On the metabolism of the rod outer segments

Myelin sheath and cyanobacterial thylakoids as concentric multilamellar structures with similar bioenergetic properties

There is a surprisingly high morphological similarity between multilamellar concentric thylakoids in cyanobacteria and the myelin sheath that wraps the nerve axons. Thylakoids are multilamellar structures, which express photosystems I and II, cytochromes and ATP synthase necessary for the light-dependent reaction of photosynthesis. Myelin is a multilamellar structure that surrounds many axons in the nervous system and has long been believed to act simply as an insulator. However, it has been shown that myelin has a trophic role, conveying nutrients to the axons and producing ATP through oxidative phosphorylation. Therefore, it is tempting to presume that both membranous structures, although distant in the evolution tree, share not only a morphological but also a functional similarity, acting in feeding ATP synthesized by the ATP synthase to the centre of the multilamellar structure. Therefore, both molecular structures may represent a convergent evolution of life on Earth to fulfill fundamentally similar functions.

Functional expression of oxidative phosphorylation proteins in the rod outer segment disc

The rod Outer Segment (OS) disc, an organelle devoid of mitochondria, is specialized in phototransduction, a process requiring a continual chemical energy supply. We have shown that OS discs express functional mitochondrial electron transport chains, Fo F1 -ATP synthase and the tricarboxylic acid cycle enzymes, all mitochondrial features. Here, we focus on oxygen consumption and adenosine triphosphate (ATP) synthesis by OS discs analysing electron transport chain I-III-IV and II-II-IV pathways, supported by reduced nicotinamide adenine dinucleotide and succinate, respectively. Interestingly, respiratory capacity of discs was measurable also in the presence of 3-hydroxy-butyrrate, a typical metabolic substrate for the brain. Data were supported by a two-dimensional electrophoresis analyses conducted as our previous one, but focused to those mitochondrial proteins that are involved in oxidative phosphorylation. Carbonic anhydrase was also found active in OS discs. Moreover, colocalization of Rhodopsin with respiratory complex I and ATP synthase seems a further step in the characterization of some proteins typical of the mitochondrial inner membranes that are expressed in the rod discs. The existence of oxygen utilization in the outer retina, likely supplying ATP for phototransduction, may shed light on some retinal pathologies related to oxidative stress of the outer retina.

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.

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all-trans retinal to all-trans retinol, and the clearance of all-trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all-trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP+glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.

ATP synthesis in the disk membranes of rod outer segments of bovine retina

ATP is synthesized on the disk membrane isolated from rod outer segments of the bovine retina. Together with a slow component which accounted for a constant rate of about 22 nmol ATP/min/mg of protein and which was due to the adenylate kinase activity, a fast component with a maximal activity of about 58 nmol ATP/min/mg of protein was measured at physiological calcium concentrations. This fast activity disappeared in the presence of the Ca(2+) ionophore A23187, was inhibited by vanadate or thapsigargin but not by oligomycin, suggesting that this ATP synthesis is due to the reversal functioning of the Ca(2+)-ATPase previously found on the disk membranes.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

Calcium pump in the disk membranes isolated from bovine retinal rod outer segments

The existence of a Ca2+ pump in rod outer segment disks of bovine retina is strongly suggested by the isolation on sodium dodecyl sulfate polyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphorylated intermediate (E-P) of molecular mass of about 100 kDa as well as by measurements of active calcium transport and adenosine 5'-triphosphate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on the presence of Mg(2+)-ATP, was inhibited by vanadate or lanthanum and appeared poorly sensitive to calmodulin. ATP hydrolysis by disk membranes was a function of free Ca2+ concentration in the absence of exogenous Mg2+. The presence of a Ca2+ pump on disk membranes is discussed in terms of its possible role in Ca2+ ion buffering during photoreceptor cell functioning.

Decreased energy requirement of toad retina during light adaptation as demonstrated by 31P nuclear magnetic resonance

1. The effect of light and dark adaptation on the levels of phosphorus metabolites (nucleotide di- and triphosphates, phosphocreatine, pyridine nucleotide, inorganic phosphate, phosphodiesters, phosphomonoesters, and uridine diphosphate-glucose) in the toad (Bufo marinus) retina and retinal extracts was studied by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Spectra were acquired using an NMR probe specifically designed for superfusion and illumination of a single retina. Retinae were maintained at a steady state for up to 10 h in an electrolyte solution containing 10 mM Hepes buffer and bubbled with 98% O2-2% CO2, pH 7.8 at 20 degrees C. 3. The intracellular concentrations of the phosphorus metabolites were measured in total darkness or during prolonged exposure to light. The concentration of nucleoside triphosphates (NTP) in the dark-adapted retina was about 1.5 mM and that of phosphocreatine (PCr) was about 0.7 mM. 4. In saturating levels of light, 6.0 x 10(11) or 1.5 x 10(13) quanta s-1 cm-2 at 520 nm, the levels of PCr and phosphomonoesters rose, the levels of NTP and protons (pH) were maintained, and the levels of pyridine nucleotides and nucleotide diphosphates (NDP) fell. 5. A rise in the level of PCr in the presence of an unchanged level of NTP in the light-adapted retina indicates that the energy consumption of the retina is greater in the dark. 6. These results are in agreement with the results of oxygen consumption, glucose dependence, and electrophysiological studies which also indicate that the metabolic energy requirement of the retina decreases in light.

Nucleoside triphosphates and hydrolysis-resistant analogues: effects on PIII responses in the isolated skate retina

Responses to test flashes were recorded extracellularly from the aspartate-treated, isolated retina of the skate, before and after topical application to the retina of solutions containing GTP, ATP, or certain hydrolysis-resistant analogues. When applied to strongly light-adapted retinas (greater than or equal to 87% bleached), the analogues p(CH2)ppG, p(NH)ppG and p(CH2)ppA induced sustained decreases in threshold of the PIII response. Similar treatment of light-adapted retinas with GTP or ATP also promoted decreases in threshold, but these changes appeared relatively transient. All of the test substances lacked significant activity when applied to dark-adapted (unbleached) retinas. The results are discussed in relation to other studies examining the effects of light on GTP- and ATP-dependent processes within the photoreceptors.

Effects of ATP and taurine on calcium uptake by membrane preparations of the rat retina

The effects of ATP and taurine on the kinetics of calcium uptake in rat retinal membrane preparations were determined. ATP increased calcium uptake at low calcium ion concentrations. Addition of ATP plus taurine further increased calcium uptake. Cooperative relationships were observed for calcium uptake in the absence of ATP and taurine. In the presence of phosphate ions reciprocal plots demonstrated upward deflections from linearity, while in the absence of phosphate ions downward deflections were noted. Addition of ATP plus taurine to the incubation system appeared to obliterate the cooperativity. Two uptake systems for calcium were observed.

Nucleotide content of isolated bovine rod outer segments

Endogenous nucleotide levels of isolated intact ROS were analyzed by high pressure liquid chromatography. Intact bovine ROS showed a total nucleotide concentration of 1.0 mM, ATP and GTP being the major components (0.1-0.2 mol per mol of rhodopsin). When intact ROS resuspended in sucrose-ficoll medium were diluted in a Ringer-Krebs bicarbonate, the nucleotide ratios were markedly changed, but the total concentration remained unchanged. The concentration of cyclic nucleotides (cAMP and cGMP) was markedly enhanced when ROS were placed in Ringer buffer, whereas ATP and GTP concentration was reduced. Total nucleotide concentration is 100% higher in intact than in leaky plasma membrane ROS. Upon illumination, no change was observed in the nucleotide levels of ROS in sucrose-ficoll medium. However ATP, GTP and cAMP levels were reduced when ROS in Ringer medium were exposed to light while cGMP concentration showed no change. Relevance of relative nucleotide content and ionic concentration to the transduction phenomenon in photoreceptor is discussed.

Glycolytic and oxidative metabolism in relation to retinal function

Measurements of lactate production and ATP concentration in superfused rat retinas were compared with extracellular photoreceptor potentials (Fast PIII). The effect of glucose concentration, oxygen tension, metabolic inhibition, and light were studied. Optimal conditions were achieved with 5-20 mM glucose and oxygen. The isolated retina had a high rate of lactate production and maintained the ATP content of a freshly excised retina, and Fast PIII potentials were similar to in vivo recordings. Small (less than 10%) decreases in aerobic and anaerobic lactate production were observed after illumination of dark-adapted retinas. There were no significant differences in ATP content in dark- and light-adapted retinas. In glucose-free medium, lactate production ceased, and the amplitude of Fast PIII and the level of ATP declined, but the rates of decline were slower in oxygen than in nitrogen. ATP levels were reduced and the amplitude of Fast PIII decreased when respiration was inhibited, and these changes were dependent on glucose concentration. Neither glycolysis alone nor Krebs cycle activity alone maintained the superfused rat retina at an optimal level. Retinal lactate production and utilization of ATP were inhibited by ouabain. Mannose but not galactose or fructose produced lactate and maintained ATP content and Fast PIII. Iodoacetate blocked lactate production and Fast PIII and depleted the retina of ATP. Pyruvate, lactate, and glutamine maintained ATP content and Fast PIII reasonably well (greater than 50%) in the absence of glucose, even in the presence of iodoacetate. addition of glucose, mannose, or 2-deoxyglucose to medium containing pyruvate and iodoacetate abolished Fast PIII and depleted the retina of its ATP. It is suggested that the deleterious effects of these three sugars depend upon their cellular uptake and phosphorylation during the blockade of glycolysis by iodoacetate.

Cyclic GMP releases calcium from disc membranes of vertebrate photoreceptors

Physiological concentrations of cyclic guanosine 3',6'-monophosphate (cGMP) inhibit 45Ca uptake and increase 45Ca release from vertebrate photoreceptor rod outer segment disc membranes. These effects are specific for cGMP. Several facts, including the independence of these effects from added triphosphates, suggest that cGMP diminishes the Ca-binding capacity of the disc membranes. Preliminary data show that the apparent affinity constant of the cGMP-dependent Ca-binding sites of the disc membranes is of the same (or even higher) order of magnitude as that of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. As expected, the observed cGMP effects are not dependent on the light or dark conditions of the disc membranes.

Molecular mechanisms of photoreception. IV. Ca2+-inhibited GTPase of rod outer segments of the frog retina

Ca2+-dependent GTPase activity is found to be present in the rod outer segments of frog retina. GTPase localization in rod outer segments is shown by fractionating the rod outer segment preparation in the sucrose density gradient. The enzyme is readily washed out of cells with isotonic NaCl solution. The Km is 0.6 mM for GTP. The activity is inhibited by 78 +/- 12% with the increase in Ca2+ concentration from 10(-9) to 10(-7) M. GTP hydrolysis is inhibited by the same concentrations of Ca2+ which block the sodium conductivity of the rod outer segment cytoplasmic membrane.

Cyclic GMP and the permeability of the disks of the frog photoreceptors

1. The diffusion of sodium, potassium and rubidium (not chloride) ions across the disk membrane is increased by cyclic guanosine monophosphate (cyclic GMP). 2. The increase is greater for sodium than for rubidium in the 0.01-0.1 mM concentration range. 3. Cyclic adenosine monophosphate (cyclic AMP) is less efficient than cyclic GMP; GMP and guanosine triphosphate are without effect. 4. The effect is present with either 1.8 mM calcium ions or 4 mM-EGTA in the perfusion fluid. 5. The presence of the cyclic GMP phosphodiesterase on the disk membranes is not needed for this effect. 6. The effect is present in both unbleached and fully bleached membranes.

Calcium translocation and storage of isolated intact cattle rod outer segments in darkness

Bovine rod outer segments (rods), isolated with an intact plasma membrane and a stable calcium exchange and storage capacity, contain 2-3 mol endogenous calcium/mol rhodopsin. By means of 45Ca accumulation experiments and concomitant 40Ca analysis, the calcium metabolism of these organelles has been studied with the following results: 1. The majority of endogenous calcium is localized within disks. 2. In the presence of the ionophore A23187 the intradiskal binding sites can be titrated with external calcium. 3. The Scatchard plot of calcium binding of rods indicates the presence of a single set of intradiskal binding sites with a maximal capacity of 8-9 mol calcium/mol rhodopsin and an affinity constant of 55 microM to calcium. 4. Without A23187 more than 99% of the rod calcium appears in a bound state in equilibrium with a free calcium concentration of 15-25 microM. 5. External calcium exchanges with endogenous calcium in a fast (t 1/2 = 12 s) process with a uniform rate constant, whereas net calcium transport is very slow (t 1/2 greater than 2 h). 6. Intact rods contain a calcium translocation system, presumably located in the plasma membrane, which performs Ca-Ca exchange with a high unidirectional flux of 2 . 10(6) calcium ions/rod per s. 7. This translocation system can be saturated by external calcium (Km = 0.5 -1 microM) and has a low Q10 (1.08). Both the calcium translocation system and the calcium binding system appear to depend on the structural integrity of the stacked disks and are very sensitive to the experimental conditions. The relevance of these findings is discussed in relation to the proposed role of calcium ions as the intracellular transmitter in vertebrate rod photoreceptor cells.

The isolation of stable cattle rod outer segments with an intact plasma membrane

A procedure is described to purify and stabilize cattle rod outer segments with an intact plasma membrane. Three criteria are applied to assess the integrity of the latter. Upon photolysis in these rod outer segments: (1) exogenous ATP cannot phosphorylate rhodopsin located in the disk membrane. (2) Endogenous cofactors (NADPH, NADPH-regenerating system) are still available in the rod cytosol and consequently retinol is the final photoproduct of photolysis of rhodopsin. (3) The rod cytosol can maintain a pH different from that of the medium, since the later stages of rhodopsin photolysis are independent of the medium pH. The stability and homogeneity of the preparation appear to be much better than those of freshly isolated frog rod outer segments, which have been used most frequently so far for experiments on the physiology of rod outer segments. In addition, these cattle rod outer segments remain intact during various manipulations and therefore considerably extend the experimental possibilities when intact rod outer segments are required.

Phosphodiesterase and GTPase in rod outer segments. Kinetics in vitro

The hydrolysis of cyclic guanosine monophosphate (cyclic GMP) and of guanosine triphosphate (GTP) by the broken rods of the frog retina after a flash of light have been studied in vitro with a constant perfusion method. The activation has an onset apparently instantaneous as observed with the existing possible time resolution of 3 s. The activation is followed by a partial inactivation that does not bring the activity back to the pre-flash level. GTP or the non-hydrolysable guanyl-5'-ylimidodiphosphate (GMP-PNP) is required for the normal light-activation of the phosphodiesterase and in its absence both the speed of activation and the sensitivity are greatly reduced. The activation speed, the sensitivity (threshold at approx. 0.00004% bleaching), and the kinetic constants do not exclude a direct role in the process of excitation for the phosphodiesterase and suggest a subsidiary but as yet undefined role for the GTPase.

Light-activated hydrolysis of GTP and cyclic GMP in the rod outer segments

1. The hydrolysis of guanosine triphosphate (GTP) and the consequent formation of guanosine diphosphate (GDP) and phosphate (P1) are activated by light in a suspension of broken retinal rods: the hydrolysis rate with GTP in the micrometer concentration range is 2.5-3.5 n-mole/min per mg of rhodopsin in the preparation. 2. The ionic composition of the medium suspending the rods is not critical: the hydrolysis is present in NaCl saline solution with MG2+ as well as in Tris-HC1 buffer solution, and with the chelating agent EDTA. 3. The ionic strength is critical: the effect is reduced when the broken rods are suspended in a low salt mannitol solution, and is altogether abolished when they are separated from the mannitol solution; it reappears when the mannitol solution is added again in the presence of salts. An element essential for the effect is thus reversibly released in the mannitol solution. No hydrolytic activity on GTP, however, is found in the mannitol soluble fraction. 4. The cyclic nucleotide phosphodiesterase is eluted from the rods in the mannitol solution, and is reaggregated to the rods in the presence of salts; once recombined with the rods, it can be activated by light. 5. The activation of the phosphodiesterase by light is present in the absence of added nucleotide triphosphates.

Metabolism of the frog outer segments: a kinetic study

1. The reaction of the high-energy phosphate esters of the frog isolated rods, with the firefly lantern extract, has been studied by recording the luminescence in a stopped-flow apparatus. The onset of the reaction was determined by the rapid mixing of the firefly lantern extract with the high-energy phosphates released from the rods fragmented during the mixing.2. The time course of the reaction, i.e. of the luminescence, was not typical of adenosine-5'-triphosphate (ATP), indicating that the major part of the rod nucleotides is not ATP.3. The isolated rods were fragmented 1 sec-2 min after a flash of light. As soon as 1 sec after illumination, a substantial decrease of the luminescent yield has been detected in a range of light bleaching from 0.007 to 20% of the rhodopsin, indicating an early reduction of the high-energy phosphate esters.4. At longer times, flashes that bleached 7 or 20% of the rhodopsin induced a progressive decrease of the luminescent yield that was half completed in 6-9 sec, and was nearly complete in 20 sec, whereas a more or less complete recovery was observed after flashes bleaching from 0.7 to 0.007% of the rhodopsin, indicating the presence of buffering mechanisms.5. Also, the time course of the reaction was modified in a complex way after stimulation of the rods, suggesting that other nucleotides beside ATP are hydrolysed.6. The light-induced reduction of the high-energy phosphates has been observed also in broken rods, in the presence of the calcium chelating agent EGTA, suggesting that calcium is not needed for this effect.7. Cyclic guanosine monophosphate (10(-4)M) interfered with the photic effect.8. The rapid effect of light on the content of high-energy phosphate esters suggests a physiological role in the mechanism of excitation.

Electrical responses of rods in the retina of Bufo marinus

1. Intracellular responses to flashes and steps of light have been recorded from the outer segment and the cell body of rods in the retina of the Bufo marinus. The identification of the origin of recorded responses has been confirmed by intracellular marking.2. Responses to flashes delivered in darkness or superimposed on a background were analysed. Responses recorded from outer segments conform to the principle of ;spectral univariance'. The shape of the response is not affected by enlarging the spot diameter from 150 to 1000 mum.3. The membrane potential measured in darkness at the outer segments varied from -15 to -25 mV. Injection of steady hyperpolarizing currents increases the size of the response to light; depolarizing currents reduce the response. The mean value of the input resistance is 97 +/- 30 MOmega in darkness and increases by 20-30% during illumination.4. The responses obtained from the cell body of rods have the same shape, time course and spectral sensitivity of those recorded at the outer segment. Injection of steady current at the cell body produces different effects than at the outer segment: hyperpolarizing currents reduce the amplitude of the response to light; depolarizing currents increase the response.5. The experimental data are fitted according to a model similar to that used to describe the responses of turtle cones (Baylor & Hodgkin, 1974; Baylor, Hodgkin & Lamb, 1974a, b).6. The model reproduces the electrical responses of the rod outer segment to a variety of stimuli: (a) brief flashes and steps of light in dark adapted conditions; (b) bright flashes superimposed on background illuminations; (c) pairs of flashes delivered at different time intervals. Responses to hyperpolarizing steps of current are also reproduced by the model.