The proliferative and apoptotic activities of E2F1 in the mouse retina

The E2F1 transcription factor controls cell proliferation and apoptosis. E2F1 activity is negatively regulated by the retinoblastoma (RB) protein. To study how inactivation of Rb and dysregulated E2F1 affects the developing retina, we analysed wild-type and Rb(-/-) embryonic retinas and retinal transplants and we established transgenic mice expressing human E2F1 in retinal photoreceptor cells under the regulation of the IRBP promoter (TgIRBPE2F1). A marked increase in cell proliferation and apoptosis was observed in the retinas of Rb(-/-) mice and TgIRBPE2F1 transgenic mice. In the transgenic mice, photoreceptor cells formed rosette-like arrangements at postnatal days 9 through 28. Complete loss of photoreceptors followed in the TgIRBPE2F1 mice but not in the Rb(-/-) retinal transplants. Both RB-deficient and E2F1-overexpressing photoreceptor cells expressed rhodopsin, a marker of terminal differentiation. Loss of p53 partially reduced the apoptosis and resulted in transient hyperplasia of multiple cell types in the TgIRBPE2F1 retinas at postnatal day 6. Our findings support the concept that cross-talk occurs between different retinal cell types and that multiple genetic pathways must become dysregulated for the full oncogenic transformation of neuronal retinal cells.

Mutant rab8 Impairs docking and fusion of rhodopsin-bearing post-Golgi membranes and causes cell death of transgenic Xenopus rods

Rab8 is a GTPase involved in membrane trafficking. In photoreceptor cells, rab8 is proposed to participate in the late stages of delivery of rhodopsin-containing post-Golgi membranes to the plasma membrane near the base of the connecting cilium. To test the function of rab8 in vivo, we generated transgenic Xenopus laevis expressing wild-type, constitutively active (Q67L), and dominant negative (T22N) forms of canine rab8 in their rod photoreceptors as green fluorescent protein (GFP) fusion proteins. Wild-type and constitutively active GFP-rab8 proteins were primarily associated with Golgi and post-Golgi membranes, whereas the dominant negative protein was primarily cytoplasmic. Expression of wild-type GFP-rab8 had minimal effects on cell survival and intracellular structures. In contrast, GFP-rab8T22N caused rapid retinal degeneration. In surviving peripheral rods, tubulo-vesicular structures accumulated at the base of the connecting cilium. Expression of GFP-rab8Q67L induced a slower retinal degeneration in some tadpoles. Transgene effects were transmitted to F1 offspring. Expression of the GFP-rab8 fusion proteins appears to decrease the levels of endogenous rab8 protein. Our results demonstrate a role for rab8 in docking of post-Golgi membranes in rods, and constitute the first report of a transgenic X. laevis model of retinal degenerative disease.

A functional rhodopsin-green fluorescent protein fusion protein localizes correctly in transgenic Xenopus laevis retinal rods and is expressed in a time-dependent pattern

To study rhodopsin biosynthesis and transport in vivo, we engineered a fusion protein (rho-GFP) of bovine rhodopsin (rho) and green fluorescent protein (GFP). rho-GFP expressed in COS-1 cells bound 11-cis retinal, generating a pigment with spectral properties of rhodopsin (A(max) at 500 nm) and GFP (A(max) at 488 nm). rho-GFP activated transducin at 50% of the wild-type activity, whereas phosphorylation of rho-GFP by rhodopsin kinase was 10% of wild-type levels. We expressed rho-GFP in the rod photoreceptors of Xenopus laevis using the X. laevis principal opsin promoter. Like rhodopsin, rho-GFP localized to rod outer segments, indicating that rho-GFP was recognized by membrane transport mechanisms. In contrast, a rho-GFP variant lacking the C-terminal outer segment localization signal distributed to both outer and inner segment membranes. Confocal microscopy of transgenic retinas revealed that transgene expression levels varied between cells, an effect that is probably analogous to position-effect variegation. Furthermore, rho-GFP concentrations varied along the length of individual rods, indicating that expression levels varied within single cells on a daily or hourly basis. These results have implications for transgenic models of retinal degeneration and mechanisms of position-effect variegation and demonstrate the utility of rho-GFP as a probe for rhodopsin transport and temporal regulation of promoter function.

Identification of an outer segment targeting signal in the COOH terminus of rhodopsin using transgenic Xenopus laevis

Mislocalization of the photopigment rhodopsin may be involved in the pathology of certain inherited retinal degenerative diseases. Here, we have elucidated rhodopsin's targeting signal which is responsible for its polarized distribution to the rod outer segment (ROS). Various green fluorescent protein (GFP)/rhodopsin COOH-terminal fusion proteins were expressed specifically in the major red rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The fusion proteins were targeted to membranes via lipid modifications (palmitoylation and myristoylation) as opposed to membrane spanning domains. Membrane association was found to be necessary but not sufficient for efficient ROS localization. A GFP fusion protein containing only the cytoplasmic COOH-terminal 44 amino acids of Xenopus rhodopsin localized exclusively to ROS membranes. Chimeras between rhodopsin and alpha adrenergic receptor COOH-terminal sequences further refined rhodopsin's ROS localization signal to its distal eight amino acids. Mutations/deletions of this region resulted in partial delocalization of the fusion proteins to rod inner segment (RIS) membranes. The targeting and transport of endogenous wild-type rhodopsin was unaffected by the presence of mislocalized GFP fusion proteins.

Fluorescent photoreceptors of transgenic Xenopus laevis imaged in vivo by two microscopy techniques

PURPOSE

To develop a method for imaging individual photoreceptors in an intact transgenic Xenopus eye, thus allowing in vivo observation of the effects of various transgenes on photoreceptor development, degeneration, or both.

METHODS

Albino and pigmented transgenic Xenopus laevis that express enhanced green fluorescent protein (GFP) in the major ("red") rods were generated. The distribution of GFP throughout the retina and within the rods was evaluated by confocal microscopy of frozen sections and immunoelectron microscopy. In vivo images of photoreceptors were obtained using conventional fluorescence microscopes to image through the lens of the eye or a laser scanning confocal microscope to image through the hypopigmented iris of albino eyes.

RESULTS

Confocal and immunoelectron microscopy of tissue sections showed that GFP was predominantly localized to the inner segments of the major rods; a smaller amount was in the outer segments. In a number of animals, not all the major rods expressed GFP. It was possible to identify these animals by obtaining fluorescence images of the retinas of intact, living tadpoles with conventional fluorescence microscopes, using the lens of the tadpole as part of the optical path. Confocal images of living animals could be used to visualize the distribution of GFP within the photoreceptors.

CONCLUSIONS

The ability to observe individual photoreceptors noninvasively allows in vivo longitudinal microscopic analysis of photoreceptor development in transgenic Xenopus tadpoles.

Apoptosis of the mammalian retina and lens

Although retinal neurons usually last the entire lifetime of an individual, many innate genetic and developmental errors and external stimuli can reduce their longevity leading to loss of visual acuity or blindness. Similarly, the lens, largely composed of denucleated fiber cells must remain transparent for life if vision is to remain clear. Apoptosis of retinal neurons and newly generated lens fiber cells contributes to retinal degeneration and cataract formation, respectively, in both humans and experimental mammals. The apoptosis is triggered by many stimuli in addition to inherited mutations and may be amenable to pharmacologic amelioration. These studies not only provide new clinical insights but also the opportunity to investigate the molecular pathways leading to apoptosis in an organ that is not required for survival. The eye, becomes, therefore, an important organ for evaluation of theories of apoptosis in vivo.

Apoptosis in the murine rd1 retinal degeneration is predominantly p53-independent

PURPOSE

To determine if p53 mediates apoptosis in photoreceptors of retinal degeneration, rd1, mice.

METHODS

The rd1/rd1 mice were interbred with p53 null mice to generate p53-/- rd1/rd1 and p53+/+ rd1/rd1 mice. Rates of loss and incidence of apoptosis in rod photoreceptors were analyzed at appropriate ages (postnatal days 12, 14 and 16).

RESULTS

The extent and kinetics of photoreceptor cell loss in rd1 mice were nearly indistinguishable in the p53+/+ and p53 null mice.

CONCLUSIONS

Photoreceptor cell apoptosis in the rd1 mouse model occurs by a predominantly p53-independent molecular pathway.

Post-Golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis

Post-Golgi vesicles budding from the trans-Golgi network (TGN) are involved in the vectorial transport and delivery of rhodopsin to photoreceptor rod outer segments (ROS). We report here that newly synthesized docosahexaenoyl (DHA) phospholipids are sequestered and cotransported by rhodopsin-bearing post-Golgi vesicles to ROS. Frog retinas were pulse-labeled with [35S]methionine/cysteine and [3H]DHA prior to ROS isolation and subcellular fractionation. After a 1-h pulse, relatively uniform [3H]DHA-lipid labeling (DPM/microg protein) was observed in all fractions enriched in post-Golgi vesicles, TGN, Golgi, and endoplasmic reticulum (ER) membranes. During the subsequent 2-h chase translocation of free [3H]DHA from ROS to the photoreceptor inner segment contributed to an additional overall increase in labeling of lipids. The specific activity (dpm/nmol DHA) in ER-enriched fraction was similar or higher than in other subcellular fractions after both the pulse and the chase, indicating that the bulk of [3H]DHA-lipids was synthesized in the ER. After the chase a 2-fold increase in labeling of lipids in the ER and Golgi and a 2.6-fold in lighter TGN-enriched fractions was observed. The highest labeling was in the post-Golgi vesicle fraction (4-fold increase), with [3H]DHA-phosphatidylcholine and [3H]DHA-phosphatidylethanolamine showing the greatest increase. At the same time, newly synthesized [35S]rhodopsin shifted from the ER and Golgi toward TGN and post-Golgi fractions. Therefore, sequestration and association of [35S]rhodopsin and [3H]DHA-lipids in a TGN membrane domain occurs prior to their exit and subsequent vectorial cotransport on post-Golgi vesicles to ROS. Labeling of ROS lipids was very low, with phosphatidylinositol and diacylglycerols displaying the highest labeling. This indicates that other mechanisms by-passing Golgi, i.e. facilitated by lipid carrier proteins, may also contribute to molecular replacement of disc membrane DHA-phospholipids, particularly phosphatidylinositol.

rab8 in retinal photoreceptors may participate in rhodopsin transport and in rod outer segment disk morphogenesis

Small GTP-binding protein rab8 regulates transport from the TGN to the basolateral plasma membrane in epithelial cells and to the dendritic plasma membrane in cultured hippocampal neurons. In our approach to identify proteins involved in rhodopsin transport and sorting in retinal photoreceptors, we have found, using [32P]GTP overlays of 2D gel blots, that six small GTP-binding proteins are tightly bound to the post-Golgi membranes immunoisolated with a mAb to the cytoplasmic domain of frog rhodopsin. We report here that one of these proteins is rab8. About 50% of photoreceptor rab8 is membrane associated and approximately 13% is tightly bound to the post-Golgi vesicles. By confocal microscopy, antibody to rab8 specifically labels calycal processes and the actin bundles of the photoreceptor inner segment that extend inward to the junctional complexes that comprise the outer limiting membrane. Anti-rab8 shows a striking periodicity of high density labeling at 1 +/- 0.12 microns intervals along the actin bundles. Rhodopsin-bearing post-Golgi membranes cluster around the base of the cilium where rab8 and actin are also co-localized, as revealed by confocal microscopy of retinal sections double labeled with anti-rab8 and phalloidin. Microfilaments have been implicated in rod outer segment (ROS) disk morphogenesis. Our data suggest that rab6, which we have previously localized to the post-Golgi compartment, and rab8 associate with the post-Golgi membranes sequentially at different stages of transport. rab8 may mediate later steps that involve interaction of transport membranes with actin filaments and may participate in microfilament-dependent ROS disk morphogenesis.

Alpha A- and alpha B-crystallin in the retina. Association with the post-Golgi compartment of frog retinal photoreceptors

alpha A- and alpha B-Crystallins are significant contributors to maintaining the transparency of the vertebrate lens. We have found that both alpha A- and alpha B-crystallins are also present, at approximately equimolar concentrations, in frog retinal cells. They were identified by sequencing portions of each polypeptide, by immunochemical cross-reactivity with antibodies to bovine alpha-crystallins, and by their relative mobility in two-dimensional gel electrophoresis. Retinal alpha-crystallins form macromolecular multimeric complexes similar to those found in the lens, and they are abundant both in soluble and membrane-associated forms. A surprising finding is that alpha-crystallins bind specifically to the photoreceptor post-Golgi membranes that mediate transport of newly synthesized rhodopsin. Upon treatment of post-Golgi membranes with urea or Triton X-114, a portion of the bound alpha B-crystallin remains tightly associated, indicating that the alpha B-form may mediate membrane binding of an alpha-crystallin multimeric complex. Both subunits are synthesized in vitro by isolated frog retinas, but alpha B-crystallin appears to have a higher renewal rate. Newly synthesized alpha-crystallins become associated with the post-Golgi membranes concurrently with newly synthesized rhodopsin. Association of alpha-crystallins with newly synthesized rhodopsin suggests that they may participate in photoreceptor outer segment membrane renewal. Our findings implicate an important function for both alpha A- and alpha B-crystallins in the same, extralenticular, tissue.

Apoptosis or retinoblastoma: alternative fates of photoreceptors expressing the HPV-16 E7 gene in the presence or absence of p53

A transgenic mouse model for retinoblastoma was produced previously by directing SV40 T antigen expression to retinal photoreceptor cells using the promoter of the interstitial retinol-binding protein (IRBP) gene. This gene becomes active prior to the terminal differentiation of photoreceptors. Because T antigen-transforming activity is attributable, at least in part, to the inactivation of the retinoblastoma (pRb) and p53 tumor suppressor proteins, we addressed the role of p53 in the development of retinoblastoma in mice. Transgenic mice expressing HPV-16 E7 under the control of the IRBP promoter were generated to inactivate pRb in photoreceptors while leaving p53 intact. Rather than developing retinoblastomas, the retinas of these mice degenerate due to photoreceptor cell death at a time in development when photoreceptors are normally undergoing terminal differentiation. The dying cells exhibit the histological and ultrastructural features of apoptosis and contain fragmented DNA. p53 is required for the induction of apoptosis in this model, because mice expressing E7 in a p53 nullizygous background develop retinal tumors instead of undergoing retinal degeneration.

Loss of diurnal arrestin gene expression in rds mutant mouse retinas

Despite the important advances in the molecular analysis of retinal degenerations, the causes of photoreceptor cell death in these conditions are unclear. To explore the metabolic impact of the rds mutation, we have continued our investigations on the expression of arrestin. Normal BALB/c mouse retinas have a diurnal variation in arrestin gene expression and protein biosynthesis, with low levels in dark-adapted and high levels in the light-adapted retinas. In contrast, arrestin is expressed in rds retinas at high levels throughout the diurnal cycle. A lack of a distinct diurnal cycle and continuously high expression of arrestin in rds retinas might be a reflection of metabolic derangements in these cells which are not an obvious consequence of the underlying molecular defect in the rds/peripherin gene.

Rab6 is associated with a compartment that transports rhodopsin from the trans-Golgi to the site of rod outer segment disk formation in frog retinal photoreceptors

The biogenesis of light sensitive membranes in retinal rod photoreceptors involves polarized sorting and targeting of newly synthesized rhodopsin to a specialized domain, the rod outer segment (ROS). We have isolated and characterized the population of post-Golgi membranes that mediate intracellular transport of rhodopsin. In the present study we have examined the association of small (20-25 kDa) GTP-binding (G) proteins with these membranes. We found that one of the small G proteins, rab6, behaves like an integral membrane protein of the post-Golgi vesicles, although approximately 30% of rab6 is soluble. The distribution of the membrane-associated and the soluble forms is highly polarized. By confocal and EM immunocytochemistry it can be seen that most of rab6 is associated with the photoreceptor trans-Golgi cisternae, trans-Golgi network (TGN) and post-Golgi vesicles. The photoreceptor axon and synaptic terminal are unlabeled, but dendrites of deeper retinal layers are labeled. The distribution of rab6 across sucrose density gradient fractions parallels the distribution of sialyltransferase (a TGN marker) activity. About 9% of membrane-bound rab6 is associated, however, with the rhodopsin-bearing sialyltransferase-free post-Golgi vesicles, which represent a very small fraction (< 1%) of the total retinal membranes. Rab6 is absent from the mature ROS disk membranes but it is present at the sites of new ROS disk formation and in the ROS cytoplasm. This suggests that rab6 becomes soluble upon disk membrane formation. Therefore, rab6 may function not only as a component of the sorting machinery of photoreceptors that delivers rhodopsin to its appropriate subcellular domain but may also participate in some aspects of ROS disk morphogenesis.

Expression of opsin and IRBP genes in mutant RCS rats

The retinal pigment epithelium of RCS rats bearing the autosomal recessive rdy mutation fails to ingest shed rod outer segment tips. Accumulation of disk debris in the subretinal space of the maturing mutant retina causes a secondary degeneration of photoreceptor cells. Two hypotheses have been offered as possible explanations of the death of photoreceptor cells in this disorder: (1) photoreceptors are starved for amino acids, retinal, oxygen, etc; and (2) that IRBP levels and synthesis may be decreased and interfere with retinal transport and this deficiency is lethal to these cells. To test these hypotheses, we have studied the effect of this mutation on the levels of expression of opsin and IRBP genes, and gene products and on rates of synthesis at various ages in dystrophic RCS p+ rats and compared the results to those obtained with normal Long Evans rats. The mutant rats and normal controls had comparable amounts of opsin and IRBP mRNA transcripts and rates of synthesis up to post-natal day 45 (P45) but opsin transcripts were barely detectable at P60 and thereafter. IRBP mRNA levels were also very low after P62 although somewhat higher than opsin mRNA. Opsin could be detected immunochemically, albeit at lower levels, at all the ages studied up to P310, but IRBP levels fell below detection after P45. We localized opsin and IRBP in the retina by post-embedding EM immunocytochemical procedures and found that opsin is present in the remnants of rod outer segment debris, even at P390, long after detectable opsin synthesis had ceased. These data suggest that expression of opsin and IRBP genes is not influenced by the shape and state of the outer segments, and that the rdy mutation does not influence the expression of the opsin and IRBP in these retinas until the photoreceptor cells are profoundly damaged. Thus, neither hypothesis about the causes of cell death in this disorder is supported.

Functional heterogeneity of mutant rhodopsins responsible for autosomal dominant retinitis pigmentosa

Thirteen mutant rhodopsins responsible for autosomal dominant retinitis pigmentosa (ADRP) have been produced by transfection of cloned cDNA into tissue culture cells. Three mutants [class I: Phe-45----Leu, Gln-344----termination (deletion of C-terminal positions 344-348), and Pro-347----Leu] resemble wild-type rhodopsin in yield, regenerability with 11-cis-retinal, and plasma membrane localization. Ten mutants [class II: Thr-17----Met, Pro-23----His, Thr-58----Arg, Val-87----Asp, Gly-89----Asp, Gly-106----Trp, Arg-135----Leu, Arg-135----Trp, Tyr-178----Cys, and Asp-190----Gly] accumulate to significantly lower levels, regenerate with 11-cis-retinal variably or not at all, and are transported inefficiently to the plasma membrane, remaining primarily in the endoplasmic reticulum. These data suggest that there are at least two distinct biochemical defects associated with different rhodopsin mutants in ADRP.

Polarized sorting of rhodopsin on post-Golgi membranes in frog retinal photoreceptor cells

We have isolated a subcellular fraction of small vesicles (mean diameter, 300 nm) from frog photoreceptors, that accumulate newly synthesized rhodopsin with kinetics paralleling its appearance in post-Golgi membranes in vivo. This fraction is separated from other subcellular organelles including Golgi and plasma membranes and synaptic vesicles that are sorted to the opposite end of the photoreceptor cell. The vesicles have very low buoyant density in sucrose gradients (rho = 1.09 g/ml), a relatively simple protein content and an orientation of rhodopsin expected of transport membranes. Reversible inhibition of transport by brefeldin A provides evidence that these vesicles are exocytic carriers. Specific immunoadsorption bound vesicles whose protein composition was indistinguishable from the membranes sedimented from the subcellular fraction. Some of these proteins may be cotransported with rhodopsin to the rod outer segment; others may be involved in vectorial transport.

Opsin synthesis and mRNA levels in dystrophic retinas devoid of outer segments in retinal degeneration slow (rds) mice

Opsin gene regulation, as a function of outer segment structure, was studied in normal and mutant retinal degeneration slow (rds) mice. We investigated the level of expression of the opsin gene in the rds mutant to determine if the reduced opsin content observed in this mutation (around 3% of normal) is a consequence of lowered expression of its gene. Normal BALB/c and rds mice were analyzed for levels of opsin mRNA and opsin content by Northern and immunoblot analysis, respectively. The rate of opsin synthesis in isolated retinas was measured by 35S-methionine incorporation in vitro, followed by analysis of the radiolabeled opsin by SDS-gel electrophoresis and autoradiography. Photoreceptor cell loss at various stages of degeneration was determined by quantitation of surviving photoreceptor nuclei. Opsin was localized in the mutant photoreceptors by immunoelectron microscopy of LR gold-embedded retinas using anti-opsin and antibody gold conjugates. The results indicate that 11- and 30-d-old mutant mice have considerable levels of opsin mRNA (60-70% of normal) and opsin synthetic rates (76-92% of normal), after the data from mutant mice are corrected for photoreceptor cell loss. We conclude, therefore, that the very low level of opsin observed in rds mice (approximately 3%) is not a result of greatly reduced expression of the opsin gene. Rather, continuous turnover of newly synthesized opsin as a result of its failure to become sequestered into an intact outer segment appears to account for the low levels of opsin in the rds mutant.

Opsin gene expression during early and late phases of retinal degeneration in rds mice

Opsin mRNA levels, opsin synthetic rates and localization of opsin were studied throughout the photoreceptor's life span in the rds mice. Mutant mice 11 days to 11 months old were investigated. Opsin mRNA levels were studied by means of northern blot analysis. Opsin synthesis was measured by incorporation of [35S]methionine into newly synthesized opsin in vitro. Distribution of opsin in the retina was determined by immunoelectron microscopy. Opsin mRNA was detected in young as well as old retinas, and opsin synthesis could be detected at early phases of degeneration but not in late phases. The absence of opsin synthesis in older rds mice might be due to translational down-regulation or some other defect in the capacity to synthesize opsin. In young mice, opsin was detected in the subretinal space in opsin-laden vesicular membranes: such membranes were absent from retinas of older mice. This disappearance parallels the cessation of opsin synthesis and the consequent failure to deliver opsin to the subretinal space in retinas from older mice. Immunochemical analysis revealed the presence of small amounts of opsin in all retinas up to 11 months of age. Immunoelectron microscopy localized the residual opsin, mostly to the plasma membrane which envelops the nuclei and synaptic terminals. These opsin molecules might be a consequence of very low levels of opsin synthesis, too low to be detected by our assays, or may have been synthesized at an earlier age and retained in the plasma membrane of the old mutant photoreceptors.

Immunocytochemical reactivity of Xenopus laevis retinal rods and cones with several monoclonal antibodies to visual pigments

Immunocytochemical reactions with several antibodies to visual pigments were used to study visual cells of the Xenopus laevis retina. Monoclonal antibodies to bovine opsin "E," 1D4, and 4B4 (reactive with the N- and C-terminus and with the loop connecting transmembrane segments 5-6, respectively) and to chicken visual pigments COS-1 and OS-2 (binding to mammalian red/green and blue cones, respectively), as well as a rabbit antifrog opsin serum 11-7, were applied to semithin and thin sections of the retina. The bound antibodies were detected with the peroxidase technique at the light microscopic level; a three-stage immunogold procedure was used for electron microscopic immunocytochemistry. The overwhelming majority of rods were labeled by monoclonal antibodies "E," 4B4, 1D4, OS-2, and serum 11-7. A small fraction (2-3%) of rods did not bind monoclonal antibodies "E" and 4B4, but this minor population of rods was strongly reactive with 1D4 and to a lesser extent with OS-2, indicating the presence of different visual pigment. These rods differ in shape from the major rod type; they are thinner, shorter, and may be comparable to the blue-sensitive ("green") rods of other amphibia. Cones were morphologically heterogeneous: double cones, large single cones, and small single cones were found, and the large single and the double cones were occasionally duplicated. Double cones and large single cones (as well as their duplicated varieties) strongly bound monoclonal antibodies COS-1 and were unlabeled by all other monoclonal antibodies, except OS-2. The small single cone was remarkably unreactive with COS-1 and "E," weakly labeled by 1D4 and 4B4, and most reactive with OS-2 and 11-7. This unique pattern of immunocytochemical reactions in the small cone type indicates the uniqueness of its visual pigment from other cone types in the Xenopus retina. The present study shows the existence of two different opsins in morphologically distinct (thick and thin) rod types and at least two cone pigments in the heterogeneous cone population.

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.

Production of bovine rhodopsin by mammalian cell lines expressing cloned cDNA: spectrophotometry and subcellular localization

Cloned cDNA encoding bovine rhodopsin has been recombined into an expression vector and cotransfected with an antibiotic resistance plasmid into cultured human embryonic kidney cells. The resulting cell lines produce 100-200 micrograms of bovine opsin per liter of saturated tissue culture medium (10(9) cells). Incubation in vitro with 11-cis retinal produces a photolabile pigment the absorbance spectrum of which is indistinguishable from that of bona fide bovine rhodopsin. Expressed rhodopsin accumulates in the plasma membrane as determined by immunoelectron microscopy.

Opsin accumulation in photoreceptor inner segment plasma membranes of dystrophic RCS rats

Opsin localization in photoreceptor plasma membrane was studied in 10- to 30-day-old dystrophic RCS rats. Preembedding cytochemical procedures with antiopsin antibodies and electron microscopy were employed. In the second postnatal week, opsin was sequestered to the outer segment plasma membrane in affected rat retinas. This distribution resembled that observed in the photoreceptors of normal rats at this age. As unphagocytosed debris accumulated in the subretinal space, the outer segments degenerated, and the distribution of opsin in the plasma membrane changed. Opsin reappeared in the inner segment plasma membrane as the outer segments were lost. Loss of the regionalized distribution of opsin was not associated with visible ultrastructural changes in the inner segments or connecting cilium. Severely damaged cells invariably were labeled on their inner segments at high density. Thus, the presence of an outer segment was correlated with the clearance of opsin from the inner segment, while damage to the outer segment was followed by reappearance of opsin in the inner segment plasma membrane.

Electron microscopic immunocytochemistry of interstitial retinol-binding protein in vertebrate retinas

Interstitial retinol binding protein (IRBP) is a soluble glycoprotein found in the interphotoreceptor matrix (IPM) and implicated in shuttling retinol between retina and pigment epithelium (PE) cells. The authors have studied the distribution of IRBP by EM immunocytochemistry. Thin sections of Lowicryl K4M embedded R. pipiens, X. laevis, bovine and human retinas were labeled sequentially with affinity purified rabbit antibovine IRBP, biotinyl-sheep antirabbit F(Ab')2, and avidin-ferritin, or with avidin and biotinyl-ferritin. Antigen was in the interphotoreceptor space and intercalated into the narrow spaces between PE cell microvilli. IRBP penetration between PE cells was delimited abruptly by the PE junctional complexes. IRBP was also observed in small vacuoles in the apical cytoplasm of PE cells and in PE cell phagosomes that contained IRBP surrounding ingested rod tips. IPM was heavily but inhomogeneously labeled. Antigen was usually deposited along the ROS and COS plasma membrane in a confluent layer, but sometimes it was distributed in large (ca. 0.2-micron thick) clumps. In bovine and human retinas, the connecting cilium was ensheathed by antigen at high density but an unlabeled halo surrounded its plasma membrane. The apical plasma membrane of the inner segment aligned along the connecting cilium was also densely coated by antigen. In both frog retinas, the ridges of the periciliary ridge complex (PRC) were coated with antigen. In none of the four species examined was Golgi labeling present. In bovine retinas, labeled vacuoles (granules) in the myoid region were found in very low numbers (15 vacuoles in 358 rod cells). Amphibian retinas also contained only small numbers of myoid vacuoles labeled by anti-IRBP. Absence of antibody binding to intracellular sites of synthesis in any of the cells that abut the interphotoreceptor matrix suggests that the antigen may be masked prior to its release from the synthetic cell(s) or that its level is below limits of detection.

Immunocytochemical localization of opsin in the inner segment and ciliary plasma membrane of photoreceptors in retinas of rds mutant mice

Homozygous 020/A mutant mice bearing the rds gene for slow inherited retinal degeneration have been observed to develop normal photoreceptor inner segments connecting cilia and synaptic contacts but fail to form outer segments. Their retinas are responsive to light, however. In order to assess the sources of these physiological responses we investigated the distribution of opsin in photoreceptors by means of immunoelectron microscopy. Opsin was detected in the inner segment plasma membrane and the distal ciliary plasma membrane. Antibody also bound to lamellar and vesicular membranes in the interphotoreceptor space and, in a small fraction of the photoreceptors, to membranes projecting from the distal cilium. These membranes may represent abortive formation of rod discs in this form of retinal degeneration. Failure to form an organized outer segment may contribute to the persistence of opsin in the inner segment plasma membranes of adult mutant mice.

Immunocytochemical binding of anti-opsin N-terminal-specific antibodies to the extracellular surface of rod outer segment plasma membranes. Fixation induces antibody binding

We have examined the binding of anti-opsin antibodies to the plasma membrane of frog retinal rod outer segments (ROS) by fluorescence light microscopy and electron microscopy. Polyclonal and monoclonal antibodies specific for the N-terminal domain of opsin were observed to bind to the extracellular surface of ROS plasma membrane of aldehyde-fixed but not of unfixed retinas. This reaction was found regardless of whether purified ROS, rhodopsin, opsin, or an N-terminal peptide of opsin was used as the immunogen. The fixation-induced binding of these antibodies contrasts with the more frequently noted loss of antigenicity upon fixation. Concanavalin A, however, binds to unfixed ROS plasma membranes. Its binding sites in the plasma membrane may be oligosaccharides in the N-terminal region of opsin. These results suggest that the N-terminal domain of opsin is latent in the native membrane and that changes in conformation may account for its detectability in fixed membranes.

Biosynthesis and vectorial transport of opsin on vesicles in retinal rod photoreceptors

Retinal rod photoreceptor cells absorb light at one end and establish synaptic contacts on the other. Light sensitivity is conferred by a set of membrane and cytosol proteins that are gathered at one end of the cell to form a specialized organelle, the rod outer segment (ROS). The ROS is composed of rhodopsin-laden, flattened disk-shaped membranes enveloped by the cell's plasma membrane. Rhodopsin is synthesized on elements of the rough endoplasmic reticulum and Golgi apparatus near the nucleus in the inner segment. From this synthetic site, the membrane-bound apoprotein, opsin, is released from the Golgi in the membranes of small vesicles. These vesicles are transported through the cytoplasm of the inner segment until they reach its apical plasma membrane. At that site, opsin-laden vesicles appear to fuse near the base of the connecting cilium that joins the inner and outer segments. This fusion inserts opsin into the plasma membrane of the photoreceptor. Opsin becomes incorporated into the disk membrane by a process of membrane expansion and fusion to form the flattened disks of the outer segment. Within the disks, opsin is highly mobile, and rapidly rotates and traverses the disk surface. Despite its mobility in the outer segment, quantitative electron microscopic, immunocytochemical, and autoradiographic studies of opsin distribution demonstrate that little opsin is detectable in the inner segment plasma membrane, although its bilayer is in continuity with the plasma membrane of the outer segment. The photoreceptor successfully establishes the polarized distribution of its membrane proteins by restricting the redistribution of opsin after vectorially transporting it to one end of the cell on post-Golgi vesicles.

Vesicular transport of newly synthesized opsin from the Golgi apparatus toward the rod outer segment. Ultrastructural immunocytochemical and autoradiographic evidence in Xenopus retinas

Each day, rod photoreceptors of the vertebrate retina synthesize rhodopsin and insert it into new membranes of the rod outer segment (ROS). The authors determined which components of the rod cell transport opsin from the Golgi to the ROS by a combined EM autoradiographic and immunocytochemical study using radiolabeled amino acid precursors and antiopsin antibodies. Radiolabeled proteins in the ellipsoid region of Xenopus laevis retinal rods were localized by comparison of the distribution of silver grains with the predicted distribution generated by a hypothetical source: grain matrix. Sources of decay were not uniformly distributed. Small vesicles compressed between mitochondria and clustered beneath the connecting cilium that joins the inner to the outer segment contained more than 30% of the radiolabel and had a specific activity 17 times higher than the surrounding cytoplasm. Opsin was localized immunocytochemically on thin sections of retinas embedded in Lowicryl K4M (Polysciences; Warrington, PA) by reaction sequentially with biotinyl-rabbit antifrog opsin, biotinyl-sheep antirabbit F(ab')2, and avidin-ferritin. Golgi apparatus, intermitochondrial vesicles, and vesicles that clustered beneath the connecting cilium were prominently labeled. Subellipsoid smooth endoplasmic reticulum was labeled at background levels. These results demonstrate that intracellular vesicular membranes transport newly synthesized opsin from the Golgi to the base of the connecting cilium of X. laevis retinas. Antibody labeled the outer segment plasma membrane at a 10-fold greater density than the contiguous inner segment plasma membrane. The polarized distribution of opsin apparently involves not only vectorial transport of opsin in the inner segment but also restrictions to the randomization of opsin inserted into the inner and outer segment plasma membrane.

Correlation of Na+,K+-ATPase content and plasma membrane surface area in adapted and de-adapted salt glands of ducklings

During salt-water adaptation, an increase occurs in Na+,K+-ATPase content and surface area of the basolateral plasma membrane of the principal cell of the duck salt gland. To determine the degree to which these changes are correlated, accepted morphometric methods were used to determine numerical cell densities and plasma membrane surface densities of peripheral and principal cells. After adaptation, the plasma membrane surface area per principal cell was five times greater than in controls. Following de-adaptation, the plasma membrane content in principal cells returned to 1.9 times control levels. Two other cell constituents, mitochondria and lipid droplets, displayed similar quantitative changes. Na+,K+-ATPase content increased about fourfold with adaptation and decreased to near control levels with de-adaptation. Thus, changes in Na+,K+-ATPase content and basolateral plasma membrane surface area in adapting and de-adapting secretory epithelia of the salt gland occur nearly in parallel. These quantitative data enable Na+,K+-ATPase synthesis and degradation to be investigated in relation to membrane biogenesis.

Rapid embedding of tissues in Lowicryl K4M for immunoelectron microscopy

Lowicryl K4M (K4M) was recently introduced as an embedding medium for immunocytochemistry at the electron microscope level (BL Armbruster, E Carlemalm, R Chiovetti, RM Garavito, JA Hobot, E Kellenberger, W Villiger (1982):J Microsc 126:77 and E Carlemalm, M Garavito, W Villiger (1982):J Microsc 126:123). While earlier protocols of fixation and embedding required 4-6 days, the present method has reduced the processing time by accelerating both dehydration of tissues and polymerization of K4M so that tissues can be prepared for sectioning within 4 hr. The immunocytochemical labeling density was quantitated in order to determine relative antigen preservation in tissues embedded by the accelerated protocol as compared to slower K4M embedding techniques and to tissues embedded in glutaraldehyde-cross-linked bovine serum albumin (BSA). Thin sections of Bufo marinus kidney were labeled with rabbit antibody to Na+,K+ATPase alpha chain catalytic subunit isolated from B. marinus kidney microsomes (M Girardet, K Geering, JM Frantes, D Geser, BC Rossier, JP Kraehenbuhl, C Bron (1981):Biochemistry 20:6684). B. marinus retinas were labeled with rabbit anti-opsin. After fixation in paraformaldehyde(3%)-glutaraldehyde(3%), tissues were washed in buffer, dehydrated in 50, 75, and 90% dimethyl-formamide (DMF, 10 min each); K4M:DMF, 1:2 (15 min); K4M:DMF, 1:1, (20 min); K4M (25 min); K4M (30 min) at room temperature and transferred in fresh K4M to BEEM capsules for exposure to ultraviolet light (GE 15 watt, Black-lite, 10 cm, 45 min or less) at 4 degrees C. Thin sections were labeled successively with antibody, biotinylated sheep anti-rabbit F(ab')2 and avidin-ferritin. Ferritin labeling densities were determined by point counting. High labeling densities were observed with both antibodies, equaling or exceeding levels of labeling by slower protocols or embedment in BSA.

Actin in the photoreceptor connecting cilium: immunocytochemical localization to the site of outer segment disk formation

Actin has been localized in Rana pipiens retinas that were fixed and embedded in aldehyde cross-linked BSA. Thin sections were reacted sequentially with (a) affinity-purified antiactin antibodies induced in rabbits; (b) biotinyl-sheep anti-rabbit antibodies; and (c) avidin-ferritin conjugates. As expected, antiactin labeling density was high in the apical pigment epithelial cell processes and in the calycal processes of photoreceptors. Actin was also localized in a new site. The connecting cilium that joins the inner and outer segments of both rods and cones was heavily labeled by antiactin at its outer segment (OS), or distal, end. In this region of the cilium, the plasma membrane evaginates to form new OS disks and these basal disks were labeled in some instances. Below the new disks in rods, the cytoplasm of liplike expansions of the distal cilium was also heavily labeled. The plasma membrane and interior of the connecting cilium and the remainder of the OS were unlabeled. These findings suggest that actin may participate in the vectorial transport of opsin and other intrinsic membrane proteins that are incorporated into newly forming OS disks. The results also implicate actin in the membrane expansion involved with OS disk formation.

Preembedding labeling with biotinylated antibodies and subsequent visualization of the biotin groups exposed on thin sections

The feasibility of labeling cell membranes with biotinylated ligands and detecting the biotin groups on thin sections was investigated. Fixed retinal tissue was incubated with biotinyl- antiopsin . Half of the biotinyl-antibody labeled retinal tissue was incubated with avidin-ferritin (AvF) and embedded in Epon (preembedding reaction). The second half was embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Thin sections of this preparation were incubated with AvF to detect biotinyl-antibodies exposed by the sectioning (postembedding reaction). Biotin groups on the thin section surface could be readily visualized with AvF. Stereoscopic images demonstrated that the ferritin particles were localized only on the exposed surface of the thin section. The labeling was highly specific, with a very low background. Quantitative analysis was employed in order to determine the optimal reaction conditions for maximizing the labeling density with minimizing nonspecific binding. The possibility of using biotinylated molecules in the study of dynamic cellular events and for the subsequent intracellular localization of biotin on thin sections is suggested.

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.

Differential distribution of opsin in the plasma membrane of frog photoreceptors: an immunocytochemical study

Opsin molecules on the surface of frog photoreceptors were visualized by immunocytochemistry at the ultrastructural level. Isolated retinas were immersed in biotinyl-antibody to bovine opsin followed by avidin-ferritin conjugates. Anti-opsin bound to the plasma membrane and to the surface of the most basal discs of red rod outer segments. Inner segment plasma membranes of red rod photoreceptors were devoid of anti-opsin label except for the apical plasma membrane in the region of the recently described periciliary ridge complex. The connecting cilium surface from its base at the periciliary region to the site of new disc evagination was almost free of anti-opsin binding, an observation in consonance with prior studies of thin sectioned retinas embedded in glutaraldehyde cross-linked bovine serum albumin. These results indicate that the continuous plasma membrane of photoreceptors is highly polarized. Opsin, which is free to diffuse throughout the outer segment plasma membrane and along the discs, does not back-diffuse onto the inner segment plasma membrane. The periciliary ridge complex and the base of the connecting cilium are possible sites of restriction of opsin mobility. This study also has provided new insight into the molecular structure of frog visual pigments. Frog green rod and cone outer and inner segment plasma membranes were not labeled by this sheep antiserum to bovine opsin. In contrast, discs of green ROS and the lamellae of some cones were labeled when these antibodies were applied to albumin embedded thin sections of frog retinas. Apparently, only internal or intramembraneous domains of green ROS and cone visual pigments were recognized by this antibody while both internal and extracellular domain(s) of red ROS opsin were reactive.

Fine structure of a periciliary ridge complex of frog retinal rod cells revealed by ultrahigh resolution scanning electron microscopy

We studied the junctional region between rod inner segments (RIS) and outer segments (ROS) in frog retinas by high resolution scanning electron microscopy (SEM). Retinas of dark adapted or light exposed Rana pipiens were critical-point-dried and RIS and ROS were split and coated with ultrathin metal films of niobium and chromium--or decorated with gold--and imaged in a new SE-I imaging mode. The connecting cilium (CC) usually broke at the base of the RIS and remained attached to the ROS. The outer part of the CC plasmalemma expanded to form liplike protrusions beyond which disks evaginated with successively larger diameter until they reached the full width of the ROS. The CC rose out from an invagination of the RIS apical plasma membrane (PM). On the lateral walls of this invagination, a highly ordered complex of nine symmetrically arrayed ridges and grooves rose steeply and extended laterally approximately 0.4-1 micron on the adjacent RIS PM. On the apical plasmalemma, the ridges and grooves formed groups of three to four parallel rows that surrounded the invagination. The grooves were bridged by filaments anchored at the top edges of the ridges. This highly ordered structure we term the periciliary ridge complex (PRC). Its ninefold symmetry apparently reflects the 9 + 0 microtubule organization of the CC axoneme. The three-dimensional structure revealed by SEM was confirmed by transmission electron microscopy (TEM) of sections of Epon-embedded retinas. TEM-immunocytochemistry on thin sections of retinas embedded in glutaraldehyde cross-linked albumin suggested that the PRC and the CC may participate in opsin transport and disk morphogenesis.

Cone lamellae and red and green rod outer segment disks contain a large intrinsic membrane protein on their margins: an ultrastructural immunocytochemical study of frog retinas

In addition to rhodopsin, the disk membranes of rod outer segments (ROS) contain a large integral membrane protein (mol. wt 290,000). This protein was previously localized by immunocytochemistry to the margins and incisures of disks in frog red ROS by specific antibody applied to thin sections of bovine serum albumin embedded retinas (Papermaster et al., 1978b, J. Cell Biol. 78, 415-425). Upon further study of the reactions of this antibody with outer segments of other photoreceptor classes in frog retina, labeling of the short incisures and margins of green ROS and margins of cone outer segment lamellae is also observed. Thus the large protein participates in the structure of the edges of disks and lamellae of all photoreceptors in the frog. In addition, labeling of the inter-incisure surface of all photoreceptor classes was observed at high antibody concentration. In order to interpret this labeling, the effect of dilution on labeling density was determined and double reciprocal plots (Markham and Benton, 1931, J. Am. Chem. Soc. 53, 497) were employed to evaluate the relative affinity and heterogeneity. There was considerable deviation from linearity in the plots of labeling disk interiors compared to the relatively linear plots of disk incisure labeling which suggests that the interior sites contain a weakly cross-reacting antigen or that the serum contains a lower concentration of antibody weakly reactive with another antigen.

Messenger RNA of opsin from bovine retina: isolation and partial sequence of the in vitro translation product

Opsin, the apoprotein of the visual pigment rhodopsin, is synthesized on membranes of the rough endoplasmic reticulum and subsequently passes through the Golgi apparatus to the rod outer segment. This pathway parallels the early stages of biosynthesis of some secretory proteins and viral membrane glycoproteins. Most of these proteins are initially synthesized as precursor molecules with a short-lived hydrophobic extra peptide segment at the NH(2) terminus. Therefore we investigated whether or not the immediate translation product of opsin mRNA contains a similar short-lived NH(2)-terminal extra peptide. The mRNA coding for opsin was isolated from bovine retina polysomes precipitated by antibodies to opsin. The mRNA directed the cell-free synthesis of a protein comparable in size to opsin that was specifically precipitated by anti-opsin antibodies. Sequence analyses of the immunoprecipitated protein labeled with six radioactive amino acids (Met, Asn, Pro, Phe, Tyr, Val) provided the following result: [Formula: see text] (X is unknown). This partial sequence of the cell-free product corresponds exactly to the published NH(2)-terminal segment of native opsin (21 residues long) and extends beyond this region. Met-1 was shown to be the initiator methionine residue, because only the initiator [(35)S]Met-tRNA(1) (Met)-not the internal [(35)S]Met-tRNA(2) (Met)-donated the NH(2)-terminal methionine. This finding essentially rules out the possibility that Met-1 was preceded by a peptide that was rapidly cleaved. Thus opsin, and not a precursor, is the immediate product of opsin mRNA translation.

Immunocytochemical localization of a large intrinsic membrane protein to the incisures and margins of frog rod outer segment disks

Immunocytochemical techniques have localized a large protein which is an intrinsic membrane component of isolated frog rod outer segments (ROS). This large protein whose apparent mol wt is 290,000 daltons comprises about 1--3% of the ROS membrane mass. Its molar ratio to opsin is between 1:300 and 1:900. Adequate immune responses were obtained with less than 30 microgram (100 pmol) of antigen per rabbit. Antibodies to the large protein were used for its localization on thin sections of frog retina embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Specifically bound antibodies were detected by an indirect sequence with ferritin-conjugated antibodies. This technique detected the protein which is represented by 1,000--3,000 molecules per disk. This indicates that the procedure is sufficiently sensitive for analysis of membrane components in low molar proportions. The large protein was specifically localized to the incisures of ROS disks which divide the disks into lobes and to the disk margin. Thus, opsin is mobile within the membrane of the disk while the large protein is apparently constrained to the disk edges. This finding raises the possibility that special functions are also localized ot his unusual region of high curvature, and that collisions of bleached opsin with these edges are physiologically important in couter segment function.