathy

Diabetic retinopathy (DR) is a complication of diabetes mellitus that can lead to vision loss and blindness. It is driven by various biochemical processes and molecular mechanisms, including lipid peroxidation and disrupted aldehyde metabolism, which contributes to retinal tissue damage and the progression of the disease. The elimination and processing of aldehydes in the retina rely on the crucial role played by aldehyde dehydrogenase (ALDH) and aldo-keto reductase (AKR) enzymes. This review article investigates the impact of oxidative stress, lipid-derived aldehydes, and advanced lipoxidation end products (ALEs) on the advancement of DR. It also provides an overview of the ALDH and AKR enzymes expressed in the retina, emphasizing their growing importance in DR. Understanding the relationship between aldehyde metabolism and DR could guide innovative therapeutic strategies to protect the retina and preserve vision in diabetic patients. This review, therefore, also explores various approaches, such as gene therapy and pharmacological compounds that have the potential to augment the expression and activity of ALDH and AKR enzymes, underscoring their potential as effective treatment options for DR.

TGF-β3 Protects Neurons Against Intermittent Hypoxia-Induced Oxidative Stress and Apoptosis Through Activation of the Nrf-2/KEAP1/HO-1 Pathway via Binding to TGF-βRI

Intermittent hypoxia (IH) is the primary pathological manifestation of obstructive sleep apnea (OSA) and the main cause of OSA-induced cognitive impairment. Hippocampal neurons are considered to be critical cells affected by IH. Transforming growth factor-β3 (TGF-β3) is a cytokine with a neuroprotective effect, which plays a crucial role in resisting hypoxic brain injury, while its role in IH-induced neuronal injury is still unclear. Here, we aimed to clarify the mechanism of TGF-β3 protecting IH-exposed neurons by regulating oxidative stress and secondary apoptosis. Morris water maze results revealed that IH exposure was unable to affect the vision and motor ability of rats, but significantly affected their spatial cognition. Second-generation sequencing (RNA-seq) and subsequent experiments supported that IH decreased TGF-β3 expression and stimulated reactive oxygen species (ROS)-induced oxidative stress and apoptosis in rat hippocampus. In vitro, IH exposure significantly activated oxidative stress within HT-22 cells. Exogenous administration of Recombinant Human Transforming Growth Factor-β3 (rhTGF-β3) prevented ROS surge and secondary apoptosis in HT-22 cells caused by IH, while TGF-β type receptor I (TGF-βRI) inhibitor SB431542 blocked the neuroprotective effect of rhTGF-β3. Nuclear factor erythroid 2-related factor 2 (Nrf-2) is a transcription factor preserving intracellular redox homeostasis. rhTGF-β3 improved the nuclear translocation of Nrf-2 and activated downstream pathway. However, Nrf-2 inhibitor ML385 suppressed the activation of the Nrf-2 mechanism by rhTGF-3 and restored the effects of oxidative stress damage. These results indicate that TGF-β3 binding to TGF-βRI activates the intracellular Nrf-2/KEAP1/HO-1 pathway, reduces ROS creation, and attenuates oxidative stress and apoptosis in IH-exposed HT-22 cells.

Fisetin Ameliorates Diabetic Nephropathy-Induced Podocyte Injury by Modulating Nrf2/HO-1/GPX4 Signaling Pathway

Diabetic nephropathy (DN) is one of the most severe microvascular complications of diabetes and has become the leading cause of end-stage renal disease formation. The pathogenesis of diabetic nephropathy is very complex and is still not fully understood. Fisetin is a flavonoid polyphenolic compound that is widely found in different fruits, vegetables, and medicinal plants. Many studies have indicated that it has a variety of pharmacological activities. In this study, we investigated the mechanism of action of fisetin in the protection of DN-induced podocyte injury both in vivo and in vitro. Results showed that fisetin could reduce high glucose (HG)-induced podocyte injury and streptozotocin (STZ)-induced diabetic nephropathy in mice. According to the histopathological staining results, fisetin ameliorated DN-induced glomerular injury in a dose-dependent manner. Western blot and immunofluorescence results showed that fisetin effectively promoted the expression of podocyte functional integrity marker proteins and inhibited the expression of podocyte injury marker proteins. In addition, according to the Western blot and RT-qPCR results, fisetin activates the nuclear translocation of Nrf2 to exert antioxidative stress ability and affects the expression of downstream antioxidant enzymes HO-1, GPX4, and other ferroptosis-related markers, thereby protecting against HG-induced podocyte ferroptosis and oxidative stress injury in DN mice. In summary, this study demonstrated that fisetin could enhance the antioxidative stress capacity of DN mice by promoting the activation of the Nrf2/HO-1/GPX4 signaling pathway in renal tissues, and attenuated HG-induced podocytes injury and STZ-induced DN in mice.

The role of innate immune system mediators in the development of retinal neurodegeneration in type 2 diabetes mellitus

Purpose. To detect the levels of transform growth factors-β (TGF- β1, TGF- β2, TGF- β3), interferon-ʏ (INF- ʏ), matrix metalloproteinase-9 (MMP-9) and S100B protein in blood serum of patients with type 2 diabetes mellitus (DM) and to reveal the connection of these factors with neurodegenerative changes in the retina. Material and methods. 30 patients, averagely aged 60.3, with type 2 DM and no signs of diabetic retinopathy (DR) (the main group) and 30 healthy individuals (control group) were examined using microperimetry and optical coherence tomography. A sandwich variance estimator of solid phase enzyme-linked immunosorbent assay was used to determine the levels of TGF- β 1, TGF- β2, TGF- β3, INF- ʏ, ММР-9 and S100B protein in blood serum of the subjects examined. Results. The patients with type 2 DM were found to experience an increased level of focal loss of retinal ganglion cells and a drop in the average photosensitivity of the retina. The main group also showed a reliable increase in the level of S100B protein and in the serum level of MMP-9 against the control, but no significant difference between the groups was found in the level of TIMP-1. The level of TGF- β2 was significantly higher in the main group, which also showed a deficiency of TGF- β3. No significant difference was found between the two groups in the levels of TGF- β1 or INF- ʏ. In contrast, a positive correlation was revealed between the levels of S100B, MMP-9 and the volume of focal loss of retinal ganglion cells. Conclusion. Patients with type 2 DM and signs of neurodegeneration of the retina reveal a higher activity of some cytokines and MMP-9. This may indicate an important role of neuroinflammation and dysfunction of the immune system in the retinal neurodegeneration process of DM patients. Further research of other cytokins is required to determine early and more sensitive markers of retinal neurodegeneration.

Transforming Growth Factorβ1 Overexpression Is Associated with Insulin Resistance and Rapidly Progressive Kidney Fibrosis under Diabetic Conditions

Diabetes mellitus is a global health problem. Diabetic nephropathy is a common complication of diabetes mellitus and the leading cause of end-stage renal disease. The clinical course, response to therapy, and prognosis of nephropathy are worse in diabetic than in non-diabetic patients. The role of transforming growth factorβ1 in kidney fibrosis is undebatable. This study assessed whether the overexpression of transforming growth factorβ1 is associated with insulin resistance and the rapid progression of transforming growth factorβ1-mediated nephropathy under diabetic conditions. Diabetes mellitus was induced with streptozotocin in wild-type mice and transgenic mice with the kidney-specific overexpression of human transforming growth factorβ1. Mice treated with saline were the controls. Glucose tolerance and kidney fibrosis were evaluated. The blood glucose levels, the values of the homeostasis model assessment for insulin resistance, and the area of kidney fibrosis were significantly increased, and the renal function was significantly impaired in the diabetic transforming growth factorβ1 transgenic mice compared to the non-diabetic transgenic mice, diabetic wild-type mice, and non-diabetic mice. Transforming growth factorβ1 impaired the regulatory effect of insulin on glucose in the hepatocyte and skeletal muscle cell lines. This study shows that transforming growth factorβ1 overexpression is associated with insulin resistance and rapidly progressive kidney fibrosis under diabetic conditions in mice.

Diabetic retinopathy: Involved cells, biomarkers, and treatments

Diabetic retinopathy (DR), a leading cause of vision loss and blindness worldwide, is caused by retinal neurovascular unit dysfunction, and its cellular pathology involves at least nine kinds of retinal cells, including photoreceptors, horizontal and bipolar cells, amacrine cells, retinal ganglion cells, glial cells (Müller cells, astrocytes, and microglia), endothelial cells, pericytes, and retinal pigment epithelial cells. Its mechanism is complicated and involves loss of cells, inflammatory factor production, neovascularization, and BRB impairment. However, the mechanism has not been completely elucidated. Drug treatment for DR has been gradually advancing recently. Research on potential drug targets relies upon clear information on pathogenesis and effective biomarkers. Therefore, we reviewed the recent literature on the cellular pathology and the diagnostic and prognostic biomarkers of DR in terms of blood, protein, and clinical and preclinical drug therapy (including synthesized molecules and natural molecules). This review may provide a theoretical basis for further DR research.

Biochemical Functions and Clinical Characterizations of the Sirtuins in Diabetes-Induced Retinal Pathologies

Diabetic retinopathy (DR) is undoubtedly one of the most prominent causes of blindness worldwide. This pathology is the most frequent microvascular complication arising from diabetes, and its incidence is increasing at a constant pace. To date, the insurgence of DR is thought to be the consequence of the intricate complex of relations connecting inflammation, the generation of free oxygen species, and the consequent oxidative stress determined by protracted hyperglycemia. The sirtuin (SIRT) family comprises 7 histone and non-histone protein deacetylases and mono (ADP-ribosyl) transferases regulating different processes, including metabolism, senescence, DNA maintenance, and cell cycle regulation. These enzymes are involved in the development of various diseases such as neurodegeneration, cardiovascular pathologies, metabolic disorders, and cancer. SIRT1, 3, 5, and 6 are key enzymes in DR since they modulate glucose metabolism, insulin sensitivity, and inflammation. Currently, indirect and direct activators of SIRTs (such as antagomir, glycyrrhizin, and resveratrol) are being developed to modulate the inflammation response arising during DR. In this review, we aim to illustrate the most important inflammatory and metabolic pathways connecting SIRT activity to DR, and to describe the most relevant SIRT activators that might be proposed as new therapeutics to treat DR.

Contribution of Müller Cells in the Diabetic Retinopathy Development: Focus on Oxidative Stress and Inflammation

Diabetic retinopathy is a neurovascular complication of diabetes and the main cause of vision loss in adults. Glial cells have a key role in maintenance of central nervous system homeostasis. In the retina, the predominant element is the Müller cell, a specialized cell with radial morphology that spans all retinal layers and influences the function of the entire retinal circuitry. Müller cells provide metabolic support, regulation of extracellular composition, synaptic activity control, structural organization of the blood–retina barrier, antioxidant activity, and trophic support, among other roles. Therefore, impairments of Müller actions lead to retinal malfunctions. Accordingly, increasing evidence indicates that Müller cells are affected in diabetic retinopathy and may contribute to the severity of the disease. Here, we will survey recently described alterations in Müller cell functions and cellular events that contribute to diabetic retinopathy, especially related to oxidative stress and inflammation. This review sheds light on Müller cells as potential therapeutic targets of this disease.

Investigation on the expression regulation of RIPK1/RIPK3 in the retinal ganglion cells (RGCs) cultured in high glucose

Diabetic retinopathy (DR) represents the most typical complication of type 2 diabetes mellitus and one of the most primary oculopathy causing blindness. However, the mechanism of DR remains unknown. RIPK1/RIPK3, as homologous serine/threonine kinases, are key elements in mediating necroptosis and may have functions in DR development. To clarify the relationship between DR and RIPK1/RIPK3, this study established a model of apoptosis using high-glucose induced RGCs, which were treated with 7.5, 19.5, and 35 mM D-glucose for 12, 24, and 48 h, respectively. Subsequently, the expression of RIPK1/RIPK3 was determined and the protective effect of necrostatin-1 on RGCs injury induced by high glucose was explored. The results demonstrated that the expression of RIPK1 and RIPK3 in the cells was increased markedly following 12 h treatment with 19.5 mM D-glucose. Additionally, following an addition of 100 μM necrostatin-1 in 19.5 mM D-glucose medium for RGCs treatment 12 h, the protein expression of RIPK1 and RIPK3 was decreased markedly, and the number of Nissl bodies in cells was increased substantially. The findings of the present study indicated that high glucose could induce the expression of RIPK1/RIPK3, and necrostatin-1 could effectively protect RGCs from D-glucose-induced cell necrosis.

Differential Retinal Protein Expression in Primary and Secondary Retinal Ganglion Cell Degeneration Identified by Integrated SWATH and Target-Based Proteomics

To investigate the retinal proteins associated with primary and secondary retinal ganglion cell (RGC) degeneration and explore their molecular pathways, SWATH label-free and target-based mass spectrometry was employed to identify the proteomes in various retinal locations in response to localized optic nerve injury. Unilateral partial optic nerve transection (pONT) was performed on adult Wistar rats and their retinas were harvested 2 weeks later. To confirm the separation of primary and secondary RGC degeneration, immunohistochemistry of RNA binding protein with multiple splicing (RBPMS) and glial fibrillary acidic protein (GFAP) was performed on retinal whole-mounts. Retinal proteomes in the temporal and nasal quadrants were evaluated with high resolution hybrid quadrupole time-of-flight mass spectrometry (QTOF-MS), and SWATH-based acquisition, and their expression was compared to the corresponding retinal quadrant in contralateral control eyes and further validated by multiple reaction monitoring mass spectrometry (MRM-MS). A total of 3641 proteins (FDR < 1%) were identified using QTOF-MS. The raw data are available via ProteomeXchange with the identifier PXD026783. Bioinformatics data analysis showed that there were 37 upregulated and 25 downregulated proteins in the temporal quadrant, whereas 20 and five proteins were upregulated and downregulated, respectively, in the nasal quadrant, respectively (n = 4, p < 0.05; fold change ≥ 1.4-fold or ≤0.7). Six proteins were regulated in both the temporal and the nasal quadrants, including CLU, GFAP, GNG5, IRF2BPL, L1CAM, and CPLX1. Linear regression analysis indicated a strong association between the data obtained by means of SWATH-MS and MRM-MS (temporal, R² = 0.97; nasal, R² = 0.96). Gene ontology analysis revealed statistically significant changes in the biological processes and cellular components of primary RGC degeneration. The majority of the significant changes in structural, signaling, and cell death proteins were associated with the loss of RGCs in the area of primary RGC degeneration. The combined use of SWATH-MS and MRM-MS methods detects and quantifies regional changes of retinal protein expressions after localized injury. Future investigation with this integrated approach will significantly increase the understanding of diverse processes of progressive RGC degeneration from a proteomic prospective.

An In Vitro Model of Diabetic Retinal Vascular Endothelial Dysfunction and Neuroretinal Degeneration

BACKGROUND

Diabetic retinopathy (DR) is a leading cause of blindness in working-age populations. Proper in vitro DR models are crucial for exploring pathophysiology and identifying novel therapeutic targets. This study establishes a rational in vitro diabetic retinal neuronal-endothelial dysfunction model and a comprehensive downstream validation system.

METHODS

Human retinal vascular endothelial cells (HRMECs) and retinal ganglion cells (RGCs) were treated with different glucose concentrations with mannitol as matched osmotic controls. Cell proliferation and viability were evaluated by the Cell Counting Kit-8. Cell migration was measured using a transwell migration assay. Cell sprouting was assessed by a tube formation assay. The VEGF expression was assessed by ELISA. RGCs were labeled by neurons and RGC markers TUJ1 and BRN3A for quantitative and morphological analysis. Apoptosis was detected using PI/Hoechst staining and TUNEL assay and quantified by ImageJ.

RESULTS

Cell proliferation and migration in HRMECs were significantly higher in the 25 mM glucose-treated group (p < 0.001) but lower in the 50 mM and 100 mM groups (p < 0.001). The permeability and the apoptotic index in HRMECs were statistically higher in the 25 mM, 50 mM, and 100 mM groups (p < 0.05). The tube formation assay found that all the parameters were significantly higher in the 25 mM and 50 mM groups (p < 0.001) concomitant with the elevated VEGFA expression in HRMECs (p = 0.016). Cell viability was significantly lower in the 50 mM, 100 mM, and 150 mM groups in RGCs (p _50mM = 0.013, p _100mM = 0.019, and p _150mM = 0.002). Apoptosis was significantly elevated, but the proportion of RGCs with neurite extension was significantly lower in the 50 mM, 100 mM, and 150 mM groups (p _50mM < 0.001, p _100mM < 0.001, and p _150mM < 0.001).

CONCLUSIONS

We have optimized glucose concentrations to model diabetic retinal endothelial (25-50 mM) or neuronal (50-100 mM) dysfunction in vitro, which have a wide range of downstream applications.