Ketogenic Diet Increases Mitophagy in a Mouse Model of Glaucoma

Purpose: We have previously shown that limiting dietary intake to high fat, low protein, and negligible carbohydrate results in mitochondrial biogenesis, and in the case of glaucoma, a reduction in neurodegeneration of retinal ganglion cells (RGCs). In this experimental follow-up study, we wanted to examine the effect of the ketogenic diet on mitophagy, or mitochondrial recycling, within the glaucomatous retina.

Methods: MitoQC mice were placed on a ketogenic diet or standard rodent chow for 5 weeks and ocular hypertension (OHT) was induced via microbead injection. The MitoQC reporter mice have a pH-sensitive mCherry-GFP tag on the outer mitochondrial membrane that results in retention of red fluorescence when mitochondria bound for recycling are engulfed by lysosomes. The FIJI (ImageJ) macro MitoQC counter was used to quantify red puncta (mitolysosomes) in sectioned retina as a measure of mitophagy within the RGCs and Müller glia.

Results: Mitophagy in RGCs, as measured by red puncta, was significantly decreased by ocular hypertension in the control retina (Control + OHT) in comparison to naïve control retina (Ctrl; p<0.0001). The ketogenic diet (KD) resulted in a significant increase in mitolysosomes in RGCs when compared to Ctrl (p<0.0001), Control + OHT (p<0.0001) and KD + OHT (p=0.0089). The ketogenic mice with OHT showed a significantly higher RGC-associated mitolysosome number than Control + OHT mice (p<0.0001). In contrast, mitolysosomes quantified in the Müller glia of Control + OHT mice were significantly higher than the naïve control mice (p=0.0127). Mice in the KD (p=0.0001) and KD + OHT(p=0.0005) groups had significantly greater mitolysosomes than the control Müller glia, however there was no difference in mitophagy between the Control + OHT, KD, and KD + OHT Müller glia groups.

Conclusion: Our data demonstrates that mitophagy is managed differently within RGCs and Müller glia of mouse retinas. The KD promoted mitophagy within the RGCs to a degree that overcame the decline of mitophagy after OHT in the control group. Within the Müller glia, the KD was redundant because OHT alone increased mitophagy to similar levels as the KD. These findings suggest a divergence of mitochondrial homeostasis in RGCs and Müller glia that may reflect the different metabolic needs of these cell types.