Extramitochondrial tricarbo xylic acid cycle in retinal rod outer segments: which is the bioenergetic relapse?

Vertebrate retinal rod Outer Segments (OS) are the site of visual transduction, an energy demanding process for which mechanisms of ATP supply are still poorly known. Glycolysis or diffusion of either ATP or phosphocreatine from the inner segment (IS) do not seem to display adequate timing to supply ATP for phototransduction [1] . Our previous data suggest the existence of an aerobic metabolism in OS, which would largely account for the light stimulated ATP need of the photoreceptor [2]. Here, by oxymetry and biochemical analyses, we show that: (i) isolated disks synthesize ATP and consume O2 in the presence of pyruvate/malate and succinate either in coupled or uncoupled conditions; (ii) OS homogenates, which contain cytosol, house the whole biochemical machinery for the aerobic degradation of glucose, i.e. glycolysis and the tricarboxylic acid cycle (TCA cycle). Activities of the 8 TCA cycle enzymes in OS were comparable to those in retinal mitochondria-enriched fractions. Disk and OS preparations were subjected to immunogold TEM analysis with specific antibodie, showing the expression therein of both the visual pigment rhodopsin and FoF1-ATP synthase. Finally, double immunofluorescence on mouse retina sections demonstrated a colocalization of some respiratory complex mitochondrial proteins with rhodopsin in rod OS, strengthening the idea that these are true components of OS. Data could shed light on those retinal pathologies (retinitis pigmentosa and age related macular degeneration) related to oxidative stress and to mutations in TCA enzymes. Moreover the high oxygen consumption by rod during dark adaptation, may substantiate rod driven hypoxia, and be the driving force of diabetic retinopathy.