The Structural Organization of Mammalian Retinal Disc Membrane

Membrane binding properties of the C-terminal segment of retinol dehydrogenase 8

Light absorption by rhodopsin leads to the release of all-trans retinal (ATRal) in the lipid phase of photoreceptor disc membranes. Retinol dehydrogenase 8 (RDH8) then reduces ATRal into all-trans retinol, which is the first step of the visual cycle. The membrane binding of RDH8 has been postulated to be mediated by one or more palmitoylated cysteines located in its C-terminus. Different peptide variants of the C-terminus of RDH8 were thus used to obtain information on the mechanism of membrane binding of this enzyme. Steady-state and time-resolved fluorescence measurements were performed using short and long C-terminal segments of bovine RDH8, comprising one or two tryptophan residues. The data demonstrate that the amphipathic alpha helical structure of the first portion of the C-terminus of RDH8 strongly contributes to its membrane binding, which is also favored by palmitoylation of at least one of the cysteines located in the last portion of the C-terminus.

Fake It 'Till You Make It-The Pursuit of Suitable Membrane Mimetics for Membrane Protein Biophysics

Membrane proteins evolved to reside in the hydrophobic lipid bilayers of cellular membranes. Therefore, membrane proteins bridge the different aqueous compartments separated by the membrane, and furthermore, dynamically interact with their surrounding lipid environment. The latter not only stabilizes membrane proteins, but directly impacts their folding, structure and function. In order to be characterized with biophysical and structural biological methods, membrane proteins are typically extracted and subsequently purified from their native lipid environment. This approach requires that lipid membranes are replaced by suitable surrogates, which ideally closely mimic the native bilayer, in order to maintain the membrane proteins structural and functional integrity. In this review, we survey the currently available membrane mimetic environments ranging from detergent micelles to bicelles, nanodiscs, lipidic-cubic phase (LCP), liposomes, and polymersomes. We discuss their respective advantages and disadvantages as well as their suitability for downstream biophysical and structural characterization. Finally, we take a look at ongoing methodological developments, which aim for direct in-situ characterization of membrane proteins within native membranes instead of relying on membrane mimetics.

Calmodulin gene expression in the neural retina of the adult rat

Calmodulin (CaM) mRNAs are expressed with low abundancy in the adult rat neural retina. However, when digoxigenin (DIG)-labeled cRNA probes specific for each CaM mRNA population were hybridized at slightly alkaline pH (pH 8.0), the widespread distribution of CaM mRNA-expressing cells was revealed, with similar abundance for all three CaM genes. The CaM genes displayed a uniquely similar, layer-specific expression throughout the retina, and no significant differences were found in the distribution patterns of the CaM mRNA populations or the labeled cell types. The strongest signal for all CaM mRNAs was demonstrated in the ganglion cell layer and the inner nuclear layer, where the highest signal intensity was found within the inner sublamina. Similarly intermediate signal intensities for all CaM genes were detected in the inner and outer plexiform layers, within the vicinity of the outer limiting membrane and in the retinal pigment epithelium. A very low specific signal was characteristic in the outer nuclear layer and the photoreceptor inner segment layer, while no specific hybridization signal was observed in the photoreceptor outer segment layer. In summary, all CaM genes exhibited a similar and a characteristically layer-specific expression pattern in the adult rat retina.

Lectin cytochemical analysis of glycoconjugates in photoreceptor cell membranes of Lampetra japonica

Seven types of ferritinized lectin were used to examine the distribution of glycoconjugates on the outer segment membranes of lamprey photoreceptor cells. Ultrastructural pre-embedding labeling revealed that peanut agglutinin, soybean agglutinin and Ricinus communis agglutinin I were preferentially bound to the proximal, lateral and luminal surfaces of the long cell outer segments, whereas Griffonia simplicifolia agglutinin II and concanavalin A agglutinin were bound to the corresponding surfaces of the short cell outer segments. The results indicate that there is marked difference in the composition of glycoconjugates over the outer segment membranes between long and short photoreceptors.

Amplification and kinetics of the activation steps in phototransduction

We can summarize our investigation of amplification in the activation steps of vertebrate phototransduction as follows. (1) A theoretical analysis of the activation steps of the cGMP cascade shows that after a brief flash of phi photoisomerizations the number of activated PDE molecules should rise as a delayed ramp with slope proportional to phi, and that, as a consequence, the cGMP-activated current should decay as a delayed Gaussian function of time (Eqn. 20). (i) Early in the response to a flash, the normalized response R(t) can be approximated as rising as 1/2 phi At2 (after a short delay), where A is the amplification constant characteristic of the individual photoreceptor. (ii) The delayed ramp behavior of PDE activation and the consequent decline of current in the form of the delayed Gaussian are confirmed by experiments in a variety of photoreceptors; the analysis thus yields estimates of the amplification constant from these diverse photoreceptors. (iii) Eqn. 20 further predicts that the response-intensity relation at any fixed time should saturate exponentially, as has been found experimentally. (2) The amplification constant A can be expressed as the product of amplification factors contributed by the individual activation steps of phototransduction, i.e., A = nu RG cGP beta sub n (Eqns. 9 and 21), where (i) nu RG is the rate of G* production per Rh*; (ii) cGP is the efficiency of the coupling between G* production and PDE* production; (iii) beta sub is the increment in hydrolytic rate constant produced by one PDE*, i.e., a single activated catalytic subunit of PDE; and (iv) n is the Hill coefficient of opening of the cGMP-activated channels. (3) The amplification factor beta sub includes the ratio kcat/Km, which characterizes the hydrolytic activity of the PDE in vivo where cG << Km. Two different analyses based upon photocurrents were developed which provide lower bounds for kcat/Km in vivo; these analyses establish that kcat/Km probably exceeds 10(7) M-1 s-1 (and is likely to be higher) in both amphibian and mammalian rods. Few biochemical studies (other than those using trypsin activation) have yielded such high values. A likely explanation of many of the relatively low biochemical estimates of kcat/Km is that Km may have been overestimated by a factor of about 4 in preparations in which stacks of disks are left intact, due to diffusion with hydrolysis in the stacks.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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)

Cyclic GMP and calcium: the internal messengers of excitation and adaptation in vertebrate photoreceptors

The roles of cyclic GMP (cGMP) and calcium (Ca2+) in vertebrate rod phototransduction are reviewed, with the emphasis on developments since the discovery of the cGMP-activated conductance of the rod outer segment. The first hypothesis subjected to critical examination is that cGMP acts as the sole internal messenger of excitation. This hypothesis is evaluated with a formal, quantitative model of the biochemical actions of cGMP. Application of the model shows a remarkable agreement between independent electrophysiological and biochemical measurements of the resting dark amounts of (1) total cGMP (2) free cGMP (3) fraction of open cGMP-activated channels and (4) the rate of cGMP hydrolysis. The second hypothesis examined is that Ca2+ acts as an internal messenger in rod light adaptation. Recent electrophysiological evidence has shown minimization of the normal light-induced reduction of free Ca2+ prevents rods from exhibiting the change in sensitivity and speed characteristic of light adaptation. Physiological effects, formerly attributed to a role of calcium as an excitational messenger are shown to be consistent with a biochemical model in which Ca2+ serves as the cytoplasmic signal in a powerful feedback loop that acts to restore the concentration of cGMP both during and after exposure to light. Residual problems facing the "cGMP cascade theory of phototransduction" are reviewed. Issues are itemized that will have to be resolved quantitatively before it will be possible to develop a fully comprehensive theory of photoreceptor excitation, restoration and adaptation combining the roles of Ca2+ and cGMP.

Photoreceptor disk membranes of Lampetra japonica

With the aim of characterizing photoreceptor outer segments and obtaining in situ observation of macromolecular variations due to cell types as well as adaption, we counted the number of outer segment disk membranes using electron micrographs of ultrathin sections as well as intramembrane particles on the complementary replicas of the retina of Lampetra japonica. Long photoreceptor cells (LPCs, cone-type cells) numbered 10,000/mm2 in the central as well as peripheral regions, while short ones (SPCs, rod-type cells) numbered 30,000/mm2 in the same regions. The LPC outer segment exhibited 306 disks on average during the light cycle versus 364 during the dark cycle. 12.0% of the LPC disks during the light cycle versus 13.4% during the dark cycle represented the "open" disks. The SPC outer segment exhibited 470 disks on average during the light cycle versus 507 during the dark cycle. 11.1% of the SPC disks during the light cycle versus 13.6% during the dark cycle represented the "open" disks. The LPC disk membrane contained 44.3 particles/0.01 microns 2 during the light cycle versus 39.5 particles during the dark cycle, 95% of which were derived from the protoplasmic fracture (PF) face. The SPCs contained 36.0 particles/0.01 micron 2 during the light cycle versus 43.6 during the dark cycle, 90% of which were derived from the PF-face. The present findings contradict the frequently cited hypothesis that an "open" disk, retaining continuity with the plasmalemma, is preserved characteristically into later stages by the cone outer segment. The significance of the intramembrane particles for the activity of the photoreceptor membrane is discussed.

Structural features of the terminal loop region of frog retinal rod outer segment disk membranes: II. Organization of the terminal loop complex

In addition to a lipid bilayer component (Corless, Fetter, and Costello: J. Comp. Neurol. 257:1-8, '87), the terminal loop region of frog rod outer segment (ROS) disks displays a clustering of discrete elements referred to as the terminal loop complex. It consists of (1) semicircular or crescentic densities within the terminal loop, (2) linear interdisk densities spanning the cytoplasm near terminal loops, and (3) distinctive freeze-fracture particles associated with the terminal loop, located between 1 and 2. The linear interdisk densities are organized on a two-dimensional lattice that appears to ensheath completely the lamellar domains of all ROS disks. Indirect evidence is presented for a net axial alignment of intraloop densities. We interpret the large freeze-fracture particles of the terminal loop region to reflect transmembrane components that connect the interdisk and intraloop densities. Thus, we propose that the entire terminal loop (TL) complex is organized on a two-dimensional net. We further infer that each TL complex is organized as a dimeric unit and that such dimers interact axially and laterally to generate the observed lattice structure. It is suggested that one component of the terminal loop complex is the high molecular weight protein localized along the disk perimeter by Papermaster, Schneider, Zorn, and Kraehenbuhl (J. Cell. Biol. 78:415-425, '78).

Signal mechanisms of phototransduction in retinal rod

The levels of intracellular molecules are modulated by illumination of rod photoreceptor cells in the vertebrate retina. Among these are Ca ions, cyclic nucleotides (cGMP in particular), and phosphate nucleotides (ATP and GTP). It is presumed now that at least two of these molecules, Ca and cGMP, may function as chemical linkers between the absorption of light by rhodopsin and the ionic channels of the plasma membrane of the rod outer segment that close when the rod is illuminated. The manuscript will review the physiology of the rod cell, the evidence in support of light-dependent changes in the intracellular levels of various small molecules, and the role of these changes in coupling rhodopsin excitation to the control of the light-sensitive membrane channels in the rod.

A 12 A resolution X-ray diffraction study of the profile structure of isolated bovine retinal rod outer segment disk membranes

Electron density profiles of disk membranes isolated from bovine retinal rod outer segments have been determined to 12 A resolution by analysis of the X-ray diffraction from oriented multilayers, in the absence of lipid phase separation. Data were collected on both film and a two-dimensional TV-detector; both detectors yielded identical patterns consisting of relatively sharp lamellar reflections of small mosaic spread. The unit cell repeat was reversibly varied over the range of 143 to 183 A. The diffraction patterns changed dramatically at 150 A; consequently, the low (less than 150 A) and high (greater than 150 A) periodicity data were independently analyzed via a swelling algorithm. The high periodicity data yielded two statistically equivalent phase choices corresponding to two symmetric, but different membrane profiles. The low periodicity data yielded essentially one, characteristically asymmetric profile. These profiles have been modeled with regard to the separate profiles of rhodopsin, lipid and water, subject to the known composition of the isolated disk membranes.

The ultrastructure of the developing inner and outer segments of the photoreceptors of chick embryo retina as revealed by the rapid-freezing and deep-etching techniques

The ultrastructure of retina cell receptors of chick embryos and of one to three week old chicks was examined paying special attention to the membrane structure of receptor discs, mitochondria, cell membrane and other cell organelles, such as the endoplasmic reticulum and the Golgi apparatus. The retinas were rapid-frozen with a liquid-propane jet, deep-etched, and rotary-shadowed replicas produced. The structure of the photolamellae membranes is asymmetrical. The fracture faces showed a smooth (E-face) and a rough (P-face) surface. Both true surfaces ( interdiscal and intradiscal) were also observable by deep-etching. Transverse fractures of the discs showed the globular structure of their membrane. Spherical or polyhedral particles, probably rhodopsin-associated particles, occupying the width of the membrane are 12 nm in diameter and are constituted by 6 subunits of 1.5-2.0 nm arranged around a channel. These particles seem to extend into the membrane of the photolamellae during the last days of incubation and were also found in variable positions in the width of the disc membrane. When observed in transversal fractures of the photolamellae , they were sometimes seen to protrude into the collapsed intradiscal space and into the cytoplasmic surface. Filament-like or particulate structures connect the discs both to each other and to the cell membrane. During development a relationship between the forming discs and the cell membrane was not observed. The mitochondria aggregated in the ellipsoid are connected by filament-like structures that form during development of the inner segment. The structure of the inner cristae membrane of the mitochondria is characterized by the presence of stalked particles as previously described by Fern andez -Morán (1961) using negative staining. An intracristal space is not present. The fracture of the receptor cell membrane shows a particulate cytoplasmic half with particle-free patches. The glycogen granula situated in the cytoplasm between the smooth ER cisternae show a rosette-like composition.

Interactions in mixed monolayers between distearoyl-L-phosphatidylethanolamine, rod outer segment phosphatidylethanolamine and all-trans retinal. Effect of pH

The interactions in mixed monolayers between distearoyl-L-phosphatidylethanolamine, natural phosphatidylethanolamine purified from bovine rod outer segments and all-trans retinal have been studied at the nitrogen/water interface at 21.0 +/- 0.5 degrees C. Seven mixtures of each phospholipid with all-trans retinal, covering the whole range of molar fractions, were studied. The monolayers were spread on a 1 X 10(-3) M phosphate buffer subphase at three different pH values, 5.5, 7.1 and 8.2. The results for the two series of mixtures are strikingly different. The surface phase rule shows that all-trans retinal is miscible with the natural phospholipid at the interface. Small, negative deviations with respect to the additivity rule are observed in this case. The excess free energies of mixing were also calculated as a function of concentration for this system at four different surface pressures, 5, 7, 10 and 13 mN X m-1. They are negative for the four surface pressures considered and symmetrical with respect to the mole fraction. On the other hand, when distearoyl-L-phosphatidylethanolamine is mixed with all-trans retinal, the components are no longer miscible at the interface. This marked difference in behaviour between the two lipids reflects the importance of hydrophobic interactions in the mixed monolayers of phospholipids with retinals. Furthermore, for the two series of mixtures, the surface pressure isotherms do not show any significant shift when the subphase pH is changed from 5.5 to 8.2. This behaviour raises questions about the formation of a Schiff base between phosphatidylethanolamine and retinal at the interface. It is suggested that, owing to the nature of the disk membranes, such an effect would also be observed in vivo. The possible implications of this are discussed, particularly with respect to questions pertaining to the stability of the retinal chromophore.