Structural features of the terminal loop region of frog retinal rod outer segment disk membranes: I. Organization of lipid components

A simplified mass-transfer model for visual pigments in amphibian retinal-cone outer segments

When radiolabeled precursors and autoradiography are used to investigate turnover of protein components in photoreceptive cone outer segments (COSs), the labeled components--primarily visual pigment molecules (opsins)--are diffusely distributed along the COS. To further assess this COS labeling pattern, we derive a simplified mass-transfer model for quantifying the contributions of advective and diffusive mechanisms to the distribution of opsins within COSs of the frog retina. Two opsin-containing regions of the COS are evaluated: the core axial array of disks and the plasmalemma. Numerical solutions of the mass-transfer model indicate three distinct stages of system evolution. In the first stage, plasmalemma diffusion is dominant. In the second stage, the plasmalemma density reaches a metastable state and transfer between the plasmalemma and disk region occurs, which is followed by an increase in density that is qualitatively similar for both regions. The final stage consists of both regions slowly evolving to the steady-state solution. Our results indicate that autoradiographic and cognate approaches for tracking labeled opsins in the COS cannot be effective methodologies for assessing new disk formation at the base of the COS.

Differential requirements for retinal degeneration slow intermolecular disulfide-linked oligomerization in rods versus cones

It is commonly assumed that the ultrastructural organization of the rim region of outer segment (OS) discs in rods and lamellae in cones requires functional retinal degeneration slow/rod outer segment membrane protein 1 (Rds/Rom-1) complexes. Cysteine-150 (C150) in Rds has been implicated in intermolecular disulfide bonding essential for functional Rds complexes. Transgenic mice containing the Rds C150S mutation (C150S-Rds) failed to form higher-order Rds oligomers, although interactions between C150S-Rds and Rom-1 occurred in rods, but not in cones. C150S-Rds mice exhibited marked early-onset reductions in cone function and abnormal OS structure. In contrast, C150S-Rds expression in rods partly rescued the rds(+/-) phenotype. Although C150S-Rds was detected in the OSs in rods and cones, a substantial percentage of C150S-Rds and cone opsins were mislocalized to different cellular compartments in cones. The results of this study provide novel insights into the importance of C150 in Rds oligomerization and the differences in Rds requirements in rods versus cones. The apparent OS structural differences between rods and cones may cause cones to be more susceptible to the elimination of higher-order Rds/Rom-1 oligomers (e.g. as mediated by mutation of the Rds C150 residue).

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.

Role of Peripherin/rds in Vertebrate Photoreceptor Architecture and Inherited Retinal Degenerations

The vertebrate photoreceptor outer segment (OS) is a highly structured and dynamic organelle specialized to transduce light signals. The elaborate membranous architecture of the OS requires peripherin/rds (P/rds), an integral membrane protein and tetraspanin protein family member. Gene-level defects in P/rds cause a broad variety of late-onset progressive retinal degenerations in humans and dysmorphic photoreceptors in murine and Xenopus models. Although proposed to fulfill numerous roles related to OS structural stability and renewal, P/rds molecular function remains uncertain. An increasingly resolved model of this protein's oligomeric structure can account for disease inheritance patterns and severity in some instances. Nonetheless, the pathogenic mechanisms underlying the uniquely broad spectrum of retinal diseases associated with P/rds defects are not currently well understood. Recent findings point to the possibility that P/rds acts as a multifunctional scaffolding protein for OS architecture and that partial-loss-of-function mutations contribute to the hallmark phenotypic heterogeneity associated with inherited defects in RDS.

Downregulation of a unique photoreceptor protein correlates with improper outer segment assembly

A unique photoreceptor protein has been characterized. This protein, termed XAP-1 antigen, is expressed by photoreceptors exclusively under conditions in which the outer segment membranes are properly assembled. When the retinal pigment epithelium is adherent to the underlying neural retina, the XAP-1 antigen is localized to the plasma membrane that surrounds the inner and outer segments in the areas juxtaposed to the subretinal space. A similar labeling pattern is detected in retinal pigment epithelium-deprived retinas in which assembly of nascent outer segments is supported by lactose. In retinas that undergo degeneration subsequent to the removal of the retinal pigment epithelium, the expression of this protein is completely downregulated. Immunohistochemical analyses and subcellular fractionation along with Western blot analysis, indicate that the XAP-1 antigen is a membrane-associated soluble protein. Mass spectrometric analysis indicates that the XAP-1 antigen shares homology via 12 tryptic peptide masses with the gamma-crystallin (lens structural protein) subclasses, although it does not immunolocalize to the same ocular structures as reported for the gamma-crystallins. We propose that XAP-1 antigen is a unique protein that is expressed extensively by healthy photoreceptor cells; the expression of the XAP-1 antigen exclusively by photoreceptors with organized outer segments suggests that this protein may play a critical role in outer segment assembly.

Analysis of the rds/peripherin.rom1 complex in transgenic photoreceptors that express a chimeric protein

Mice homozygous for the retinal degeneration slow (rds) mutation completely lack photoreceptor outer segments. The rds gene encodes rds/peripherin (rds), a membrane glycoprotein in the rims of rod and cone outer segment discs. rds is present as a complex with the related protein, rom1. Here, we generated transgenic mice that express a chimeric protein (rom/D2) containing the intradiscal D2 loop of rds in the context of rom1. rom/D2 was N-glycosylated, formed covalent homodimers, and interacted non-covalently with itself, rds, and rom1. The rds.rom/D2 interaction was significantly more stable than the non-covalent interaction between rds and rom1 by detergent/urea titration. Analysis of mice expressing rom/D2 revealed that rds is 2.5-fold more abundant than rom1, interacts non-covalently with itself and rom1 via the D2 loop, and forms a high order complex that may extend the entire circumference of the disc. Expression of rom/D2 fully rescued the ultrastructural phenotype in rds+/- mutant mice, but it had no effect on the phenotype in rds-/- mutants. Together, these observations explain the striking differences in null phenotypes and frequencies of disease-causing mutations between the RDS and ROM1 genes.

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)

Cloning of the cDNA for a novel photoreceptor membrane protein (rom-1) identifies a disk rim protein family implicated in human retinopathies

The molecules essential to the continual morphogenesis and shedding of the opsin-containing disks of vertebrate photoreceptors are largely unknown. We describe a 37 kd protein, rom-1, which is 35% identical and structurally similar to peripherin/retinal degeneration slow (rds). Like peripherin, rom-1 is a retina-specific integral membrane protein localized to the photoreceptor disk rim. The two proteins are similarly oriented in the membrane, and each has a highly conserved (15/16 residues) cysteine- and proline-rich domain in the disk lumen. Although both rom-1 and peripherin form disulfide-linked dimers, they do not form heterodimers with each other, but appear to associate noncovalently. These results suggest both that rom-1 and peripherin are functionally related members of a new photoreceptor-specific protein family and that rom-1, like peripherin, is likely to be important to outer segment morphogenesis. The association of mutations in RDS with retinitis pigmentosa indicates that ROM1 is a strong candidate gene for human retinopathies.

Modulation of disk margin structure during renewal of cone outer segments in the vertebrate retina

In the process of disk renewal in retinal cone outer segments (COSs), apical displacement of disks must be coupled to systematic reductions in disk area and perimeter in order to retain overall conical geometry. We have quantified these changes in disk area and perimeter segment lengths by morphometric analyses of cross sections of fully formed disks taken from basal to apical ends of COSs. Specifically excluded from these analyses are data arising from partial or incomplete disks within the COS, which do not conform to the conical geometry and which constitute a minor fraction of the COS disk population. Thus, our results address the long-range pattern of structural changes affecting the major population of disks along the length of the COS. Our data indicate that decreases in total disk margin length associated with apical displacement of fully formed disks are due to decreases in the length of the margin opposite the cilium, i.e., the open margin segment. In contrast, the average length of the closed margin segment remains constant or increases slightly in the apical direction. The open margins of frog COS disks have recently been shown to possess a distinctive lattice of membrane-associated components (Fetter and Corless: Invest. Ophthalmol. Vis. Sci. 28:646-657, '87). We have also examined COSs by the freeze-fracture, deep-etch technique for evidence of a mechanism whereby measured changes in open margin length may be accommodated while maintaining the overall organization of the open margin segments. In regions of membrane continuity between open margins and the COS plasma membrane, we have observed elevated ridges on the plasma membrane that 1) tend to lie parallel to the open margin segments, 2) have a similar axial spacing, 3) occasionally demonstrate interconnecting filaments similar to those of the open margin lattice, and 4) appear to have a particulate substructure. The mechanism proposed for reducing open margin length involves tangential displacement of the lateral edges of the open margin lattice to the adjacent plasma membrane. These shifted lattice domains initially give rise to the plasmalemmal ridges, which subsequently disassemble, and whose components become redistributed in the COS plasma membrane. These structural features of COS open margins suggest several revisions of our earlier model of disk morphogenesis (Corless and Fetter: J. Comp. Neurol. 257:24-38, '87), which was based on the margin structure of ROS disks alone. Eckmiller (J. Cell Biol. 105:2267-2277, '87) has recently proposed that partial-disks observed within the COS represent sites of new disk formation.(ABSTRACT TRUNCATED AT 400 WORDS)

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).

Structural features of the terminal loop region of frog retinal rod outer segment disk membranes: III. Implications of the terminal loop complex for disk morphogenesis, membrane fusion, and cell surface interactions

The perimeter of rod outer segment (ROS) disks displays a two-dimensional lattice of components referred to as the terminal loop complex (Corless, Fetter, Zampighi, Costello, and Wall-Buford: J. Comp. Neurol. 257:9-23, '87b). We take the view that this pattern of structural organization reflects the mechanism(s) whereby the disk perimeter is defined and constructed. Herein we develop and partially evaluate a generalized template mechanism of disk perimeter development, to account for the structure and the axial alignment of both marginal and incisural domains. Components of the terminal loop complex are conceived as the morphogens that determine the location and guide the differentiation of the disk perimeter. Briefly, we postulate that transmembranous components of the terminal loop complex are present within the reflection of plasmalemma that forms the base of the rod outer segment. These components interact with the cytoplasmic template provided by the perimeter lattice present along the lower surface of the most basal disk, thereby propagating the lattice and creating an extracellular template. The latter is then available to interact with corresponding elements distributed within the apical surface of the adjacent disk precursor evagination. The progressive interaction and alignment of these extracellular domains form the scaffolding that guides the restructuring of the plasmalemma, to form the mature disk margin topology. Successive repetitions of this process are seen to produce an axial stacking of disks whose perimeters are aligned and ensheathed by a two-dimensional net.