This audio article is from VisualFieldTest.com. Read the full article here: https://visualfieldtest.com/en/glaucoma-and-glutamine-is-there-a-real-link-through-glutamate-retinal-metabolism-and-neurodegeneration Test your visual field online: https://visualfieldtest.com Support the show so new episodes keep coming: https://www.buzzsprout.com/2563091/support Excerpt: Executive Summary Glutamine is a common amino acid in the body, but current evidence does not show that glutamine itself causes or treats glaucoma. Instead, glutamine is part of the normal glutamate–glutamine cycle in the nervous system, including the retina (). In glaucoma (a disease where retinal ganglion cells and the optic nerve degenerate), researchers have wondered whether excitotoxic damage from too much glutamate may play a role. Since glutamine is the main precursor for glutamate, it is studied as an indirect marker of this process. Some experimental studies (mostly in animals or lab models) show changes in glutamine handling by retinal glial cells when pressure or blood flow is disturbed. A few small human studies found glaucoma patients had slightly higher glutamine in the eye’s fluids (), while others found no difference () (). Overall the human data are limited and inconsistent. Glutamine supplements have not been shown to help glaucoma, and no clinical trial has tested this. There is also no evidence that taking or avoiding glutamine changes eye pressure or disease. In practical terms, the main proven treatment for glaucoma remains lowering eye pressure (with drops, laser, or surgery), not dietary changes. What is Glutamine? Glutamine (Gln) is one of the body’s most abundant free amino acids. It serves many roles: a building block for proteins, a fuel for immune and gut cells, and a carrier of nitrogen between tissues (). Under stress or illness, cells use glutamine quickly and it can become “conditionally essential” (meaning we may need more from food or supplements) (). Glutamate (Glu) is a closely related amino acid that acts as a major excitatory neurotransmitter in the brain and retina. In contrast, glutamine itself is not an excitatory neurotransmitter. Instead, it is a “converter” or storage form. Neurons use glutamine mostly to re-synthesize glutamate. High extracellular glutamate can be toxic to neurons (a process called excitotoxicity), but glutamine is not toxic and does not directly activate glutamate receptors (). The glutamate–glutamine cycle: In the retina (and brain), neurons and glial cells recycle glutamate and glutamine in a tight loop (). For example: A neuron (such as a retinal ganglion cell) releases glutamate at its synapse. Nearby Müller glial cells (the main support cells in the retina) quickly take up this glutamate and convert it into glutamine (). The Müller cell then releases glutamine back to neurons. Neurons take up glutamine and convert it back into glutamate for future signaling. In effect, glutamine is a “safe” way to mop up excess glutamate. It keeps the fast-acting glutamate neurotransmitter within neurons and prevents glutamate from lingering too long outside cells, which could be harmful (). The cycle is illustrated conceptually below: Neuron releases glutamate → Glial cell converts glutamate → glutamine → Glial cell sends glutamine back → Neuron converts glutamine back to glutamate. () This recycling ensures that neurotransmitter levels remain balanced. Importantly, disturbances in this cycle (for example if glial cells fail to clear glutamate) can allow glutamate buildup and potentially cause excitotoxic damage to neurons. Why Could Glutamine Matter in Glaucoma? Glaucoma basics: Glaucoma is a group of eye diseases leading to optic nerve damage and vision loss, usually by death of retinal ganglion cells (RGCs). The most common form is primary open-angle glaucoma (POAG), often associated with elevated intraocular pressure (IOP). Another form is normal-tension glaucoma, where nerve damage occurs at normal pressures. Regardless of pressure, glaucoma involves progressive RGC loss. The National Eye Institute and others describe glaucoma as an optic neuropathy (nerve disease) that leads to peripheral vision loss and eventual blindness if untreated () (). Excitotoxicity hypothesis: Because glutamate is known to kill retinal neurons in lab studies (for example, injecting glutamate into the eye causes RGC death), scientists have long hypothesized that elevated glutamate could contribute to glaucoma damage. Some early studies reported higher vitreous (eye fluid) glutamate in glaucomatous eyes, suggesting an “excitotoxic” mechanism () (). In one review, it was noted that glaucoma patients had about 27 μM glutamate in vitreous vs 11 μM in controls, enough to harm RGCs (). However, other studies (including Honkanen et al. 2003) found no significant increase in ocular glutamate or glutamine in glaucoma patients () (). The role of glutamate excitotoxicity in human glaucoma remains unproven. Glutamine’s indirect role: Because glutamine is the precursor and breakdown product of glutamate, it is studied indirectly. If glutamate were accumulating, one might see changes in glutamine too. For example, one recent hypothesis is that in glaucoma, Müller glial cells may raise glutamine production in order to keep free glutamate levels low and protect neurons (). In effect, more glutamine in eye fluids might reflect an attempt to buffer glutamate. This is only speculative. The frontiers study (Lillo et al.) mentions that higher aqueous glutamine in glaucoma “could be a means of keeping the concentration of glutamate under control, thus avoiding [neuron] death” (). But whether this happens or matters in patients is unknown. Müller cell and astrocyte changes: Glial cells (Müller cells in retina, astrocytes in optic nerve head) normally regulate glutamate-glutamine recycling. In animal glaucoma models, these glial cells sometimes become reactive or dysfunctional. For instance, experimental glaucoma in monkeys led to higher glutamine labeling in Müller cells (), suggesting they were still converting extra glutamate to glutamine. In rat studies, raising intraocular pressure briefly actually blocked the increase in glial glutamine-synthetase (GS) that would normally follow glutamate exposure (). Only after one week of continued pressure did Müller cells resume raising GS as before. This hints that acute pressure spikes might temporarily impair glial glutamate clearance (). Such mechanistic findings show that the glutamate–glutamine cycle can be altered by glaucoma-like conditions, but they do not prove that glutamine itself is toxic or protective. They simply underscore that late-stage RGC death in glaucoma could involve metabolic stress in glial cells. Human Research: Glutamine/Glutamate Levels in Glaucoma Studies in humans have looked for differences in glutamine or related metabolites in the eye or blood of glaucoma patients. The results are mixed and generally not definitive: Aqueous humor (eye fluid) studies: New metabolomics analyses of aqueous humor (the fluid in the front of the eye) found that glaucoma patients had higher glutamine levels than controls. For example, a 2022 Frontiers in Medicine study reported median glutamine ~697 μM in glaucoma patients vs ~563 μM in cataract controls (). This was statistically significant and the authors noted glutamine (but not glutamate) was elevated in treated glaucoma. They suggested this might help keep glutamate low in the eye (). However, older analyses of aqueous humor (and vitreous) have not consistently confirmed this. A systematic review of glaucoma metabolomics noted that some studies found glutamine increased (e.g. Buisset et al. 2019; Tang et al. 2021) while others saw it decreased or unchanged (e.g. Myer et al. 2020) (). In meta-analysis of multiple aqueous humor studies in open-angle glaucoma, glutamine was often reported as an affected metabolite, but the findings went in opposite directions in different studies (). Overall, aqueous humor data suggest there are metabolic changes in glaucoma, but the specific role of glutamine is uncertain. Vitreous humor (eye gel) studies: Vitreous samples from glaucoma eyes have been measured in a few small studies. Honkanen et al. (2003) measured 16 amino acids (including glutamate and glutamine) in vitreous from glaucoma patients undergoing vitrectomy (usually for other eye problems) versus controls. They found no significant difference in glutamine (and no significant difference in glutamate) between groups (). The average glutamine was ~1200 μM in both glaucoma and control eyes, with p>.99 (). This argues against a large buildup of glutamate or its precursor glutamine in human glaucoma vitreous. (Earlier, Dreyer 1996 had reported higher glutamate in vitreous of glaucoma patients (), but that finding was not replicated by Honkanen.) In experimental eyes, a rabbit model of optic nerve ischemia (simulating glaucoma) also showed no change in vitreous glutamine, even though glutamate tripled (). So human vitreous data to date do not support a glutamine difference. Blood/serum studies: There is little data on glutamine in the blood of glaucoma patients. Metabolomics studies of patient plasma have identified many molecules altered in glaucoma, but glutamine specifically has not emerged as a clear marker in blood. For example, Tang et al. (2021) profiled plasma metabolites in POAG versus cataract controls and found some energy-related changes (like purine metab Support the show
