Evidence for aerobic metabolism in retinal rod outer segment disks.
The disks of the vertebrate retinal rod Outer Segment (OS), devoid of mitochondria, are the site of visual transduction, a very energy demanding process. In a previous proteomic study we reported the expression of the respiratory chain complexes I-IV and the oxidative phosphorylation Complex V (F(1)F(0)-ATP synthase) in disks. In the present study, the functional localization of these proteins in disks was investigated by biochemical analyses, oxymetry, membrane potential measurements, and confocal laser scanning microscopy. Disk preparations, isolated by Ficoll flotation, were characterized for purity. An oxygen consumption, stimulated by NADH and Succinate and reverted by rotenone, antimycin A and KCN was measured in disks, either in coupled or uncoupled conditions. Rhodamine-123 fluorescence quenching kinetics showed the existence of a proton potential difference across the disk membranes. Citrate synthase activity was assayed and found enriched in disks with respect to ROS. ATP synthesis by disks (0.7 micromol ATP/min/mg), sensitive to the common mitochondrial ATP synthase inhibitors, would largely account for the rod ATP need in the light. Overall, data indicate that an oxidative phosphorylation occurs in rod OS, which do not contain mitochondria, thank to the presence of ectopically located mitochondrial proteins. These findings may provide important new insight into energy production in outer segments via aerobic metabolism and additional information about protein components in OS disk membranes.
Panfoli
Isabella
I
DIBIO, University of Genoa, Viale Benedetto XV, 5, 16132 Genova, Italy. Isabella.Panfoli@unige.it
Calzia
Daniela
D
Bianchini
Paolo
P
Ravera
Silvia
S
Diaspro
Alberto
A
Candiano
Giovanni
G
Bachi
Angela
A
Monticone
Massimiliano
M
Aluigi
Maria Grazia
MG
Barabino
Stefano
S
Calabria
Giovanni
G
Rolando
Maurizio
M
Tacchetti
Carlo
C
Morelli
Alessandro
A
Pepe
Isidoro M
IM
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000332:Aerobiosis; D000818:Animals; D000968:Antimycin A / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D019069:Cell Respiration / Q000187:drug effects; D002478:Cells, Cultured; D004579:Electron Transport / Q000502:physiology; D008565:Membrane Proteins / Q000378:metabolism*; D018613:Microscopy, Confocal; D025261:Mitochondrial Proton-Translocating ATPases; D009243:NAD / Q000494:pharmacology; D010085:Oxidative Phosphorylation; D010101:Oxygen Consumption / Q000187:drug effects; D011190:Potassium Cyanide / Q000494:pharmacology; D055214:Retinal Photoreceptor Cell Outer Segment / Q000201:enzymology*; D017948:Retinal Rod Photoreceptor Cells / Q000201:enzymology* / Q000648:ultrastructure; D012402:Rotenone / Q000494:pharmacology; D014475:Uncoupling Agents / Q000494:pharmacology
D002352:Carrier Proteins; D008565:Membrane Proteins; D014475:Uncoupling Agents; D012402:Rotenone; D009243:NAD; D000968:Antimycin A; D000251:Adenosine Triphosphatases; D025261:Mitochondrial Proton-Translocating ATPases; C020439:oligomycin sensitivity-conferring protein; D011190:Potassium Cyanide