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Background: Progressive retinal ganglion cell (RGC) dysfunction and death are common characteristics of retinal neurodegenerative diseases. Recently, hydroxycarboxylic acid receptor 1 (HCA1R, GPR81) was identified as a key modulator of mitochondrial function and cell survival. Thus, we aimed to test whether activation of HCA1R with 3,5-Dihydroxybenzoic acid (DHBA) also promotes RGC survival and improves energy metabolism in mouse retinas. Methods: Retinal explants were treated with 5 mM of the HCA1R agonist, 3,5-DHBA, for 2, 4, 24, and 72 h. Additionally, explants were also treated with 15 mM of L-glutamate to induce toxicity. Tissue survival was assessed through lactate dehydrogenase (LDH) viability assays. RGC survival was measured through immunohistochemical (IHC) staining. Total ATP levels were quantified through bioluminescence assays. Energy metabolism was investigated through stable isotope labeling and gas chromatography-mass spectrometry (GC-MS). Lactate and nitric oxide levels were measured through colorimetric assays. Results: HCA1R activation with 3,5-DHBAincreased retinal explant survival. During glutamate-induced death, 3,5-DHBA treatment also increased survival. IHC analysis revealed that 3,5-DHBA treatment promoted RGC survival in retinal wholemounts. 3,5-DHBA treatment also enhanced ATP levels in retinal explants, whereas lactate levels decreased. No effects on glucose metabolism were observed, but small changes in lactate metabolism were found. Nitric oxide levels remained unaltered in response to 3,5-DHBA treatment. Conclusion: The present study reveals that activation of HCA1R with 3,5-DHBA treatment has a neuroprotective effect specifically on RGCs and on glutamate-induced retinal degeneration. Hence, HCA1R agonist administration may be a potential new strategy for rescuing RGCs, ultimately preventing visual disability.

PurposeRetinal ganglion cell (RGC) death is a common characteristic for ocular neurodegenerative diseases such as glaucoma and optic neuropathies. Recently, GPR81 agonist treatment has been identified as a key modulator of mitochondrial function and cell survival. Thus, we aimed to test whether GPR81 agonist treatment likewise promotes RGC survival and energy metabolism in retinal explants and wholemounts from mice.MethodsRetinal explants were treated with 5 mM of the GPR81 agonist, 3,5‐DHBA, for two, four, 24 & 72 hours, and compared to conditions with no treatment. Additionally, explants were also treated with 15 mM of L‐glutamate to induce toxicity and simultaneously treated with GPR81 agonist. Tissue survival was assessed through lactate dehydrogenase (LDH) viability assays. Retinal ganglion cell survival was measured in murine wholemount retinas through immunohistochemical staining (IHC). Total ATP levels were quantified through bioluminescence assays.ResultsGPR81 agonist treatment increased retinal explant survival after 24 and 72 hours of exposure. No significant effect was seen in retinal explants survival after GPR81 agonist treatment alone for two hours. However, during glutamate‐induced death, supplemented GPR81 agonist treatment increased survival compared to conditions with glutamate toxicity. IHC analysis revealed that GPR81 agonist treatment for two hours promoted RGC survival in retinal wholemounts compared with no treatment. GPR81 agonist treatment also enhanced ATP levels in retinal explants after two, 24, and 72 hr of exposure.ConclusionsThe present study reveals that GPR81 agonist treatment has a neuroprotective effect on specifically RGCs and on glutamate‐induced retinal degeneration. Hence, GPR81 agonist administration may be a potential new strategy to sustain RGCs, ultimately preventing visual disability as a consequence of RGC death.

PurposeVisual changes are some of the earliest symptoms that patients with Alzheimer’s disease (AD) experience1. Pathophysiological processes such as amyloid‐β plaque formation, vascular changes, neuroinflammation, and loss of retinal ganglion cells (RGCs) have been detected in the retina of AD patients and animal models1. However, little is known about the molecular processes that underlie retinal neurodegeneration in AD. The cellular architecture and constant sensory activity of the retina impose high metabolic demands2. We thus hypothesized that energy metabolism might be compromised in the AD retina similarly to what has been observed in the AD brain3.Methods/ ResultsTo address this question, we explored cellular alterations and retinal metabolic activity in the 5xFAD mouse model of AD. We used 8‐months old female 5xFAD mice, in which the AD‐related pathology has been shown to be apparent. We observed that RGC density is selectively affected in the retina of 5xFAD mice. To map retinal metabolic activity, we incubated isolated retinal tissue with [U‐13C]glucose and analyzed tissue extracts by gas chromatography‐mass spectrometry. We found that the retinas of 5xFAD mice exhibit glucose hypometabolism. Moreover, we detected decreased glutamine synthesis in 5xFAD retinas but no changes in the expression of markers of Müller glia, the main glial cell type responsible for glutamate uptake and glutamine synthesis in the retina.ConclusionsThese findings suggest that AD presents with metabolic alterations not only in the brain but also in the retina that may be detrimental to RGC activity and survival, potentially leading to the visual impairments that AD patients suffer. Chiquita, S. et al. The Retina as a Window or Mirror of the Brain Changes Detected in Alzheimer’s Disease: Critical Aspects to Unravel. Molecular Neurobiology vol. 56 5416–5435 (2019). Oesch, N. W., Wade Kothmann, W. & Diamond, J. S. Illuminating synapses and circuitry in the retina. Current Opinion in Neurobiology vol. 21 238–244 (2011). Andersen, J. V. et al. Deficient astrocyte metabolism impairs glutamine synthesis and neurotransmitter homeostasis in a mouse model of Alzheimer’s disease. Neurobiol. Dis. 148, (2021).