Purpose: Mitochondrial stress impairs the function of photoreceptors and can lead to retinal degeneration and blindness. Microtubules and the cytoplasmic dynein complex are necessary for proper trafficking and localization of cargo, including mitochondria. We have identified Nuclear Distribution Protein C (NUDC), a known regulator of cytoplasmic dynein, to be critical in rod and cone photoreceptor health. We hypothesize that the absence of NUDC causes a dysregulation in dynein movement along microtubules leading to mitochondrial and protein mislocalization, as well as cellular stress and cell death.

Methods: We generated a floxed Nudc mouse and bred it to the rod-specific Opsin-iCre75 mouse to produce Nudc^+/- or Nudc^-/- in rods. To characterize the effect of NUDC loss on mitochondria in the retina of these mice, we performed a combination of assays including: Optical Coherence Tomography (OCT) to uncover retinal thickness in vivo; Transmission Electron Microscopy (TEM) to investigate the ultrastructure of mitochondria in rods; quantitative RT-PCR (qRT-PCR) to measure changes in transcripts of selected mitochondrial genes; immunohistochemistry (IHC) to reveal localization of key proteins involved in phototransduction, mitochondrial fission/fusion, and unfolded protein response (UPR) pathways; and seahorse assays to assess mitochondrial respiration in freshly dissected ex vivo retinal tissues.

Results: At P21, OCT analysis revealed retinal degeneration progressing through P42. TEM demonstrated an increased number of mitochondria scattered throughout the inner segment in Nudc^-/- rods, rather than localized in the ellipsoid as in wild type. qRT-PCR showed no change or decrease in mRNA levels of genes associated with mitochondrial fission, but IHC revealed an upregulation of gliosis via GFAP in Müller glia and rhodopsin mislocalization in photoreceptors in the absence of NUDC. Additionally, altered mitochondrial respiration and mitochondrial reserve capacity were observed with Nudc deletion in P21 rods.

Conclusions: We have demonstrated that, in the absence of NUDC, mitochondria are mislocalized and exhibit compromised function in rod photoreceptors. We detect an upregulation of proteins involved in the UPR and in the inflammatory response concomitant with retinal degeneration, underscoring the importance of microtubule trafficking by NUDC in the overall health of photoreceptors.