Signal mechanisms of phototransduction in retinal rod

Chemistry and biology of the initial steps in vision: the Friedenwald lecture

Visual transduction is the process in the eye whereby absorption of light in the retina is translated into electrical signals that ultimately reach the brain. The first challenge presented by visual transduction is to understand its molecular basis. We know that maintenance of vision is a continuous process requiring the activation and subsequent restoration of a vitamin A-derived chromophore through a series of chemical reactions catalyzed by enzymes in the retina and retinal pigment epithelium (RPE). Diverse biochemical approaches that identified key proteins and reactions were essential to achieve a mechanistic understanding of these visual processes. The three-dimensional arrangements of these enzymes' polypeptide chains provide invaluable insights into their mechanisms of action. A wealth of information has already been obtained by solving high-resolution crystal structures of both rhodopsin and the retinoid isomerase from pigment RPE (RPE65). Rhodopsin, which is activated by photoisomerization of its 11-cis-retinylidene chromophore, is a prototypical member of a large family of membrane-bound proteins called G protein-coupled receptors (GPCRs). RPE65 is a retinoid isomerase critical for regeneration of the chromophore. Electron microscopy (EM) and atomic force microscopy have provided insights into how certain proteins are assembled to form much larger structures such as rod photoreceptor cell outer segment membranes. A second challenge of visual transduction is to use this knowledge to devise therapeutic approaches that can prevent or reverse conditions leading to blindness. Imaging modalities like optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) applied to appropriate animal models as well as human retinal imaging have been employed to characterize blinding diseases, monitor their progression, and evaluate the success of therapeutic agents. Lately two-photon (2-PO) imaging, together with biochemical assays, are revealing functional aspects of vision at a new molecular level. These multidisciplinary approaches combined with suitable animal models and inbred mutant species can be especially helpful in translating provocative cell and tissue culture findings into therapeutic options for further development in animals and eventually in humans. A host of different approaches and techniques is required for substantial progress in understanding fundamental properties of the visual system.

Direct effects of temperature on phospholipid and polyunsaturated fatty acid metabolism in isolated brain cells from rainbow trout, Oncorhynchus mykiss

1. The direct effects of temperature on the metabolism of [1-14C]18:2(n-6), [1-14C]18:3(n-3), [1-14C]20:4(n-6) and [1-14C]20:5(n-3) were studied in isolated brain cells from rainbow trout, Oncorhynchus mykiss. 2. Recovery of radioactivity from all the polyunsaturated fatty acids (PUFA) in total lipid was significantly greater at 5 and 15 degrees C than at 25 degrees C. 3. The lower incubation temperatures decreased the relative net incorporation of all the 14C-labelled PUFA into phosphatidylcholine (PC) and increased the relative incorporation of the PUFA into the other phosphoglycerides, especially phosphatidylethanolamine (PE). 4. The effects on PC were generally more significant between 25 and 15 degrees C, whereas the effects on PE were generally significant both between 25 and 15 degrees C and between 15 and 5 degrees C. 5. This suggests that the lysophospholipid acyltransferases responsible for the incorporation of PUFA into different phosphoglycerides may have differential sensitivities to temperature. 6. In contrast, the acyltransferase activities showed fatty acyl preferences that were independent of temperature. 7. Although a trend towards decreased activity at 5 degrees C was apparent, temperature generally had little significant effect on the relative percentages of the PUFA metabolized via the desaturase pathways.

Differential expression and localization of brain-type and mitochondrial creatine kinase isoenzymes during development of the chicken retina: Mi-CK as a marker for differentiation of photoreceptor cells

The expression and the cellular- as well as subcellular-distribution of brain-type B-CK and mitochondrial Mi-CK during development of the chicken retina was studied by immunoblotting, immunofluorescence and immunogold methods. B-CK expression and accumulation in retina was high from early stages of embryonic development on, decreased slightly around hatching and remained high again during adulthood. At early stages of development (days 2-5), B-CK was more or less evenly distributed over the entire retina with the exception of ganglion cells, which were stained more strongly for B-CK than other retinal precursor cells. Then, at around day 10, the beginning of stratified immunostaining by anti-B-CK antibody was noted concomitant with progressing differentiation. Finally, a dramatic increase in staining of the differentiating photoreceptor cells was seen before hatching (day 18) with weaker staining of other cell types. At hatching, as in the adult state, most of the B-CK was localized within rods and cones. Thus, during retinal development marked changes in the immunostaining pattern for B-CK were evident. By contrast, Mi-CK expression was low during development in ovo and rose just before hatching with a predominant accumulation of this isoenzyme within the ellipsoid portion of the inner photoreceptor cell segments. Mi-CK accumulation in the retina coincided with functional maturation of photoreceptors and therefore represents a good marker for terminal differentiation of these cells. B-CK, present from early stages of retina development, seems to be relevant for the energetics of retinal cell proliferation, migration and differentiation, whereas the simultaneous expression of both B- and Mi-CK around the time of hatching indicates a coordinated function of the two CK isoforms as constituents of a PCr-circuit involved in the energetics of vision, which, in autophagous birds, has to be operational at this point in time.

Phototransduction: different mechanisms in vertebrates and invertebrates

The photoreceptor cells of invertebrate animals differ from those of vertebrates in morphology and physiology. Our present knowledge of the different structures and transduction mechanisms of the two animal groups is described. In invertebrates, rhodopsin is converted by light into a meta-rhodopsin which is thermally stable and is usually re-isomerized by light. In contrast, photoisomerization in vertebrates leads to dissociation of the chromophore from opsin, and a metabolic process is necessary to regenerate rhodopsin. The electrical signals of visual excitation have opposite character in vertebrates and invertebrates: the vertebrate photoreceptor cell is hyperpolarized because of a decrease in conductance and invertebrate photoreceptors are depolarized owing to an increase in conductance. Single-photon-evoked excitatory events, which are believed to be a result of concerted action (the opening in invertebrates and the closing in vertebrates) of many light-modulated cation channels, are very different in terms of size and time course of photoreceptors for invertebrates and vertebrates. In invertebrates, the single-photon events (bumps) produced under identical conditions vary greatly in delay (latency), time course and size. The multiphoton response to brighter stimuli is several times as long as a response evoked by a single photon. The single-photon response of vertebrates has a standard size, a standard latency and a standard time course, all three parameters showing relatively small variations. Responses to flashes containing several photons have a shape and time scale that are similar to the single-photon-evoked events, varying only by an amplitude scaling factor, but not in latency and time course. In both vertebrate and invertebrate photoreceptors the single-photon-evoked events become smaller (in size) and faster owing to light adaptation. Calcium is mainly involved in these adaptation phenomena. All light adaptation in vertebrates is primarily, or perhaps exclusively, attributable to calcium feedback. In invertebrates, cyclic AMP (cAMP) is apparently another controller of sensitivity in dark adaptation. The interaction of photoexcited rhodopsin with a G-protein is similar in both vertebrate and invertebrate photoreceptors. However, these G-proteins activate different photoreceptor enzymes (phosphodiesterases): phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates. In the photoreceptors of vertebrates light leads to a rapid hydrolysis of cGMP which results in closing of cation channels. At present, the identity of the internal terminal messenger in invertebrate photoreceptors is still unsolved.(ABSTRACT TRUNCATED AT 400 WORDS)

Light-evoked changes in extracellular calcium concentration in frog retina

Light-evoked changes in extracellular Ca2+ concentration were recorded with Ca2(+)-selective microelectrodes in the retina of the frog eyecup. A Ca-decrease at light onset and offset was found in the inner plexiform layer, and its properties are consistent with it resulting from Ca2+ influx into activated neuronal terminals. In the subretinal space, a Ca-increase at light onset and a Ca-decrease at offset were observed, and these likely arise directly from photoreceptors. A slower ON Ca-decrease was also seen here. Because it survives pharmacological isolation of the photoreceptors from post-synaptic interactions, but not physical isolation of the retina from the pigment epithelium, this component probably depends on pigment epithelial activity.

Calmodulin and calmodulin binding proteins in amphibian rod outer segments

The calmodulin (CaM) content of fully intact frog rod outer segments (ROS) has been measured. The molar ratio between rhodopsin and total CaM in ROS is 800:1. This is in good agreement with the data reported for bovine ROS CaM [Kohnken, R. E., Chafouleas, J. G., Eadie, D. M., Means, A. R., & McConnell, D.G. (1981) J. Biol. Chem. 256, 12517-12522]. In the absence of Ca2+, the ROS membrane fraction contains only 4% of total ROS CaM. In contrast, in the presence of Ca2+, 15% of total ROS CaM is found in the membrane fraction. For half-maximal binding of CaM to CaM-depleted ROS membranes, 3 X 10(-7) M Ca2+ is required. This CaM binding is inhibited by trifluoperazine. CaM binding proteins in the ROS membrane fraction are identified by using two different methods: the overlay method and the use of 3,3'-dithiobis(sulfosuccinimidyl propionate) (DTSSP), a bifunctional cross-linking reagent. Ca2+-dependent CaM binding proteins with apparent molecular weights of 240,000, 140,000, 53,000, and 47,000 are detected in the ROS membrane fraction by the overlay method. Anomalous, Ca2+-independent CaM binding to rhodopsin is also detected with this method, and this CaM binding is inhibited by the presence of Ca2+. With the bifunctional cross-linking reagent, DTSSP, three discrete proteins with molecular weights of 240,000, 53,000, and 47,000 are detected in the native ROS membrane fraction. CaM binding to rhodopsin is not detected with this method. Moreover, while the Mr 140,000 band is not detected with DTSSP, a smeared band with a molecular weight between 78,000 and 93,000 is identified (with DTSSP) in the ROS membrane fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Transduction persists in rod photoreceptors after depletion of intracellular calcium

We have examined the role of Ca++ in phototransduction by manipulating the intracellular Ca++ concentration in physiologically active suspensions of isolated and purified rod photoreceptors (OS-IS). The results are summarized by the following. Measurement of Ca++ content using arsenazo III spectroscopy demonstrates that incubation of OS-IS in 10 nM Ca++-Ringer's solution containing the Ca++ ionophore A23187 reduces their Ca++ content by 93%, from 1.3 to 0.1 mol Ca++/mol rhodopsin. Virtually the same reduction can be accomplished in 10 nM Ca++-Ringer's without ionophore, presumably via the plasma membrane Na/Ca exchange mechanism. Hundreds of photoresponses can be obtained from the Ca++-depleted OS-IS for at least 1 h in 10 nM Ca++-Ringer's with ionophore. The kinetics and light sensitivity of the photoresponse are essentially the same in the presence or absence of the ionophore in 10 nM Ca++. The addition of A23187 in 1 mM Ca++-Ringer's results in a Ca++ influx that rapidly suppresses the dark current and the photoresponse. This indicates that there is an intracellular site at which Ca++ can modulate the light-regulated conductance. Both the current and photoresponse can be restored if intracellular Ca++ is reduced by lowering the external Ca++ to 10 nM. During the transition from high to low Ca++, the response duration becomes shorter, which suggests that it can be regulated by a Ca++-dependent mechanism. If the dark current and the photoresponse are suppressed by adding A23187 in 1 mM Ca++-Ringer's, the subsequent addition of the cyclic GMP phosphodiesterase inhibitor isobutylmethylxanthine can restore the current and photoresponse. This implies that under conditions where the rod can no longer control its intracellular Ca++, the elevation of cyclic GMP levels can restore light regulation of the channels. The persistence of normal flash responses under conditions where intracellular Ca++ levels are reduced and perturbed suggests that changes in the intracellular Ca++ concentration do not cause the closure of the light-regulated channel.

High content of creatine kinase in chicken retina: compartmentalized localization of creatine kinase isoenzymes in photoreceptor cells

Two isoforms of creatine kinase (CK; ATP:creatine N-phosphotransferase, E.C. 2.7.3.2), brain type (BB-CK) and mitochondrial type (MiMi-CK), but not the muscle types (MM- or hybrid MB-CK), were identified by cellulose polyacetate electrophoresis and immunoblots in retina from adult chickens. Indirect immunofluorescence labeling of cryosections of retinas revealed high concentrations of BB-CK in both rod and cone photoreceptor cells. Most of the fluorescence staining with anti-B-CK antibodies was found within the myoid and the ellipsoid portions of inner segments and the peripheral region of the outer segments. Significant staining with anti-B-CK antibodies was also found in horizontal cells and in the optical nerve fibers, with additional stratified staining in the inner plexiform layer. MiMi-CK was solely demonstrated in the ellipsoid portion of the photoreceptor cells. The presence of high concentrations of compartmentalized CK isoenzymes within photoreceptor cells (approximately equal to 30 enzyme units/mg) as well as the relatively high concentration of total creatine in these cells (approximately equal to 10-15 mM) indicates an important physiological function for CK and phosphocreatine in the energy transduction of vision.

In situ cGMP phosphodiesterase and photoreceptor potential in gecko retina

The possible involvement of phosphodiesterase (PDE) activation in phototransduction was investigated in gecko photoreceptors by comparing the in situ PDE activity with the photoreceptor potential. In the dark, intracellular injection of cGMP into a gecko photoreceptor caused a long-lasting depolarization. An intense light flash given during the depolarization phase repolarized the cell with a short latency comparable to that of the light-evoked hyperpolarizing response, which indicates that the activation of PDE in situ is rapid enough to generate the photoreceptor potential. PDE activity in situ was estimated quantitatively from the duration of the cGMP-induced depolarization, since it was expected that the higher the PDE activity, the shorter the duration. Under steady illumination, the enzyme exhibited a constant activity. On exposure to a light flash, PDE became activated, but recovered in the dark with a time course that was dependent on the intensity of the preceding stimulus. When PDE activity and photoreceptor sensitivity to light were measured in the same cell after a light flash, both recovery processes showed similar kinetics. Theoretical analysis showed that the parallelism in the recovery time courses could be explained if cGMP is the transduction messenger. These results suggest that PDE activation is involved not only in the generation but also in the adaptation mechanisms of the photoreceptor potential.

Sodium-calcium exchange in the outer segments of bovine rod photoreceptors

Intact rod outer segments (r.o.s.) isolated from bovine retinas were used to measure net Ca2+ fluxes using the optical Ca2+ indicator Arsenazo III. Ca2+ fluxes were observed, which could change the internal Ca2+ content of isolated r.o.s. by as much as 0.5 mM s-1. The Ca2+ content of isolated intact r.o.s. was strongly dependent on the Na/Ca ratio in the isolation medium, and could be made less than 0.1 mol Ca2+ mol-1 rhodopsin (zero Ca2+ in isolation medium) or up to 7 mol Ca2+ mol-1 rhodopsin (zero Na+ in isolation medium). Ca2+ efflux from r.o.s. rich in Ca2+ was observed only when Na+ was added to the external medium (as opposed to any other alkali cation); in Ca2+-depleted r.o.s. Ca2+ uptake required the presence of internal Na+ and was inhibited selectively by external Na+. These results suggest that Na-Ca exchange across the plasma membrane operated freely in both directions and controlled the internal Ca2+ concentration in r.o.s. Na+-stimulated Ca2+ efflux depended on the external Na+ concentration in a sigmoidal way. This suggests that the simultaneous binding of two Na ions is rate limiting for transport. In Ca2+-depleted r.o.s. and in the absence of external Na+, 1 mol Ca2+ mol-1 rhodopsin (or 3 mM-total Ca2+) could be taken up within 1 min by intact r.o.s. at a free external Ca2+ concentration of about 1 microM. Only part of the internal Ca2+ was available for Na-Ca exchange. The external Na+ and K+ concentration as well as the temperature were factors controlling the accessibility of internal Ca2+ to participate in Na-Ca exchange. Ca2+ fluxes in r.o.s. with a permeabilized plasma membrane but intact disk membranes were very similar to those observed in intact r.o.s.; Na-Ca exchange could operate in both directions across the disk membrane. In addition to Na-Ca exchange, leaky r.o.s. also showed a guanosine 3', 5'-cyclic monophosphate (cyclic GMP)-induced Ca2+ release that was about 1/20 of the rate of Na-Ca exchange. Na-Ca exchange could release 1.5 mol Ca2+ mol-1 rhodopsin from disks as compared with a cyclic-GMP-induced release of 0.15 mol Ca2+ mol-1 rhodopsin.

Calcium content and calcium exchange in dark-adapted toad rods

We have used laser-activated micro mass analysis (l.a.m.m.a.) and energy-dispersive X-ray analysis (e.d.x.) to measure Ca content and Ca movements in 'red' rod photoreceptors in the dark-adapted retina of the toad, Bufo marinus. Measurements with both l.a.m.m.a. and e.d.x. show that intact rod outer segments contain 4-5 mmol total Ca/l wet tissue volume, or 1-2 Ca per rhodopsin. We could detect no significant variation in the total Ca as a function of distance across or up and down the outer segment. In the inner segment, Ca could be detected only within the mitochondria-rich ellipsoid body, where the total Ca concentration was of the order of 100-400 mumol/l wet tissue volume. To measure the exchange of Ca in outer segments from intact photoreceptors, we exposed the dark-adapted retina to Ringer containing the stable isotope 44Ca. Since l.a.m.m.a. can measure separately the concentrations of each of the isotopes of the elements, and since native rods contain almost exclusively 40Ca, the increase in 44Ca and decrease in 40Ca could be used as a measure of Ca influx and efflux. Ca exchange in intact rod outer segments in darkness is very slow. The rate of accumulation of 44Ca was only 10(5) Ca/rod.s, or about 10% of the total outer segment Ca/h. This slow rate of exchange is apparently not the result of restricted movement of Ca across the plasma membrane. Ca exchange was also measured in outer segments which were either partially or entirely detached from the rest of the photoreceptor. In broken-off outer segments, Ca exchange is faster than in the intact organelles, and in 1 h, half of the 44Ca exchanges for 40Ca. When the retina was incubated in Ringer for which all of the Na was substituted with Li or choline, there was an increase in the rate of 44Ca accumulation in intact outer segments, probably due to an inhibition of Na-Ca counter transport across the plasma membrane. Our measurements indicate that the great majority of the Ca in the rod appears to be inaccessible to exchange under physiological conditions, probably because it is sequestered within the disks which in intact rods appear to be nearly impermeable to Ca in darkness.

Ca2+ buffer sites in intact bovine rod outer segments: introduction to a novel optical probe to measure ionic permeabilities in suspensions of small particles

The nature of the Ca2+ buffer sites in intact rod outer segments isolated from bovine retinas (ROS) was investigated. The predominant Ca2+ buffer in intact ROS was found to be negatively charged groups confined to the surface of the disk membranes. Accordingly, Ca2+ buffering in ROS was strongly influenced by the electrostatic surface potential. The concentration of Ca2+ buffer sites was about 30 mM, 80% of which were located at the membrane surface in the intradiskal space. A comparison with observations in model systems suggests that phosphatidylserine is the major Ca2+ buffer site in ROS. Protons and alkali cations could replace Ca2+ as mobile counterions for the fixed negatively charged groups. At physiological ionic strength, the total number of these diffusible, but osmotically inactive, counterions was as large as the number of osmotically active cations in ROS. The surface potential is dependent on the concentration of cations in ROS and can be measured with the optical dye neutral red. Addition of cations to the external solution led to the release of the internally bound dye as the cations crossed the outer membrane. The chemical and spectral properties of the dye enable its use as a real-time indicator of cation transport across the outer envelope of small particles in suspension. In this study, the dye method is illustrated by the use of well-defined ionophores in intact ROS and in liposomes. In the companion paper this method is used to describe the cation permeabilities native to ROS.