On the light-stimulated coupling between rhodopsin and its disk membrane environment

Cryo-electron microscopy of nervous tissue. A review

The present work attempts to demonstrate that cryofixation is a valuable method for the study of the nervous tissue. The use of the newly developed methods of cryofixation and freeze-etching without fixatives or cryoprotectants allows new exciting perspectives for the electron microscopical observation of cellular components, emphasizing their three-dimensional morphological structures. Significant contributions have been made on the fine structure of the cytoskeleton, cell membranes and cell organelles. The components of the cytoskeleton are distributed in different composition through the perikarya, dendrites and axon. The ubiquitous presence of the cytoskeleton suggests a crucial role in the functional activities of the neurons, especially in relation to the intracellular communication and to developmental and regeneration processes. Vitrified cellular membranes of myelin sheaths and rod outer segments have been observed in hydrated state by using cryofixation and cryotransfer techniques. These procedures allow new insights into the supramolecular structure and an approximation of morphological data to the present biophysical membrane model including a critical comparison with the current descriptions gained by conventional electron microscopy.

Watching G proteins at work

It has been known for over a century that rod photoreceptors in the living retina contract and swell in response to light. Although it is still not known whether this structural light-response is of any functional significance, it has recently been possible to correlate the underlying molecular processes with the activation and deactivation of the photoreceptor G protein, transducin. The technique of light-scattering allows the monitoring of minute changes in cell dimensions, and using this non-invasive experimental approach it can be shown that certain properties of the coupling between transducin and rhodopsin are different in a structurally well-preserved system as compared with rod material used for conventional biochemical studies. Thus, not unlike a psychiatrist, who often learns more about a patient's 'interiors' by observing the body language than by direct interrogation, a biochemist, studying the 'body language' of a cell, may extract information about delicate 'cell interior processes' that would be perturbed by more direct experimental approaches.

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

ATP can cause dramatic structural changes in the outer segment of rod photoreceptors. These changes can be visualized by means of a concomitant light-scattering signal AD, a decrease in scattered light intensity of over 20%. The large size of the signal suggests that major structural changes occur. The underlying molecular events may reflect an important, yet still unknown, part of the photoreceptor machinery. AD signals reflect ATPase-driven transmembrane events which occur in and at the disk membrane. Their only structural prerequisite is the structural integrity of the disk compartment. The angular dependence of AD, which can be mimicked by an osmotically-induced disk-swelling, suggests that the disk compartment swells during the production of the AD signal. AD signals proceed with first-order kinetics (half-life = 1 min at 20 degrees C and ATP concentrations of greater than 100 microM) and are accompanied by the hydrolysis of approximately 4 mol ATP (mol rhodopsin)-1. The AD signal is inhibited by a number of transport ATPase inhibitors (quercetin, NBD.Cl, vanadate, DCCD), but not by oligomycin, azide and ouabain. The sensitivity to DCCD, together with the fact that except magnesium no other cation has to be present, points to a proton translocation. This proton transport appears to be electrogenic, since AD signals require the presence of a permeant anion. In physiological saline this is chloride, and the chloride flux is facilitated by a DIDS-sensitive anion transport unit in the disk membrane.

Light- and nucleotide-dependent binding of phosphodiesterase to rod disk membranes: correlation with light-scattering changes and vesicle aggregation

Under conditions in which large guanosine cyclic 3',5'-phosphate (cGMP)- and phosphodiesterase (PDE)-dependent changes in near-infrared transmission and vesicle aggregation and disaggregation occur, we have observed a striking change in the binding of PDE to rod disk membranes. The change in PDE binding is nucleotide and light dependent as are the light-scattering changes. The cGMP- and PDE-dependent light-scattering signal can be produced by a 500-nm light flash which bleaches 1/(1 X 10(7] rhodopsin molecules. Mg ions are an essential cofactor for the nucleotide-dependent PDE binding and light-scattering changes. 3-Isobutyl-1-methylxanthine and other competitive inhibitors of PDE hydrolytic activity support increased PDE binding to the disk membrane, vesicle aggregation, and the light-scattering signal. However, treatments which block GTP-dependent activation of PDE hydrolytic activity (colchicine, GDP, or ethylenediaminetetraacetic acid) also block these phenomena. Thus, GTP-dependent activation of PDE rather than its hydrolytic activity appears to be correlated with the light-scattering signal.

cGMP- and phosphodiesterase-dependent light-scattering changes in rod disk membrane vesicles: relationship to disk vesicle-disk vesicle aggregation

Visible light activates a large guanosine cyclic 3',5'-phosphate (cGMP)- and phosphodiesterase (PDE)-dependent infrared light-scattering change in suspensions of photoreceptor disk membranes. Reconstitution experiments show that this signal requires bleached rhodopsin, G protein (three polypeptide subunits of Mr 39 000, 37 000, and 6000 which comprise the GTPase), phosphodiesterase, cGMP, and GTP. The lowest light intensity which elicits the light-scattering signal bleaches 0.002% rhodopsin. cGMP and GTP hydrolysis occurs more slowly than the initial phase of the scattering signal, and the kinetics of nucleotide hydrolysis do not correlate with any phase of the signal. Hydrolysis-resistant analogues of cGMP and GTP support the initial decreasing phase of the signal. Thus, the signal apparently depends upon nucleotide binding rather than hydrolysis. Microscopic observations made under the same conditions as light-scattering experiments show that vesicle-vesicle aggregation and disaggregation occur. The data suggest that light and nucleotide activations of the cyclic nucleotide cascade enzymes are responsible for the vesicle aggregation process and nucleotide hydrolysis for vesicle disaggregation. The vesicle aggregation-disaggregation phenomenon appears likely to be the physical basis of the cGMP- and PDE-dependent changes in infrared transmission.

Temperature and pH dependence of the metarhodopsin I-metarhodopsin II kinetics and equilibria in bovine rod disk membrane suspensions

The kinetics of the relaxation of bleached bovine rod disk membrane suspensions from metarhodopsin I into the equilibrium between metarhodopsins I and II were determined at pHs between 5.9 and 8.1 and at temperatures between -1 and 15 degrees C. From these data, thermodynamic equations were generated by two-way linear regression that simultaneously describe the functional dependence on pH and temperature of the pseudo-first-order and true forward rate constants, the reverse and observed rate constants, and the equilibrium constant. Using these equations, we obtained the thermodynamic parameters and the apparent net proton uptake for the transitions from metarhodopsin I to metarhodopsin II and from metarhodopsin I to the activated intermediate. The reversibility of this equilibrium and the effect of aging of the preparation on the measured rate constants were investigated.

ATP causes a structural change in retinal rod outer segments: disc swelling is not involved

Mg2+-ATP was found to produce a 15 to 30% drop in the turbidity of suspensions of broken retinal rod outer segments from the toad Bufo marinus, prepared by washing or flotation in sucrose. This in vitro process has a half-time of about two minutes and appears to be irreversible. It is not affected by the bleaching of rhodopsin. Direct measurements show that the drop in turbidity is not due to swelling of the disc internal space measured in outer segments recovered by centrifugation. Instead, the total packed volume of the outer segments increases following incubation in Mg2+-ATP. Under the specific conditions of these experiments, the total pellet volume increase was 26 +/- 22% (13 experiments) when corrected for the percent of rhodopsin recovered in the centrifugal pellet. The magnitude of the ATP effect on turbidity suggests that the majority of the discs are involved in some kind of structural change. Vanadium in the +5 oxidation state (vanadate) is an inhibitor of the Mg2+-ATP effect on turbidity at a half-maximal concentration of 0.2 to 0.4 microM, and inhibition is rapidly reversed by norepinephrine, which complexes vanadate. A Mg2+-ATPase activity in extensively washed outer segment membranes, previously shown to be activated as much as twofold following light exposure of the membranes, is not sensitive to vanadate at the concentrations which block the ATP-dependent change of turbidity.

Light-induced interaction between rhodopsin and the GTP-binding protein. Metarhodopsin II is the major photoproduct involved

We have previously described [H, Kühn et al. (1981) Proc. Natl Acad. Sci. USA, 78, 6873-6877] a light-induced scattering change ('binding signal') associated with a stoichiometric binding between photoexcited rhodopsin and a peripheral membrane protein, the GTP-binding protein, in bovine rod outer segment suspensions. We have attempted here to identify the rhodopsin intermediate R* which is responsible for this interaction, by studying its dependence on pH, temperature and ionic strength. The results strongly suggest that the active state is metarhodopsin II (M II). 1. The initial phase of the binding signal is slightly slower than the formation of metarhodopsin II (2-37 degrees C, pH 5.5-9). 2. The kinetics of the decay of the active rhodopsin state are similar to those of the metarhodopsin II leads to metarhodopsin III transition (37 degrees C, pH 7.3). 3. All conditions which lead to light-induced binding of the GTP-binding protein to R* also lead to the formation of M II. At 2 degrees C, pH 8.3, in particular where no M II is formed in the absence of GTP-binding protein, binding signals and light-induced attachment of the GTP-binding protein to the membrane are still observed. Consistently, addition of GTP-binding protein to a suspension of extracted membranes bleached at 2 degrees C (pH 8.3) shifts the metarhodopsin I in equilibrium metarhodopsin II equilibrium towards metarhodopsin II. The shift is reversed by GTP, which dissociates the rhodopsin--GTP-binding protein complex. 4. At low ionic strength, where the GTP-binding protein is soluble in the dark (instead of being associated to the membrane as in the above experiments) M II still induces the binding whereas M I does not, indicating a much lower affinity of the GTP-binding protein for MI.

Modeling the rod outer segment birefringence change correlated with metarhodopsin II formation

A rapid birefringence loss associated with metarhodopsin II formation, delta (delta n) MII, is produced when frog rod outer segments are exposed to a bleaching light flash. To analyze the nature of the underlying structure change, measurements of delta (delta n) MII were made in rod outer segments perfused with glycerol solutions to increase the refractive index of the cytoplasmic and intradisk spaces. Comparisons of experimental results with computed changes in the form birefringence component using two- and three-dielectric outer segment models for several putative structure changes were made. It is concluded that delta (delta n) MII can be due to either a change in the intrinsic birefringence component caused by the reorientation of anisotropic molecules, or to a change in the form birefringence component caused by small changes in the cytoplasmic and/or intradisk volumes.

Concurrent birefringence and forward light-scattering measurements of flash-bleached rod outer segments

A microretardometer--nephelometer was constructed to measure birefringence and forward-direction light scattering concurrently in single retinal rod outer segments (ROS). The relative contributions of light-induced birefringence and light-scattering changes to observed far-red transmission changes measured normal to the cell axis were evaluated in isolated, whole, flash-bleached ROS. No light-scattering transients were found corresponding to the previously reported rapid birefringence loss associated with the metarhodopsin I lead to metarhodopsin II reaction even under conditions in which such a light-scattering change has been reported for outer-segment fragments.

Interfacial potentials at the disk membranes of isolated intact cattle rod outer segments as a function of the occupation state of the intradiskal cation-exchange binding sites

Two different methods have been used to determine the interfacial potential at the disk membranes of intact isolated bovine rod outer segments: (1) The photolysis products of rhodopsin are known to be dependent on pH. We have used this property in order to probe the interfacial potential at disk membranes which is considered to change the surface pH at the disk membrane seen by rhodopsin. (2) The pK value of the amphiphilic pH-indicating dye neutral red (uncharged basic form) in water is 6.6, but adsorbed to disk membranes at least 7.8. This makes the distribution of neutral red between disk membranes and bulk water dependent on the interfacial potential at the disk membrane if the pH in the bulk solution is less than 7.8. Both methods yielded comparable results on the influence of ions and ion carriers on the interfacial potential at disk membranes. In particular, we have studied the effect of different occupation states on the internal binding capacity (of rod outer segments) for divalent cations. In the presence of the ionophore A23187, addition of EDTA to a suspension of intact rod outer segments removed all endogenous divalent cations (Schnetkamp, P.P.M. (1979) Biochim. Biophys. Acta 554, 441--459) and resulted in an interfacial pH at the disk membrane surface of about 6.4, whereas the bulk pH was 7.4. Subsequent addition of 2 mM Mn2+ saturated the internal binding capacity and resulted in an apparent shift towards alkaline pH of the surface pH at the disk membrane by 1.0--1.1 pH units. This could indicate a change of the interfacial potential by 60--65 mV. The same change of ionic conditions resulted in a change of the interfacial potential by 72 mV as determined from the partitioning behaviour of neutral red. These results were independent of the presence of H+ ionophores such as carbonyl cyanide p-trifluoromethoxy-phenylhydrazone and gramicidin. We conclude that the above results can be explained by the presence of fixed net negative charges (charge density: 0.5--1.5 electronic charges/rhodopsin molecule) at the intradiskal membrane surface. That the above charge density can be attributed to the intradiskal membrane surface is inferred from the observation that the presence of A23187 was required for access of divalent cations to the membrane interface involved in both rod outer segments with an intact as well as with a leaky plasma membrane.

Kinetics and saturation of light-induced near-infrared scattering changes in isolated bovine rod outer segments

The axial and radial shrinkage of bovine rod outer segments, monitored by near-infrared scattering changes (P-signal), is investigated in dependence on the intensity of the activating flash. Suspensions of axially oriented and randomly oriented rod outer segments were measured. In the latter case, axial and radial effects are superimposed to another. The following results are obtained: 1. The axial signal (Pa, tau approximately 10 ms) and the radial signal (Pr, tau = 40-100 ms), simultaneously measured on axially oriented rod outer segments, are similarly saturated with a half-saturation at a rhodopsin turnover of 3.5%. 2. For the saturation of the signal amplitude, measured on randomly oriented rod outer segments, a good fit is obtained by: P(rho) approximately 1 - e-beta rho, rho: relative rhodopsin turnover by the flash; beta is found in the range 23 less than or equal to beta less than or equal to 27 in all measurements. 3. The kinetics of the signal, also measured on the isotropic sample, depends on the rhodopsin turnover, the apparent time constant becoming faster with increasing turnover, The distortion of the signal cannot be fitted by a sum of exponentials with a fixed set of time constants. The signals from the isotropic sample are fitted by a phenomenological model. It introduces three first order processes concatenated in series; the first step is assumed as a rhodopsin transition inducing the two further processes. The distortion of the signals with increasing rho is then described assuming a rho-dependent quenching of this induction, according to the measured amplitude saturation. The time constants remain thereby unchanged. The fit yields the values in 2/k = 4, 11, and 45 ms with mean square deviations of 20%.

Light and GTP effects on the turbidity of frog visual membrane suspensions

The time course of turbidity changes of frog visual membranes, dependent on osmotic shocks, on light and on nucleotide substrates or effectors of enzyme activities, were measured as absorption changes in a rapid mixing stopped-flow spectrophotometer. As a result of studies on different preparations, it is concluded that light can cause both rapid (within 50 msec) and slow (within 90 sec) changes in the turbidity of visual membranes, not associated with permeability changes, and that they are affected by GTP or its analog guanyl-5'-yl imidodiphosphate; however, the light and GTP effects are lost when a water soluble fraction containing the light-sensitive enzyme cGMP-phosphodiesterase, is removed from the rod outer segments membranes. It is suggested that the fast light and GTP-sensitive response is related to the activation of cGMP-phosphodiesterase.

Investigations of the rhodopsin/Meta I and rhodopsin/Meta II transitions of bovine rod outer segments by means of kinetic infrared spectroscopy

We have applied our recently developed technique of flash induced kinetic infrared spectroscopy to the rhodopsin/Meta I and rhodopsin/Meta II transitions. Features of the infrared spectrum reflecting the C = C-vibration and the isomeric form of the chromophore are in agreement with resonant Raman experiments. Different results are obtained for the C = N-vibration of the Schiff base retinal opsin link. They are interpreted in terms of a Schiff base protonated via an hydrogen bond. A proton transfer in the excited state is suggested to explain the deviating results. In addition we have obtained spectral changes which cannot be attributed to molecular changes in the chromophore. We assume that these spectral features reflect molecular events in the protein part of rhodopsin.

A possible role of rhodopsin in maintaining bilayer structure in the photoreceptor membrane

31P-NMR measurements demonstrate that at 37 degrees C, independent of the photolytic state of the photopigment rhodopsin, the lipids in the photo-receptormembrane are almost exclusively organised in a bilayer. In strong contrast, the 31P-NMR spectra of the extracted lipids are characteristic for the hexagonal HII phase and an isotropic phase. The isotropic phase is characterised by freeze-fracture electron microscopy as particles and pits on smooth surfaces, possibly indicating inverted micelles. These results suggest a structural role for rhodopsin in maintaining the photoreceptor membrane lipids in a bilayer configuration.