The rhodopsin cycle in the developing vertebrate retina

Phototransduction in mouse rods and cones

Phototransduction is the process by which light triggers an electrical signal in a photoreceptor cell. Image-forming vision in vertebrates is mediated by two types of photoreceptors: the rods and the cones. In this review, we provide a summary of the success in which the mouse has served as a vertebrate model for studying rod phototransduction, with respect to both the activation and termination steps. Cones are still not as well-understood as rods partly because it is difficult to work with mouse cones due to their scarcity and fragility. The situation may change, however.

The persistence of cone photoreceptors within the dorsal retina of aged retinally degenerate mice (rd/rd): implications for circadian organization

Rod- and cone-opsin specific antibodies were used in an attempt to immunolabel remaining photoreceptor cells in the mutant rd (retinal degeneration) mouse retina. We identified a region-specific distribution in the pattern of photoreceptor degeneration, with the dorsal retina showing markedly less photoreceptor degeneration than the ventral retina. All rod and cone immunoreactive cells disappeared in the ventral retina by 100-120 days of age. By contrast, both cone and a small number of rod immunopositive cells were identified in the dorsal retina at this time. By 200 days all rod immunoreactive cells had disappeared. At 360 days numerous cone immunoreactive cells remained within a restricted region of the dorsal retina. As rd mice show unattenuated circadian responses to light, these remaining photoreceptor cells within the dorsal retina become candidates for the regulation of circadian physiology by light.

Synthesis and secretion of interphotoreceptor retinoid-binding protein (IRBP) and developmental expression of IRBP mRNA in normal and rd mouse retinas

The synthesis and secretion of interphotoreceptor retinoid-binding protein (IRBP) was quantitatively assessed in retinas of normal and rd mutant mice using short-term organ culture with [35S]methionine. Retinas were studied at ages P9-P12, time points prior to and immediately after the onset of the degeneration of the rd retina. Soluble proteins of the retinal pellet and the incubation medium were subjected to SDS-polyacrylamide gel electrophoresis. Analysis of labeled protein bands utilized a radioactivity scanning system to quantify [35S]methionine incorporation into newly synthesized IRBP. The synthesis and secretion into the incubation medium of IRBP by rd mouse retinas was comparable to normal retinas at P9-P10 but decreased by more than 50% by P12. IRBP mRNA levels were evaluated in retinas of normal and rd mice ages P7-P14. Although IRBP mRNA expression increased in the rd mouse through P10, it decreased markedly thereafter. Previously reported immunocytochemical studies suggested that IRBP was not secreted in the rd mouse retina. The results of this study indicate, however, that rd mouse retinas, when removed from the eye, have the capacity to synthesize and secrete IRBP.

Acylation and glycosylation of rhodopsin in the rd mouse

Retinas of 9-10-day-old rd and control mice were incubated for 2 hr with [14C]leucine along with either tritiated palmitic acid or galactose to investigate the acylation or glycosylation, respectively, of rhodopsin. Although other laboratories have reported that phosphorylation of rhodopsin is not detectable in rd retinas, the two post-translational modifications of rhodopsin investigated in the present work are detectable. The rod outer segments (ROS) were separated from the retinal debris containing the rough endoplasmic reticulum (RER) of photoreceptor cells by vortexing and then by linear sucrose gradients. The rhodopsin from the RER was purified by affinity chromatography and gel electrophoresis. In the acylation studies, the mean ratio of palmitate to leucine in the rd mouse was nearly twice that of controls (11.73 +/- 2.84 v. 6.81 +/- 1.04). Possible explanations for the disparity between the two groups could include: (1) a diminished internal pool size of the fatty acid; or (2) acylation of amino acids such as serine or threonine which normally are not acylated in rhodopsin. Treatment of purified rhodopsin with 1 M hydroxylamine released similar amounts of palmitate from the rd mice and controls. Hence, the higher ratio of palmitate to leucine in rd mice is apparently due to a diminished internal pool size. In the glycosylation studies, the ratio of galactose to leucine was very similar between rd mice and controls, 1.7 +/- 0.43 v. 2.47 +/- 0.74. Protein content and specific activity were determined for the crude ROS preparations and for the remaining retinal debris. Although the amount of ROS protein differed significantly between the two groups, the specific activities did not.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and distribution of opsin-like immunoreactivity in the dystrophic retinas of rdle mutant mice

The opsin-like immunoreactivity in the retinas of C57BL/6J rdle mutant mice has been studied by light-microscopic immunocytochemistry. Positively labeled cells were found in the normal heterozygous and homozygous mutant mouse outer nuclear layer (ONL) as early as postnatal day 3 (PN3). Beginning at PN10, in the retinas of the homozygous mutant mice, labeled photoreceptor cells rapidly decreased in number and disappeared after PN42. In the decreased ONL, remaining opsin-positive cells were labeled at higher density than those of controls. The retinal pigment epithelium was also moderately labeled during the loss of opsin-positive photoreceptor cells. In addition, sparse opsin-immunoreactive cells were demonstrated in the inner nuclear layer (INL) in the retinas of both the mutant and non-dystrophic mice as early as PN10 and are presumed to be ectopic photoreceptor cells. However, these displaced photoreceptor cells disappeared by PN28 in mutants along the same time course as those in the ONL but were still present in the PN28 retina of controls and seemed to be more abundant at later adult ages. There was no difference in the developmental regulation of opsin in the heterozygous and normal controls.

Opsin distribution and synthesis in degenerating photoreceptors of rd mutant mice

The distribution of opsin and the capacity of photoreceptors to synthesize opsin was studied in retinas of mice bearing the rd (retinal degeneration) mutation and compared to control normal mice. Opsin was localized by means of pre-embedding and post-embedding immunocytochemistry and electron microscopy. Cones were identified with anti-cone antibodies and peanut agglutinin lectin which labels cone matrix sheaths. Opsin synthesis was measured by incorporation of [35S]methionine into opsin which was detected by immunoblots. Immunocytochemistry revealed that degeneration of rod outer segments was accompanied by accumulation of opsin in the plasma membrane enveloping the inner segment, nuclei and synaptic terminals. Rod photoreceptors degenerated faster than cones. By post-natal day 19 (P19), 87% of the remaining inner segments were cones. Opsin synthesis in rd mice could no longer be measured after P15. However, opsin molecules could be detected both by immunocytochemistry and immunoblotting up to P30. Between P20 and P30 all detectable opsin was localized in the plasma membrane which envelopes nuclei and synaptic terminals. Unlabeled surviving nuclei after P30 are probably cones. The opsin which is detected in rod inner segment plasma membranes might be derived, by back diffusion, from degenerating outer segments and represent molecules synthesized at an earlier time. Alternatively opsin accumulation might be a result of low levels of opsin synthesis, undetectable by [35S]methionine incorporation, which continues in the absence of outer segments.

A developmental study of interphotoreceptor retinoid-binding protein (IRBP) in single and double homozygous rd and rds mutant mouse retinae

Interphotoreceptor retinoid-binding protein (IRBP) was studied using immunochemical and immunocytochemical techniques in retinae of mice with allelic combinations at the rd and rds loci at different stages of development and degeneration. Until postnatal day 7 (P7), IRBP is located intracellularly in developing retinae of the different genotypes. Thereafter, IRBP is present mainly in the interphotoreceptor matrix. As previously noted, cell death is slowest in the heterozygous +/+,rds/+ mutant with loss increasing in order in +/+,rds/rds, rd/rd, rds/rds and rd/rd,+/+ animals. The IRBP content of the total retina also approximates this pattern, with lowest amounts by far in rd/rd, rds/rds and rd/rd,+/+ mutants (after P14). Interestingly though, IRBP loss significantly precedes visual cell loss in the rd/rd,rds/rds retina. In all the mutants, the remaining rod cells in the outer nuclear layer exhibit synthesis of intracellularly located IRBP at late stages of degeneration. In the single homozygous rd/rd,+/+ and the double homozygous rd/rd,rds/rds mutants, IRBP is present intracellularly during the entire degenerative process with somewhat less intracellular IRBP in the rd/rd,rds/rds mutant. Retinae of homozygous +/+,rds/rds and heterozygous +/+,rds/+ animals exhibit a normal distribution pattern of IRBP immunoreactivity until loss of photoreceptor cells becomes pronounced at later stages of the disease. Many of the remaining cells at this time are probably cone elements although they are structurally changed. Double labeling with IRBP and S-antigen demonstrates, in many but not all, the presence of both proteins in the same cell body. Immunocytochemistry clearly demonstrated the presence of IRBP in remaining photoreceptor cells at late stages of the disease. Thus, the biochemically measured loss of IRBP appears to be a complex process neither directly dependent on the loss of photoreceptor outer segments and reduced interphotoreceptor matrix space (e.g. there is a sustained IRBP level in rodless rds mutants) nor simply due to cell death (e.g. in the rd/rd,rds/rds mutant, IRBP loss significantly precedes cell loss). That this IRBP is mainly intracellular, however, may indicate an abnormality in secretion which, combined with other factors, induces a degenerated and less differentiated phenotype.

Rhodopsin, 11-cis vitamin A, and interstitial retinol-binding protein (IRBP) during retinal development in normal and rd mutant mice

Biochemical and immunological techniques were used to determine the emergence of interstitial retinol binding protein (IRBP), rhodopsin, and stored retinyl esters (all-trans and 11-cis) during retinal development in normal and rd mice. IRBP could be demonstrated at embryonic Day 17 (E17), corresponding to an early stage of inner segment development. Although all-trans retinyl esters were present earlier, 11-cis retinyl esters did not appear until postnatal Days 6-7 (P6-P7), corresponding to rod outer segment (ROS) disc formation. Rhodopsin was detected at the same developmental stage. The proportion of 11-cis retinyl esters reached a maximum of 40-50% at P15-P20. Thereafter, the proportion dropped, due to more rapid accumulation of the all-trans isomer. Rhodopsin and IRBP increased in parallel with ROS elongation up to P25, when the ROS had reached their mature lengths. The increases then continued up to P40-P50. In rd (retinal degeneration) mice, IRBP and rhodopsin were identical with the controls until P12, but then dropped as the photoreceptors degenerated. Synthesis and secretion of IRBP in vitro was less than 10% of the controls in rd retinas at P26, when only 4-5% of the photoreceptors survived. The quantities of retinyl esters (mainly stearate and palmitate in the ratio of 6:1, respectively) stored in dark-adapted mouse eyes progressively increased as the animals aged, representing 0.5 mole eq. of the rhodopsin at 8 months. Although retinyl esters (11-cis and all-trans) also accumulated in rd mouse eyes up to P12, little further increase occurred. At P93, the retinyl esters (0.01 nmole X eye-1) were only 4% of the controls at P91. A peak in the proportion of 11-cis isomer occurred at P10-P20, but it averaged only 15% of the total ester and declined to 5% at P93. These findings support the hypothesis that IRBP is synthesized by the rods and cones, and suggest that its synthesis and secretion are initiated when the photoreceptor inner segments start to differentiate. 11-cis Retinoids and rhodopsin do not appear until the outer segments start to form. It is suggested that in the rd mouse the absence of photoreceptors, perhaps coupled with lack of normal interphotoreceptor matrix, leads to a loss in the ability of the pigment epithelium to store retinyl esters.

Immunoassay of rod visual pigment (opsin) in the eyes of rds mutant mice lacking receptor outer segments

In 020/A mice, homozygous for the retinal degeneration slow (rds) gene, the photoreceptor cells fail to develop outer segments, and in the absorption spectra of retinal extracts the rhodopsin peak is lacking. Application of an enzyme-linked immunoassay using antisera against bovine opsin shows, however, that opsin is present in the homozygous mutant retina (0.010 nmol/eye) at 3% of the level of the normal retina (0.38 nmol/eye) of Balb/c mice. In the retina of heterozygous mice the opsin level (0.19 nmol/eye) is about half of the normal. Detection of opsin in the rds mutant retina demonstrates the functional basis for the reported electroretinographic response and light-mediated reduction in cyclic nucleotide levels in this mutant.

Development and degeneration of retina in rds mutant mice: the electroretinogram

In mice, homozygous for the retinal degeneration slow (rds) gene, the photoreceptor cells lack outer segment disc structures, contain low amounts of rhodopsin, and degenerate slowly, while the inner retinal layers remain intact. The electroretinogram (ERG) from 1-month-old mutant mice shows a lower than normal amplitude but the time-to-peak is normal. The ERGs from mutant mice of 2-3 months and 6-7 months of age show a further decline in response amplitude as the receptor cell population is depleted with progress of degeneration while the time-to-peak increases. The ERG is absent in 1-year-old mutant mice which have lost their receptor cells completely. The possible morphological correlations of the ERG and its components in the rds mutant mice are discussed.

Immunocytochemical localization of opsin in the cell membrane of developing rat retinal photoreceptors

Mature retinal rod photoreceptors sequester opsin in the disk and plasma membranes of the rod outer segment (ROS). Opsin is synthesized in the inner segment and is transferred to the outer segment along the connecting cilium that joins the two compartments. We have investigated early stages of retinal development during which the polarized distribution of opsin is established in the rod photoreceptor cell. Retinas were isolated from newborn rats, 3-21 d old, and incubated with affinity purified biotinyl-sheep anti-bovine opsin followed by avidin-ferritin. At early postnatal ages prior to the development of the ROS, opsin is labeled by antiopsin on the inner segment plasma membrane. At the fifth postnatal day, as ROS formation begins opsin was detected on the connecting cilium plasma membrane. However, the labeling density of the ciliary plasma membrane was not uniform: the proximal cilium was relatively unlabeled in comparison with the distal cilium and the ROS plasma membrane. In nearly mature rat retinas, opsin was no longer detected on the inner segment plasma membrane. A similar polarized distribution of opsin was also observed in adult human rod photoreceptor cells labeled with the same antibodies. These results suggest that some component(s) of the connecting cilium and its plasma membrane may participate in establishing and maintaining the polarized distribution of opsin.

Absence of receptor outer segments in the retina of rds mutant mice

In mice homozygous for the newly reported rds (retinal degeneration slow) gene, a progressive loss of visual cells occurs throughout life. Histogenesis of the retina proceeds normally with the exception of the receptor layer which remains rudimentary. Electron microscopic observations of the retina at different stages of development have revealed that the outer segments of the visual cells in this mutant fail to develop and that the receptor layer consists of inner segments only. In the absorption spectra of the retinal extract from the mutant the characteristic rhodopsin peak is lacking.

Studies of visual function and its decay in mice with hereditary retinal degeneration

Functional implications of mouse hereditary retinal degeneration have been studied at the level of the superior colliculus and visual cortex in the C57BL/6J-le rd strain. On autoradiography at a light-microscopic level, following eye injection with radioactive compounds, central visual structures appeared normal. A slight reduction in ipsilateral retinal projection was probably related to reduced retinal pigmentation associated with the light ear (le) mutation. In recordings from visual cortex and tectum in rd mice older than five months the cells discharged with highly rhythmic maintained activity. This ongoing activity depended on retinal input, since temporary asphyxia of the eye stopped it immediately. The frequency of the rhythm was influenced by the anesthesia. In these older mice no visual receptive fields could be mapped, but in a few tectal recordings it was possible to suppress the maintained activity by diffuse, very intense illumination. As in normal mice, no auditory or somatosensory responses were observed in the visual cortex or upper tectal layers. In recordings from tectum before the age of three weeks retinotopic topography and receptive fields were normal. By day 24 no receptive fields could be recorded from parts of the tectum representing the central 90--100 degrees of the visual field, whereas within a peripheral ring responses were still roughly normal under photopic conditions. Over the following four months these peripheral responses faded away slowly. Incremental thresholds, especially in the scotopic range, were elevated, rising slowly to unmeasurable values. Similarly during dark adaptation the thresholds fell to values several log units above those reached in normal mice; these values of dark adapted thresholds in rd mice rose with age. This is consistent with morphological changes known to occur in the retina as a consequence, of the rd mutation the rods degenerating before the cones.

In utero development and maturation of the retina of a non-primate mammal: a light and electron microscopic study of the guinea pig

A light and electron microscopic examination of retinogenesis in the fetal guinea pig has revealed an early development of synapses and photoreceptor cells. Differentiation of the neural retina begins around day 23 of gestation. By 34 days the retina reaches its maximum thickness. It differentiates an inner plexiform layer in which vesicle-containing processes and primitive synapses are evident. Synaptic ribbons are found in processes of this layer by 43-45 days of gestation. An outer plexiform layer develops within the neuroblast layer at 40 days of gestation; from its first appearance the outer plexiform layer contains synapses complete with synaptic ribbons. Receptor terminals of the alpha, paranuclear and beta type are present well before birth. Photoreceptor cells form inner segments by 40 days; the formation of outer segments is indicated by 45 days but not widespread until 49 days. The retina appears mature by day 51-57. It is clear that the primate is not unique in the early differentiation of its retinal synapses relative to the time of maturation of its photoreceptor cells. The potential functional capacities of precocious retinae, and the mechanisms of synapse development are discussed.

Kinetics of rod outer segment renewal in the developing mouse retina

The kinetics of rod outer segment renewal in the developing retina have been investigated in C57BL/6J mice. Litters of mice were injected with [(3)H]amino acids at various ages and killed at progressively later time intervals. Plastic 1.5 microm sections of retina were studied by light microscope autoradiography. The rate of outer segment disk synthesis, as judged by labeled disk displacement away from the site of synthesis, is slightly greater than the adult level at 11-13 days of age; it rises to more than 1.6 times the adult rate between days 13 and 17, after which it falls to the adult level at 21-25 days. The rate of disk disposal, as measured by labeled disk movement toward the site of disposal, is less than 15% of the adult level at 11-13 days of age; it rises sharply to almost 70% of the adult level by days 13-15 and then more gradually approaches the adult rate. The net difference in rates of synthesis and disposal accounts for the rapid elongation of rod outer segments in the mouse between days 11 and 17 and the subsequent, more gradual elongation to the adult equilibrium length reached between days 19 and 25. The changing rate of outer segment disk synthesis characterizes the late stages of cytodifferentiation of the rod photoreceptor cells.