Oxidative Stress Pathways from High Sugar Exposure: Nrf2, Mitochondria, and RGC Survival

This audio article is from VisualFieldTest.com.Read the full article here: https://visualfieldtest.com/en/oxidative-stress-pathways-from-high-sugar-exposure-nrf2-mitochondria-and-rgc-survivalTest your visual field online: https://visualfieldtest.comSupport the show so new episodes keep coming: https://www.buzzsprout.com/2563091/supportExcerpt:Oxidative Stress Pathways from High Sugar Exposure: Nrf2, Mitochondria, and Retinal Ganglion Cell Survival Introduction High sugar exposure is often discussed as though it has one simple effect on the eye. The biology is more complicated. In experimental studies, elevated D-glucose can increase reactive oxygen species, disrupt mitochondrial energy production, alter antioxidant defenses, activate retinal glial cells, and promote retinal ganglion cell injury. Most of the evidence comes from two types of research: Cell-culture studies, in which retinal cells are exposed to glucose concentrations commonly ranging from 20 to 40 millimoles per liter, sometimes for several days. Diabetic animal models, in which chronic hyperglycemia occurs together with vascular dysfunction, inflammation, lipid abnormalities, advanced glycation products, and tissue hypoxia. These models do not prove that one high-sugar meal directly kills retinal ganglion cells. They do, however, support a biological pathway in which persistent or repeated hyperglycemia increases oxidative pressure and reduces the ability of ocular cells to recover. The most consistent model is: > High glucose → increased reactive oxygen and nitrogen species → mitochondrial stress and impaired energy production → inadequate nuclear factor erythroid 2-related factor 2 antioxidant activity → inflammation, glial reactivity, and retinal ganglion cell apoptosis. The strength of each link varies by cell type, glucose concentration, exposure time, and whether the experiment involves diabetes, glaucoma-related stress, or isolated high glucose. --- The central redox system: reactive oxygen species and nuclear factor erythroid 2-related factor 2 Reactive oxygen species are chemically reactive oxygen-containing molecules. In small amounts, they participate in normal cell signaling. When their production exceeds antioxidant capacity, they can oxidize proteins, membrane lipids, mitochondrial DNA, and other cellular structures. The main protective pathway discussed in this context is controlled by nuclear factor erythroid 2-related factor 2, commonly called Nrf2. Under resting conditions, Nrf2 is held in the cytoplasm by Kelch-like ECH-associated protein 1, or Keap1, which promotes its degradation. When oxidative stress is sensed, Nrf2 can stabilize, move into the nucleus, and stimulate antioxidant-response genes. These include genes involved in glutathione production, superoxide removal, hydrogen peroxide breakdown, thioredoxin recycling, and detoxification. () Important Nrf2-regulated defenses in retinal cells include: Glutamate-cysteine ligase, which supports glutathione production Superoxide dismutase 2, a mitochondrial antioxidant enzyme Catalase Thioredoxin Heme oxygenase-1 NAD(P)H quinone oxidoreductase 1 A key finding across the literature is that high glucose does not always completely turn off Nrf2. Instead, the response may be delayed, transient, cell-specific, or insufficient for the amount of oxidative stress produced. --- How high glucose increases oxidative stress in ocular cells Increased metabolic pressure Retinal neurons and glial cells are metabolically active. When excess glucose enters the cell, more carbon is directed through glycolysis and mitochondrial energy pathways. This can increase the supply of reducing equivalents to the mitochondrial respiratory chain. If electron transport becomes overloaded or poorly coordinated, electrons can leak and generate superoxide. Experimental retinal studies have also identified non-mitochondrial contributors, including: Activation of nicotinamide adenine dinucleotide phosphate oxidase Increased inflammatory signaling through nuclear factor kappa B Thioredoxin-interacting protein induction Nitric oxide and reactive nitrogen species production Disruption of glutathione and antioxidant enzyme systems In Müller glial cells, high glucose increased reactive oxygen species within hours and increased reactive nitrogen species even earlier. The same study found increased nuclear factor kappa B signaling and increased inducible nitric oxide synthase, linking redox imbalance to inflammation. () Oxidative stress can become self-reinforcing Mitochondria are both a source and a target of reactive oxygen species. Once mitochondrial proteins, membranes, or mitochondrial DNA are damaged, the respiratory chain may become less efficient. This can produce still more oxidant leakage, creating a feedback loop: > High glucose → mitochondrial stress → more reactive oxygen species → further mitochondrial damage. In Müller cells, high glucose increased mitochondrial superoxide, disrupted mitochondrial membrane potential, and increased the mitochondrial localization of thioredoxin-interacting protein. These changes were associated with mitochondrial dysfunction and impaired mitochondrial quality control. () --- Evidence for glucose-induced mitochondrial dysfunction Müller glial cells One of the clearest mitochondrial studies used a rat Müller cell line exposed to 30 millimoles per liter glucose for seven days. High glucose caused: Mitochondrial fragmentation Greater variation in mitochondrial membrane potential Lower basal and maximal oxygen consumption Lower extracellular acidification Cytochrome c release Increased apoptosis These findings are important because they connect mitochondrial structural changes with functional failure and cell death rather than merely showing an increase in a laboratory oxidative-stress marker. () A related study found that high glucose increased thioredoxin-interacting protein, mitochondrial superoxide, and mitochondrial membrane-potential abnormalities in rat Müller cells. The changes were accompanied by impaired mitophagy, the process that removes damaged mitochondria. () More recent work suggests that mitochondrial remodeling is not always the same in every stage of disease. Under diabetic conditions, Müller cells may initially use mitochondrial fragmentation as an adaptive response to remove damaged segments. In more advanced neurodegeneration, mitochondrial hyperfusion and defective turnover may develop instead. This means that mitochondrial shape alone cannot be interpreted without considering exposure duration and disease stage. () Retinal ganglion cells Retinal ganglion cells are especially vulnerable to mitochondrial stress because they have long axons and substantial energy requirements. In high-glucose retinal ganglion cell models, investigators have reported: Increased reactive oxygen species Loss or abnormality of mitochondrial membrane potential Cytochrome c release Reduced antioxidant enzyme activity Increased B-cell lymphoma-2-associated X protein and caspase signaling Increased terminal deoxynucleotidyl transferase dUTP nick-end labeling, a marker of DNA fragmentation Reduced cell survival In one study, L-carnitine, a mitochondrial-supporting molecule, reduced glucose-induced reactive oxygen species and lipid peroxidation. It also restored mitochondrial membrane potential, reduced cytochrome c release, lowered caspase-9 and caspase-3 activation, and shifted the B-cell lymphoma-2-associated X protein to B-cell lymphoma-2 balance toward cell survival. () A 2024 study using a retinal precursor cell model and diabetic mice found that high glucose or diabetes was associated with mitochondrial dysfunction, reactive oxygen species production, abnormal antioxidant protein expression, and retinal ganglion cell apoptosis. Increasing DJ-1, a protein involved in mitochondrial protection and redox control, improved mitochondrial function and reduced oxidative injury. () Mitochondrial DNA damage Mitochondrial DNA is located close to the respiratory chain and has less protection than nuclear DNA. A recent retinal ganglion cell study reported that high glucose was associated with mitochondrial structural abnormalities, increased reactive oxygen species, abnormal mitochondrial membrane potential, lower oxygen consumption, lower adenosine triphosphate production, and mitochondrial DNA damage. The authors also observed reduced expression of proteins involved in DNA damage repair. () These results support a model in which mitochondrial damage is not simply a consequence of apoptosis. It may occur early enough to contribute to the decision of a retinal ganglion cell to enter an apoptotic pathway. --- Evidence for impaired or inadequate Nrf2 antioxidant responses A transient Nrf2 failure in Müller cells A detailed time-course study exposed primary rat Müller cells to 25 millimoles per liter glucose for one to 48 hours. The findings show why the phrase “Nrf2 impairment” needs to be used carefully: Reactive oxygen species increased from approximately 12 to 48 hours. Reactive nitrogen species rose earlier. Nrf2 protein and nuclear Nrf2 temporarily decreased after approximately three hours. Glutamate-cysteine ligase, superoxide dismutase 2, and thioredoxin messenger RNA fell during the early exposure period. Glutathione remained low even after some Nrf2 measures recovered. Nrf2 and several antioxidant genes increased later, suggesting a delayed compensatory response. Thus, high glucose produced an early window in which oxidative stress increased while the Nrf2 antioxidant program was tempSupport the show