Pharmacological Depletion of Retinal Mononuclear Phagocytes is Neuroprotective in a Mouse Model of Mitochondrial Optic Neuropathy
Purpose: The Vglut2-Cre;ndufs4loxP/loxP mouse strain with retinal ganglion cell (RGC)-specific mitochondrial complex I dysfunction develops severe RGC degeneration by postnatal day 90 (P90), with accompanying retinal mononuclear phagocyte (MNP) accumulation. We have reported that continuous exposure to hypoxia partially rescues RGC death in these mice, with minimal effect on MNP abundance. We hypothesized that pharmacological depletion of MNPs with the colony-stimulating factor-1 receptor inhibitor pexidartinib would enhance RGC neuroprotection by hypoxia. Methods: Iba1+ retinal MNP depletion was assessed in C57Bl/6J mice fed control or pexidartinib-infused chow beginning at P25. Subsequently, Vglut2-Cre;ndufs4loxP/loxP mice and control littermates were raised under normoxia or hypoxia and fed control or pexidartinib chow from P25 to P90. The neuroprotective effect of pexidartinib and hypoxia alone and in combination was assessed by quantifying RGC soma and axon survival in retinal flat mounts and optic nerve cross sections. Results: Pexidartinib completely depleted retinal MNPs within one week of treatment. Untreated Vglut2-Cre;ndufs4loxP/loxP mice exhibited the expected ~50% reduction of RGC soma and axon survival at P90 (p<0.0001 for both). Hypoxia or pexidartinib monotherapy each reduced RGC degeneration by more than one-half, while their combination resulted in complete RGC neuroprotection (p<0.001 for all three treatments). Normal myelination patterns were restored in mice receiving dual therapy. Conclusions: Pexidartinib effectively depletes retinal MNPs and is neuroprotective in the setting of severe RGC mitochondrial dysfunction. This therapeutic effect is additive to that of hypoxia. Combating retinal neuro-inflammation may therefore be a useful adjunct therapy in mitochondrial optic neuropathies like Leber hereditary optic neuropathy.