Can Nicotinamide Treatment Overcome the Effect of Monocarboxylate Transporter 2 Loss on Retinal Ganglion Cell Survival and Function? dm

dc.creatorMurinda, Kudakwasheen_US
dc.creatorInman, Deniseen_US
dc.creatorKiehlbauch, Charlesen_US
dc.creatorMorgan, Autumnen_US
dc.description.abstractPurpose: There is currently no cure for the vision loss in glaucoma that is characterized by retinal ganglion cell (RGC) loss and irreversible optic neuropathy. Monocarboxylate transporter 2 (MCT2), which transports pyruvate, lactate, and ketone bodies, is exclusively found in neurons such as the RGCs. We have previously shown that MCT2 is lost during glaucoma, in advance of RGC loss, and MCT2 overexpression protects RGC number and function. We sought to determine if MCT2 is necessary for RGC survival by knocking it out, and to establish whether providing oral nicotinamide (NAM) could compensate for the anticipated metabolic disruption to RGCs. Methods: To test these hypotheses, we injected tamoxifen into Thy1-ERT2-cre: MCT2fl/fl mice to conditionally knock out MCT2 from Thy1-positive RGCs. Control mice carried the MCT2 flox’d allele but were Thy1-ERT2-cre-negative. Control and experimental mice were subjected to ocular hypertension using the magnetic microbead model; separate naïve controls from each genotype were also evaluated. To test the effect of nicotinamide intervention, we repeated the same groups but added the administration of oral nicotinamide to each before inducing ocular hypertension. Intraocular pressure (IOP) was measured using the TonoLab rebound tonometer. Pattern electroretinogram (PERG) and Visual Evoked Potential (VEP) were used to analyze the RGC function. We used unbiased stereology (Stereo Investigator, Micro Brightfield) to count the number of retinal ganglion cells in the wholemount retina, and ATP levels in the retina were also measured. Axon counts were done from plastic-embedded optic nerves. Results: IOP was higher in the ocular hypertension (OHT) groups. MCT2 knockout alone did not impact IOP, nor did it exacerbate RGC function loss post-OHT. After OHT, PERG amplitude was significantly lower in the OHT and KO + OHT treatment groups (p<0.005). RGC function was preserved in the KO + NAM and OHT+NAM groups but was significantly decreased in the KO+OHT group. After OHT, MCT2 KO alone did not alter RGC density but OHT and KO + OHT groups had significantly decreased RGC density (p<0.005). There was no significant decline in RGC density in any of the nicotinamide groups. ATP production in the KO + OHT group was significantly higher (1.81 +/- 0.89 µg/µl) than in the naïve control group (0.68 +/- 0.42 µg/µl). Conclusions: MCT2 knockout alone from RGCs did not change IOP, RGC density, or PERG, suggesting that MCT2 is not necessary for RGC function and survival. Ocular hypertension decreased PERG amplitude and RGC density, and the magnitude of the decrease was not significantly worsened by MCT2 knockout. The nicotinamide groups had no significant loss in RGC density, supporting the proposed neuroprotective effect of NAM administration. These data suggest that RGCs can meet their immediate metabolic needs through means beyond MCT2, and nicotinamide can rescue RGCs in the context of glaucoma.en_US
dc.description.sponsorshipNIH EY026662 (DMI).en_US
dc.titleCan Nicotinamide Treatment Overcome the Effect of Monocarboxylate Transporter 2 Loss on Retinal Ganglion Cell Survival and Function? dmen_US