Mitochondrial Complex I Dysfunction in Rods Does Not Impair Light Signaling

Abstract

Purpose : The importance of oxidative metabolism in meeting photoreceptor energy needs remains controversial. We previously demonstrated that knockout mice lacking NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4), an accessory subunit of mitochondrial complex I, exhibited ~50% reduction of electroretinogram (ERG) a- and b-wave amplitudes in vivo, yet light responses were normal in ex vivo ERGs performed on isolated ndufs4^-/- retinas. We hypothesize that the impaired in vivo light signaling was due to complex I dysfunction within retinal pigment epithelium (RPE) cells rather than photoreceptors.

Methods : To achieve cell-specific deletion of ndufs4 in rods and RPE, mice with floxed ndufs4 were crossed with iCre75 and Rpe65^CreERT2 mice, respectively. Mice with Rpe65^CreERT2 required tamixofen induction at postnatal day 30 (P30). ERG recordings were performed on dark-adapted iCre75;ndufs4^loxP/loxP and Rpe65^CreERT2;ndufs4^loxP/loxPmice and littermate controls (n≧5 per genotype) at several time points. Depletion of NDUFS4 protein within ocular tissues was assessed via Western blot. Photoreceptor survival was assessed by outer retinal nuclei counts in retinal cross sections. Statistical comparisons between cohorts were made using two-tailed t tests.

Results : NDUFS4 protein content was depleted by >99% in the photoreceptors of iCre75;ndufs4^loxP/loxP mice by P60. ERG recordings at P90 showed indistinguishable scotopic a- and b-wave amplitudes between iCre75;ndufs4^loxP/loxP and control ndufs4^loxP/loxP mice (ns for all stimulus intensities). Photoreceptor nuclei counts remained normal in iCre75;ndufs4^loxP/loxP mice thorugh P180 (ns at 8 different retinal locations). In contrast, in Rpe65^CreERT2;ndufs4^loxP/loxP mice, only ~25% depletion of NDUFS4 was achieved within RPE-choroid complex 120 days after tamoxifen induction. Nevertheless, there was a trend toward reduced a- and b-wave amplitudes compared to control ndufs4^loxP/loxP littermates (15-45% reductions over multiple stimulus intensities; p-values ranging from 0.07 to 0.16).

Conclusions : Rods are not dependent on intact complex I for normal light signaling and survival, supporting the prevailing concept that rods primarily rely on glycolysis. Although limited by suboptimal Cre recombination efficiency, the trend toward decreased ERG amplitudes in mice with RPE-specific deletion of ndufs4 may suggest that RPE oxidative metabolism is key in supporting photoreceptor light signaling.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.