New aspects of the ultrastructure of frog rod outer segments

Rhodopsin Forms Nanodomains in Rod Outer Segment Disc Membranes of the Cold-Blooded Xenopus laevis

Rhodopsin forms nanoscale domains (i.e., nanodomains) in rod outer segment disc membranes from mammalian species. It is unclear whether rhodopsin arranges in a similar manner in amphibian species, which are often used as a model system to investigate the function of rhodopsin and the structure of photoreceptor cells. Moreover, since samples are routinely prepared at low temperatures, it is unclear whether lipid phase separation effects in the membrane promote the observed nanodomain organization of rhodopsin from mammalian species. Rod outer segment disc membranes prepared from the cold-blooded frog Xenopus laevis were investigated by atomic force microscopy to visualize the organization of rhodopsin in the absence of lipid phase separation effects. Atomic force microscopy revealed that rhodopsin nanodomains form similarly as that observed previously in mammalian membranes. Formation of nanodomains in ROS disc membranes is independent of lipid phase separation and conserved among vertebrates.

Modeling the flexural rigidity of rod photoreceptors

In vertebrate eyes, the rod photoreceptor has a modified cilium with an extended cylindrical structure specialized for phototransduction called the outer segment (OS). The OS has numerous stacked membrane disks and can bend or break when subjected to mechanical forces. The OS exhibits axial structural variation, with extended bands composed of a few hundred membrane disks whose thickness is diurnally modulated. Using high-resolution confocal microscopy, we have observed OS flexing and disruption in live transgenic Xenopus rods. Based on the experimental observations, we introduce a coarse-grained model of OS mechanical rigidity using elasticity theory, representing the axial OS banding explicitly via a spring-bead model. We calculate a bending stiffness of ∼10(5) nN⋅μm2, which is seven orders-of-magnitude larger than that of typical cilia and flagella. This bending stiffness has a quadratic relation to OS radius, so that thinner OS have lower fragility. Furthermore, we find that increasing the spatial frequency of axial OS banding decreases OS rigidity, reducing its fragility. Moreover, the model predicts a tendency for OS to break in bands with higher spring number density, analogous to the experimental observation that transgenic rods tended to break preferentially in bands of high fluorescence. We discuss how pathological alterations of disk membrane properties by mutant proteins may lead to increased OS rigidity and thus increased breakage, ultimately contributing to retinal degeneration.

Transport and localization of signaling proteins in ciliated cells

Most cells in the human body elaborate cilia which serve a wide variety of functions, including cell and tissue differentiation during development, sensing physical and chemical properties of the extracellular milieu and mechanical force generation. Common among cilia is the transduction of external stimuli into signals that regulate the activities of the cilia and the cells that possess them. These functions require the transport and localization of specialized proteins to the cilium, a process that many recent studies have shown to be vital for normal cell function and, ultimately, the health of the organism. Here we discuss several mechanisms proposed for the transport and localization of soluble and peripheral membrane proteins to, or their exclusion from the ciliary compartment with a focus on how the structure of the cytoplasm and the size and shape of proteins influence these processes. Additionally, we examine the impact of cell and protein structure on our ability to accurately measure the relative concentrations of fluorescently tagged proteins amongst various cellular domains, which is integral to our understanding of the molecular mechanisms underlying protein localization and transport.

Impact of signaling microcompartment geometry on GPCR dynamics in live retinal photoreceptors

G protein–coupled receptor (GPCR) cascades rely on membrane protein diffusion for signaling and are generally found in spatially constrained subcellular microcompartments. How the geometry of these microcompartments impacts cascade activities, however, is not understood, primarily because of the inability of current live cell–imaging technologies to resolve these small structures. Here, we examine the dynamics of the GPCR rhodopsin within discrete signaling microcompartments of live photoreceptors using a novel high resolution approach. Rhodopsin fused to green fluorescent protein variants, either enhanced green fluorescent protein (EGFP) or the photoactivatable PAGFP (Rho-E/PAGFP), was expressed transgenically in Xenopus laevis rod photoreceptors, and the geometries of light signaling microcompartments formed by lamellar disc membranes and their incisure clefts were resolved by confocal imaging. Multiphoton fluorescence relaxation after photoconversion experiments were then performed with a Ti–sapphire laser focused to the diffraction limit, which produced small sub–cubic micrometer volumes of photoconverted molecules within the discrete microcompartments. A model of molecular diffusion was developed that allows the geometry of the particular compartment being examined to be specified. This was used to interpret the experimental results. Using this unique approach, we showed that rhodopsin mobility across the disc surface was highly heterogeneous. The overall relaxation of Rho-PAGFP fluorescence photoactivated within a microcompartment was biphasic, with a fast phase lasting several seconds and a slow phase of variable duration that required up to several minutes to reach equilibrium. Local Rho-EGFP diffusion within defined compartments was monotonic, however, with an effective lateral diffusion coefficient D_lat = 0.130 ± 0.012 µm²s⁻¹. Comparison of rhodopsin-PAGFP relaxation time courses with model predictions revealed that microcompartment geometry alone may explain both fast local rhodopsin diffusion and its slow equilibration across the greater disc membrane. Our approach has for the first time allowed direct examination of GPCR dynamics within a live cell signaling microcompartment and a quantitative assessment of the impact of compartment geometry on GPCR activity.

Steric volume exclusion sets soluble protein concentrations in photoreceptor sensory cilia

Proteins segregate into discrete subcellular compartments via a variety of mechanisms, including motor protein transport, local binding, and diffusion barriers. This physical separation of cell functions serves, in part, as a mechanism for controlling compartment activity by allowing regulation of local protein concentrations. In this study we explored how soluble protein size impacts access to the confined space within the retinal photoreceptor outer segment signaling compartment and discovered a strikingly steep relationship. We find that GFP monomers, dimers, and trimers expressed transgenically in frog rods are present in the outer segment at 1.8-, 2.9-, and 6.8-fold lower abundances, relative to the cell body, than the small soluble fluorescent marker, calcein. Theoretical analysis, based on statistical-mechanical models of molecular access to polymer meshes, shows that these observations can be explained by the steric hindrance of molecules occupying the highly constrained spaces between outer segment disc membranes. This mechanism may answer a long-standing question of how the soluble regulatory protein, arrestin, is effectively excluded from the outer segments of dark-adapted rods and cones. Generally, our results suggest an alternate mode for the control of protein access to cell domains based on dynamic, size-dependent compartmental partitioning that does not require diffusion barriers, active transport, or large numbers of immobile binding sites.

Diffusion of a soluble protein, photoactivatable GFP, through a sensory cilium

Transport of proteins to and from cilia is crucial for normal cell function and survival, and interruption of transport has been implicated in degenerative and neoplastic diseases. It has been hypothesized that the ciliary axoneme and structures adjacent to and including the basal bodies of cilia impose selective barriers to the movement of proteins into and out of the cilium. To examine this hypothesis, using confocal and multiphoton microscopy we determined the mobility of the highly soluble photoactivatable green fluorescent protein (PAGFP) in the connecting cilium (CC) of live Xenopus retinal rod photoreceptors, and in the contiguous subcellular compartments bridged by the CC, the inner segment (IS) and the outer segment (OS). The estimated axial diffusion coefficients are D_CC = 2.8 ± 0.3, D_IS = 5.2 ± 0.6, and D_OS = 0.079 ± 0.009 µm² s⁻¹. The results establish that the CC does not pose a major barrier to protein diffusion within the rod cell. However, the results also reveal that axial diffusion in each of the rod’s compartments is substantially retarded relative to aqueous solution: the axial diffusion of PAGFP was retarded ∼18-, 32- and 1,000-fold in the IS, CC, and OS, respectively, with ∼20-fold of the reduction in the OS attributable to tortuosity imposed by the lamellar disc membranes. Previous investigation of PAGFP diffusion in passed, spherical Chinese hamster ovary cells yielded D_CHO = 20 µm² s⁻¹, and estimating cytoplasmic viscosity as D_aq/D_CHO = 4.5, the residual 3- to 10-fold reduction in PAGFP diffusion is ascribed to sub-optical resolution structures in the IS, CC, and OS compartments.

Three-dimensional architecture of murine rod outer segments determined by cryoelectron tomography

The rod outer segment (ROS) of photoreceptor cells houses all components necessary for phototransduction, a set of biochemical reactions that amplify and propagate a light signal. Theoretical approaches to quantify this process require precise information about the physical boundaries of the ROS. Dimensions of internal structures within the ROS of mammalian species have yet to be determined with the precision required for quantitative considerations. Cryoelectron tomography was utilized to obtain reliable three-dimensional morphological information about this important structure from murine retina. Vitrification of samples permitted imaging of the ROS in a minimally perturbed manner and the preservation of substructures. Tomograms revealed the characteristic highly organized arrangement of disc membranes stacked on top of one another with a surrounding plasma membrane. Distances among the various membrane components of the ROS were measured to define the space available for phototransduction to occur. Reconstruction of segments of the ROS from single-axis tilt series images provided a glimpse into the three-dimensional architecture of this highly differentiated neuron. The reconstructions revealed spacers that likely maintain the proper distance between adjacent discs and between discs and the plasma membrane. Spacers were found distributed throughout the discs, including regions that are distant from the rim region of discs.

Diffusion coefficient of the cyclic GMP analog 8-(fluoresceinyl)thioguanosine 3',5' cyclic monophosphate in the salamander rod outer segment

Cyclic GMP (cGMP) is the intracellular messenger mediating phototransduction in retinal rods, with its longitudinal diffusion in the rod outer segment (ROS) likely to be a factor in determining light sensitivity. From the kinetics of cGMP-activated currents in the truncated ROS of the salamander (Ambystoma tigrinum), the cGMP diffusion coefficient was previously estimated to be approximately 60 x 10(-8) cm2 s-1. On the other hand, fluorescence measurements in intact salamander ROS using 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate (Fl-cGMP) led to a diffusion coefficient for this compound of 1 x 10(-8) cm2 s-1; after corrections for differences in size and in binding to cellular components between cGMP and Fl-cGMP, this gave an upper limit of 11 x 10(-8) cm2 s-1 for the cGMP diffusion coefficient. To properly compare the two sets of measurements, we have examined the diffusion of Fl-cGMP in the truncated ROS. From the kinetics of Fl-cGMP-activated currents, we have obtained a diffusion coefficient of 3 x 10(-8) cm2 s-1 for this analog; the cGMP diffusion coefficient measured from the same truncated ROSs was approximately 80 x 10(-8) cm2 s-1. Thus, a factor of 27 appears appropriate for correcting differences in size and intracellular binding between cGMP and Fl-cGMP. Application of this correction factor to the Fl-cGMP diffusion coefficient measurements by Olson and Pugh (1993) gives a cGMP diffusion coefficient of approximately 30 x 10(-8) cm2 s-1, in reasonable agreement with the value measured from the truncated ROS.

Cyclic GMP diffusion coefficient in rod photoreceptor outer segments

Cyclic GMP (cGMP) is the intracellular messenger that mediates phototransduction in retinal rods. As photoisomerizations of rhodopsin molecules are local events, the longitudinal diffusion of cGMP in the rod outer segment should be a contributing factor to the response of the cell to light. We have employed the truncated rod outer segment preparation from bullfrog (Rana catesbeiana) and tiger salamander (Ambystoma tigrinum) to measure the cGMP diffusion coefficient. In this preparation, the distal portion of a rod outer segment was drawn into a suction pipette for measuring membrane current, and the rest of the cell was then sheared off with a glass probe, allowing bath cGMP to diffuse into the outer segment and activate the cGMP-gated channels on the surface membrane. Addition and removal of bath cGMP were fast enough to produce effectively step changes in cGMP concentration at the open end of the outer segment. When cGMP hydrolysis is inhibited by isobutylmethylxanthine (IBMX), the equation for the diffusion of cGMP inside the truncated rod outer segment has a simple analytical solution, which we have used to analyze the rise and decay kinetics of the cGMP-elicited currents. From these measurements we have obtained a cGMP diffusion coefficient of approximately 70 x 10(-8) cm2 s-1 for bullfrog rods and approximately 60 x 10(-8) cm2 s-1 for tiger salamander rods. These values are six to seven times lower than the expected value in aqueous solution. The estimated diffusion coefficient is the same at high (20-1000 microM) and low (5-10 microM) concentrations of cGMP, suggesting no significant effect from buffering over these concentration ranges.

Free calcium concentrations in bullfrog rods determined in the presence of multiple forms of Fura-2

We employed the fluorescent calcium indicator Fura-2, loaded into intact retinas of the bullfrog Rana catesbeiana, to measure free calcium concentrations in the rod outer-segment cytosol. We determined that traditional methods of calculation yielded erroneous values of calcium. This error results from the presence of at least two distinct pools of Fura-2 in rod outer segments. Application of manganese quenches each pool, but quenching occurs at different rates. Using this fact, we show that the pools can be isolated by brief exposure to manganese and examined separately. One of these pools has the same fluorescent properties as the free salt of Fura-2 we use in our in vitro calibrations. The other source of fluorescence has more unusual properties. Although insensitive to calcium concentrations in the physiological range, it contributes significant anomalous fluorescence when cytosolic free calcium concentrations are elevated by application of IBMX. Nevertheless, the experimentally isolated, classic pool of Fura-2 is well behaved and allows us to calculate calcium concentrations relative to the Kd of Fura-2 by the usual ratio method. We show that when rods are exposed to saturating light, the free calcium concentration in their outer segments falls to a level not significantly different from zero within 20-30 s.

Diffusion coefficient of cyclic GMP in salamander rod outer segments estimated with two fluorescent probes

Experiments have demonstrated that single photoisomerizations in amphibian and primate rods can cause the suppression of 3-5% of the dark circulating current at the response peak (Baylor, D. A., T. D. Lamb, and K. W. Yau. 1979. J. Physiol. (Lond.). 288:613-634; Baylor, D. A., B. J. Nunn, and J. L. Schnapf. 1984. J. Physiol. (Lond.). 357:575-607). These results indicate that the change in [cGMP] effected by a single isomerization must spread longitudinally over at least the corresponding fractional length of the outer segment. The effective longitudinal diffusion coefficient, Dx, of cGMP is thus an important determinant of rod sensitivity. We report here measurements of the effective longitudinal diffusion coefficients, Dx, of two fluorescently labeled molecules: 5/6-carboxyfluorescein and 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate, introduced into detached outer segments via whole-cell patch electrodes. For these compounds, the average time for equilibration of the entire outer segment with the patch pipette was approximately 6 min. Fluorescence images of rods were analyzed with a one-dimensional diffusion model that included limitations on transfer between the electrode and outer segment and the effects of intracellular binding of the dyes. The analyses yielded estimates of Dx of 1.9 and 1.0 microns 2.s-1 for the two dyes. It is shown that these results place an upper limit on Dx for cGMP of 11 microns2.s-1. The actual value of Dx for cGMP in the rod will depend on the degree of intracellular binding of cGMP. Estimates of the effective buffering power for cGMP in the rod at rest range from two to six (Lamb and Pugh, 1992; Cote and Brunnock, 1993). When combined with these estimates, our results predict that for cGMP itself, Dx falls within the range of 1.4-5.5 microns 2.s-1.

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)

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.

Neutral lipids of frog and rat rod outer segments

Neutral lipids of rod outer segments (ROS) from frog and rat comprise roughly 10 mol% of the total lipids and consist of free sterol, free fatty acids, and 1,2-diglycerides, but contain no detectable sterol esters or triglycerides. Expressed as nmol per mg ROS protein, the levels of both free fatty acids and free sterol are higher in the rat than in the frog, whereas the diglycerides in frogs are about four times higher than in rats. The level of 22:6 omega 3 in the free fatty acids fraction of the frog is twice that of the rat, although the level of 22:6 omega 3 in both the frog and rat free fatty acids is lower than in any of the glycerolipids. The diglyceride fraction from both animals consists almost entirely of two molecular species: in the frog, the C-38 and C-40 type predominate in a molar distribution of 52 and 42% respectively, whereas in the rat, the C-36 and C-38 types are most abundant in a molar distribution of 28 and 60%, respectively. Comparison of the diglyceride fatty acid and molecular species compositions with those of phosphatidylinositol and phosphatidic acid led to the suggestion that ROS diglycerides are derived from phosphatidylinositol.

Imaging of outer segment periodicities in unstained cryoultramicrotomy sections of the frog retina

Frog retinas were examined following cryoultramicrotomy. The extent of glutaraldehyde cross-linking ranged from maximally 2% for 2 h to no cross-linking. When the tissue was cryoprotected, the effects of hypotonic, isotonic and hypertonic media on outer segment banding periodicities were studied. The specimen was cryosectioned at 193 K to 113 K with sections collected on a dry knife (213 K to 123 K). The sections were freeze-dried and only rehydrated with phosphotungstic acid section stain when transmission electron microscopy was used to establish the extent of gross retinal morphology for each fixation protocol employed. As the extreme of no cross-linking and no cryoprotection was approached and reached, conventional sections which displayed extensive retinal morphology were not obtained. Instead, fragments of retinal tissue resulted which showed periodicities of alternating light and dark bands. These periodicities are interpreted as photoreceptor outer segment discs with contrast formation depending upon an interplay between three factors: mass density differences between disc and interdisc space, section cutting angle, and section thickness.