Rod outer segments metabolism and retinal degenerative diseases: new perspectives

Abstract

 

Purpose

 

Based on our previous proteomic and biochemical studies on an extramitochondrial oxidative phosphorylation (OXPHOS) process occurring in rod outer segments (OS) disks, that are devoid of mitochondria, the goal of the present study was to test the presence and functionality of OXPHOS proteins in the retinal OS from animal models such as cattle, mouse and zebrafish.

 

Methods

 

Imaging studies were conducted by Atomic Force Microscopy (AFM) on isolated disks, and by Electron (TEM), fluorescence and light microscopy on mammalian retinal sections. Biochemical, hysto- and and immunocyto-chemical assays were also employed. Experiments ex vivo on living retinas, incubated with MitoTracker Deep Red 633 (MT), a fluorescent mitochondrial probe, were performed by confocal laser scanning microscopy (CLSM).

 

Results

 

We report the presence of FoF1-ATP synthase on bovine retina by TEM (Fig 1) and on isolated purified bovine disk surface by AFM. ATP synthase was active in OS, synthesizing a consistent amount of ATP, and was inhibited by Oligomycin, Resveratrol, Curcumin/Piperin. CLSM on bovine retina show that besides mitochondria, OS are stained with MT, whose fluorescence colocalizes with Rhodopsin (Rh) autofluorescence. Immunocytochemical analyses on Zebrafish (Danio rerio) retina at various stages of development show colocalization of OXPHOS proteins with Rh (Fig 2). Hystochemical analyses show that the four respiratory complexes are active on mouse retinal unfixed sections.

 

Conclusions

 

Data suggest the presence of an extramitochondrial aerobic metabolism in bovine, mouse and Zebrafish rod OS, where OXPHOS proteins are ectopically expressed. Zebrafish confirms to represent a convenient animal model to study the ectopic OXPHOS expression and their putative intraflagellar transport to the OS. Data shed light on the pathogenesis of many retinal degenerative diseases that correlate with oxidative stress and hypometabolism in the photoreceptor, and on the efficacy of empirical treatments in these pathologies, such as hyperbaric or dietary supplement therapies.

 

 

Fig 1: Immunogold TEM imaging of bovine retina.

 

 

Fig 2: Immunohistochemistry on zebrafish embryo (12 days) sections

 

Keywords: 648 photoreceptors • 689 retina: distal (photoreceptors, horizontal cells, bipolar cells) • 689 retina: distal (photoreceptors, horizontal cells, bipolar cells)