In their recent publication, Song Li and Tatjana C. Jakobs, researchers at Harvard’s Schepens Eye Research Institute in Boston, Massachusetts, report an exciting discovery. They have identified Vitamin C as a potential neuroprotective substance that could be used clinically in patients suffering from glaucoma.
The neurodegenerative disease glaucoma is characterized by retinal degradation due to ganglion cell death as well as an increased number of reactive astrocytes in the optic nerve – cells that show abnormal functional and morphological patterns. Disease progression is associated with vision loss.
Lowering increased intraocular pressure (IOP) – the only current treatment option
There are several factors that increase the risk for getting glaucoma, including family history, age, and elevated intraocular pressure (IOP) due to aqueous fluid buildup in the eye. This can either be caused by increased fluid production or faulty drainage through the trabecular meshwork. Of these risk factors, IOP is the only one that is modifiable by treatment, either through medication or surgery. However, these treatments are not always successful. It is therefore highly desirable to identify supportive treatment options that protect the retina and slow down or even prevent ganglion cell death during glaucoma.
The scientific breakthrough of Li’s and Jakobs’ research consists of identifying substances that protect the retinal tissue from degradation and can therefore postpone or prevent vision loss significantly. The study focuses on a neuroprotective state of astrocytes. Astrocytes react to retinal ganglion cell (RGC) damage by adapting a short-term neuroprotective state that is characterized by a decreased expression of toxic mediators and an up-regulation of genes associated with neuroprotection, including increased phagocytosis, oxidative phosphorylation and the release of neurotropic factors that support overall RGC survival. A potential strategy for further drug development, besides the decrease of IOP, is therefore the identification of substances that regulate and upkeep this supportive astrocyte state.
Vitamin C enhances retinal ganglion cell (RGC) survival via SPP1 mediation
In a previous study, Li and Jakobs have demonstrated that the protein SPP1 (secreted phosphoprotein 1) has strong neuroprotective effects: it promotes RGC survival and thus helps to maintain visual function. However, using SPP1 as a direct treatment agent would be difficult. It would require intravitreal injection which can cause complications and retinal damage. Therefore, in this study, Li and Jakobs set out to identify small molecules that would enhance SPP1-expression, and they successfully identified both vitamin A and C. The toxicity of high vitamin A dosage limits its application possibilities, prompting Li and Jakobs to study the effects of Vitamin C more closely.
This research approach was successful: Treating astrocytes in in vitro culture with Vitamin C or its sodium salt (sodium ascorbate) showed an upregulation in SPP1. Additionally, the astrocytes assumed their neuroprotective state upon treatment. This happened in a SPP1-dependent manner since spp1-knock out (KO) astrocytes did not adopt a neuroprotective state. Similarly, applying Vitamin C to the retina increased SPP1 expression in retinal ganglion cells, and it had the same effect on astrocytes as in culture.
Results were confirmed when applied to glaucoma mouse models
To demonstrate the beneficial effect of inducing the neuroprotective astrocyte phenotype for glaucoma disease, Li and Jakobs worked with a mouse model of glaucoma disease. In these mice, IOP was increased by injecting microbeads into one eye, which interferes with aqueous humor drainage. The mice show reduced survival of retinal ganglion cells and reduced visual function in the modified eye, while the other eye serves as a control. When Vitamin C was administered to such mice through drinking water, visual function and RGC survival showed a significant increase in comparison to control groups that did not receive Vitamin C. Li and Jakobs used Striatech’s OptoDrum to determine the visual behavior of their mice. With the OptoDrum, they could demonstrate at the behavioral level that microbead injection worsened vision specifically in the injected eye, and that Vitamin C had positive effects, measuring the visual function for each eye independently.
Vitamin C was clearly beneficial in vivo, improved the functional readouts in the treated mice, and it also reduced the IOP. Interestingly, these benefits were also observed in spp1-KO animals, which somewhat contradicts the earlier findings that Vitamin C acts through an SPP1-dependent pathway in astrocytes. One possibility was that the reduction of IOP was sufficient to increase ganglion cell survival, and that Vitamin C activation of the SPP1 pathway is not required in this model. To test this, the researchers performed an additional set of experiments with an IOP-independent mouse model of ganglion cell death, the optic nerve crush model. In this model, RGC protection by Vitamin C was detected only in wild type mice, not in spp1-KO mice, suggesting that Vitamin C can indeed have a SPP1-mediated neuroprotective effect also in vivo. This SPP1-mediated action of Vitamin C has not been known before, and it adds to many known beneficial roles of Vitamin C in the body.
The clinical potential of Vitamin C administration in humans
How is this applicable to humans? Mice can synthesize their own Vitamin C via the GULO-gene, which is only a pseudogene in humans. This means that we rely on dietary supplementation of Vitamin C in our day-to-day life. Excessive intake of Vitamin C beyond the recommended daily dose of 75-90 mg is excreted through the urine and has not shown any beneficial effects. In fact, very high (>2 g per day) Vitamin C supply can even cause gastrointestinal disturbances. The authors suggest that patients that are already optimally supplied with Vitamin C through a healthy diet may not benefit from additional Vitamin C. However, a sizable number of patients may have suboptimal Vitamin C intake, and this can increase the rate of RGC loss and hasten disease progression. Indeed, a clear correlation has been observed in patients between lower Vitamin C concentration in the serum and increased IOP. This could suggest that increasing Vitamin C intake can be beneficial in such glaucoma patients. The additional administration of Vitamin C cannot replace other treatments in glaucoma patients, but the positive effects of sticking to dietary guidelines are very likely.
Emilia Kawecka, University of Tübingen, Student Assistant at Striatech
Li, S., Jakobs, T. C. Vitamin C protects retinal ganglion cells via SPP1 in glaucoma and after optic nerve damage. Life Sci Alliance 6(8), (2023). doi:10.26508/lsa.202301976
Mohan N, Chakrabarti A, Nazm N, Mehta R, Edward DP. Newer advances in medical management of glaucoma. Indian J Ophthalmol. 2022 Jun;70(6):1920-1930. doi: 10.4103/ijo.IJO_2239_21.
Li S, Jakobs TC. Secreted phosphoprotein 1 slows neurodegeneration and rescues visual function in mouse models of aging and glaucoma. Cell Rep. 2022 Dec 27;41(13):111880. doi: 10.1016/j.celrep.2022.111880.