Diabetic retinopathy (DR) is a complication of diabetes. Several studies have implicated oxidative stress as a fundamental factor in the progression of the disease. The nuclear factor erythroid-2-related factor 2 (Nrf2) is one of the main regulators of redox homeostasis. Glia Müller cells (MC) maintain the structural and functional stability of the retina. The objective of this study was to evaluate the effect of high glucose concentrations on reactive oxygen species (ROS) production and Nrf2 expression levels in rat MC. MC were incubated with normal (NG; 5 mM) or high glucose (HG; 25 mM) for different times. Incubation with HG increased ROS levels from 12 to 48 h but did not affect cell viability. However, exposure to 3 h of HG caused a transient decrease Nrf2 levels. At that time, we also observed a decrease in the mRNA expression of Nrf2 target genes, glutathione levels, and catalase activity, all of which increased significantly beyond initial levels after 48 h of incubation. HG exposure leads to an increase in the p65 subunit of nuclear factor-κB (NF-kB) levels, and its target genes. These results suggest that high glucose concentrations lead to alteration of the redox regulatory capacity of Nrf2 mediated by NF-kB regulation.

Müller cells are the most abundant glial cells in the mammalian retina. Their morphology and metabolism enable them to be in close contact and interact biochemically and physically with almost all retinal cell types, including neurons, pericytes, endothelial cells, and other glial cells, influencing their physiology by releasing bioactive molecules. Studies indicate that Müller glial cells are the primary source of angiogenic growth factor secretion in the neuroretina. Because of this, over the past decade, it has been postulated that Müller glial cells play a significant role in maintaining retinal vascular homeostasis, with potential implications in vasoproliferative retinopathies. This review aims to summarize the current understanding of the mechanisms by which Müller glial cells influence retinal angiogenesis in health and disease, with a particular emphasis on three of the retinopathies with the most significant impact on visual health worldwide: diabetic retinopathy, retinopathy of prematurity, and age-related macular degeneration.

Diabetic retinopathy is the most common cause of vision loss among diabetic patients. Although hyperglycemia produces retinal oxidative stress in long-standing diabetes, the pathogenesis mechanism is unknown. The Nuclear factor erythroid 2-related factor 2 (Nrf2) plays a central role in cell responses against oxidative damage. We used adult Long Evans rats where diabetes was induced by streptozotocin. Normal and treated rats were sacrificed at 7, 20, and 45 days after streptozotocin injection. We analyzed Nrf2 and Keap1 expression in retinal homogenates, cytoplasmic, and nuclear retinal fractions. Normal retina showed Nrf2 expression in all retina nuclear layers. We found a transitory decrease of Nrf2 mRNA and protein expression at 7 and 20 days after the streptozotocin injection that recovered later on: moreover, the protein level increased after 45 days. Keap1 immunoprecipitation revealed similar levels as Nrf2 in normal and diabetic rat retinas, indicating that the diabetic condition did not lead to dissociation of the Keap1-Nrf2 complex. Indeed, glutathione levels and superoxide dismutase activity were not altered in the treated rat retinas. These results do not support oxidative stress in the retina shortly after diabetes induction.

Gamma‐aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). The biological effects of GABA are mediated by activating its receptors, GABA_A or GABA_B, which are distributed across various tissues, predominantly in the brain and retina. The retina is a neural tissue responsible for receiving and transducing light stimuli. Within this tissue, GABA receptors (GABARs) primarily mediate lateral inhibition in the retina. In recent years, several studies have focused on elucidating the potential role of GABARs in retinal diseases, such as diabetic retinopathy (DR), retinitis pigmentosa (RP), glaucoma, and age‐related macular degeneration (AMD). This review aims to present the most significant findings from recent years regarding the involvement of GABA receptors in retinal pathologies. Additionally, it highlights the potential of GABA receptors as targets for developing precise therapies or adjunctive strategies for treating retinal diseases.

GABA receptors are classically known for driving neuronal hyperpolarization and modulating synaptic transmission. In glial cells, however, GABA induces depolarization and triggers calcium-dependent signaling pathways. Müller glia, the principal retinal glial population, maintain retinal homeostasis and are the major source of neuroretinal VEGF-A, a key angiogenic factor in development and disease. Although GABA receptor (GABAR) activity has been proposed to influence retinal VEGF-A, it remains unclear whether this regulation occurs through Müller glial cells (MGC) and which mechanisms are involved. Here, we investigated how GABAR activation modulates VEGF-A in primary mouse MGC cultures. Cells were exposed to GABA and selective agonists or antagonists of GABA_A (muscimol, gabazine) and GABA_B receptors (baclofen, CGP55845). VEGF-A expression and secretion were analyzed by immunofluorescence, western blot, RT-qPCR, and ELISA. To assess Ca²⁺ involvement, we used Ca²⁺-free Ringer-Krebs solution and the L-type channel blocker nimodipine, and examined MAPK signaling with the ERK1/2 inhibitor FR180204. Our findings show that GABA and muscimol increased VEGF-A fluorescence intensity after 48 hours while reducing VEGF-A secretion, without altering Vegfa mRNA. Both effects were abolished by extracellular Ca²⁺ removal or nimodipine, indicating a Ca²⁺-dependent mechanism. FR180204 also attenuated GABA- and GABA_A-mediated effects, implicating MAPK signaling. Short-term assays revealed that GABA rapidly elevates VEGF-A protein and secretion within ∼30 minutes. Together, these findings identify a Ca²⁺- and GABA_A-dependent pathway through which Müller glia regulate VEGF-A production and release, providing new insight into glial signaling and neurotransmitter-driven modulation of retinal angiogenic factors.

Research on diabetic retinopathy (DR) usually emphasizes hyperglycemia and other causes like dyslipidemia, which are still not well understood. This study examined the effects of palmitic acid (PA) exposure, alone and combined with high glucose (G25), on Müller Glial Cell (MGC) dysfunction and angiogenic signaling. Primary MGC cultures were treated with G25 (25 mM), PA (250 µM), or PA + G25 for 24 and 48 h, followed by assessments of cell viability and analysis of the Vascular Endothelial Growth Factor (VEGFA)/VEGFA receptor 2 (VEGFR2) pathway through immunofluorescence, Western blot, and ELISA. Additionally, Gaussian mixture models (GMMs) were used to identify phenotypic subpopulations based on fluorescence intensity. The results showed that while hyperglycemia did not cause significant changes, PA and PA + G25 induced apoptosis-related cell death and significantly increased the expression of VEGFA, VEGFR2, HIF-α, and SP1. Although broad phenotypic diversity was observed at 24 h, by 48 h, a distinct shift towards an angiogenic phenotype was noted, with significantly elevated VEGFA/VEGFR2 levels. In summary, this research demonstrates that PA acts as a critical inducer of an angiogenic secretory phenotype in MGCs, indicating that lipid-mediated signaling plays a vital role in neovascularization in DR, possibly independent of glucose levels.