Optical probes of intradiskal processes in rod photoreceptors. I: Light-scattering study of ATP-dependent dark reactions

Are rod outer segment ATP-ase and ATP-synthase activity expression of the same protein?

Vertebrate retinal rod outer segments (OS) consist of a stack of disks surrounded by the plasma membrane, where phototransduction takes place. Energetic metabolism in rod OS remains obscure. Literature described a so-called Mg(2+)-dependent ATPase activity, while our previous results demonstrated the presence of oxidative phosphorylation (OXPHOS) in OS, sustained by an ATP synthetic activity. Here we propose that the OS ATPase and ATP synthase are the expression of the same protein, i.e., of F1Fo-ATP synthase. Imaging on bovine retinal sections showed that some OXPHOS proteins are expressed in the OS. Biochemical data on bovine purified rod OS, characterized for purity, show an ATP synthase activity, inhibited by classical F1Fo-ATP synthase inhibitors. Moreover, OS possess a pH-dependent ATP hydrolysis, inhibited by pH values below 7, suggestive of the functioning of the inhibitor of F1 (IF1) protein. WB confirmed the presence of IF1 in OS, substantiating the expression of F1Fo ATP synthase in OS. Data suggest that the OS F1Fo ATP synthase is able to hydrolyze or synthesize ATP, depending on in vitro or in vivo conditions and that the role of IF1 would be pivotal in the prevention of the reversal of ATP synthase in OS, for example during hypoxia, granting photoreceptor survival.

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.

Live imaging of mammalian retina: rod outer segments are stained by conventional mitochondrial dyes

The vertebrate retina is an array of "narrow-capture" photoreceptive elements of diverse cellular types that allow the fine spatial resolution characteristic of vision. Imaging of photoreceptors and of the whole retina has been previously reported; however, both were achieved exclusively after fixation. We report our development of a new technique for imaging live bovine retinas ex vivo. Using this technique, we conducted fluorescence confocal laser scanning microscopic imaging of bovine retinas. Eyecups were incubated with conventional fluorescent mitochondrial probes (MitoTracker and JC-1). Unexpectedly, we found that, besides the retinal mitochondria, the rod outer segments that are devoid of mitochondria were also stained. No other neuron was stained. Both protonophores, which decrease mitochondrial membrane potential, or inhibit electron transport strongly inhibited the selective association of dyes with both retinal rod outer segments and mitochondria. This is the first time that living rod outer segments were visualized by this technique. This finding may shed light on previous reports of the existence of a proton potential across the disk membranes and on the mechanism of the adenosine tri-phosphate (ATP) supply for phototransduction, which still requires investigation.

Proteomic analysis of the retinal rod outer segment disks

The initial events of vision at low light take place in vertebrate retinal rods. The rod outer segment consists of a stack of flattened disks surrounded by the plasma membrane. A list of the proteins that reside in disks has not been achieved yet. We present the first comprehensive proteomic analysis of purified rod disks, obtained by combining the results of two-dimensional gel electrophoresis separation of disk proteins to MALDI-TOF or nLC-ESI-MS/MS mass spectrometry techniques. Intact disks were isolated from bovine retinal rod outer segments by a method that minimizes contamination from inner segment. Out of a total of 187 excised spots, 148 proteins were unambiguously identified. An additional set of 61 proteins (partially overlapping with the previous ones) was generated by one-dimensional (1D) gel nLC-ESI-MS/MS method. Proteins involved in vision as well as in aerobic metabolism were found, among which are the five complexes of oxidative phosphorylation. Results from biochemical, Western blot, and confocal laser scanning microscopy immunochemistry experiments suggest that F 1F o-ATP synthase is located and catalytically active in ROS disk membranes. This study represents a step toward a global physiological characterization of the disk proteome and provides information necessary for future studies on energy supply for phototransduction.

Evidence for ATP-ase activity of arrestin from bovine photoreceptors

In vertebrate photoreceptors the soluble protein arrestin (45 kDa) is involved in controlling the light dependent activity of receptor proteins such as transducin or the cGMP-phosphodiesterase. Arrestin has further been identified as the retinal-S-antigen which is assumed to cause the autoimmune disease uveitis. In a first communication a binding of the nucleotide ATP to arrestin was described. In this subsequent study it is shown that arrestin is also able to hydrolyse ATP at a rate of (5.1 +/- 0.3) x 10(-3) U/mg.min with C1/2 = 93 +/- 5 nM and a Hill coefficient n = 1.8 +/- 0.1 at pH 7.2 and 20 degrees C. These findings suggest a new insight into the process of regulating photoreceptor activity.