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PubMed1991

Proteoglycans in the mouse interphotoreceptor matrix. IV. Retinal synthesis of chondroitin sulfate proteoglycan.

To determine whether the mouse retina contributes chondroitin sulfate proteoglycan (CS-PG) to the interphotoreceptor matrix (IPM), 35SO4(2-) was used as a tracer for newly synthesized proteoglycan by retinas maintained in vitro in the absence of pigment epithelium. Following incubation with the tracer for 3 hr, the 35S-labeled proteoglycans present in the incubation medium and associated with isolated photoreceptor outer segments were analyzed separately. Proteoglycan was extracted with 4 M guanidine, and then separated on a G-25 column followed by DEAE ion exchange chromatography in the presence of 8 M urea. The proteoglycan fraction was eluted with a linear NaCl gradient of 0.15-1.0 M. Eluted 35S-labeled macromolecules were susceptible to chondroitinase AC and ABC degradation, indicating that virtually all the 35S-labeled proteoglycan synthesized by the mouse retina and secreted into the incubation media is of the chondroitin sulfate type. In parallel autoradiographic analysis of retinas following 35SO4(2-) incubation, silver grains were present over all retinal compartments, with 41-48% associated with the photoreceptor layer. Quantitative autoradiography of retinas following chondroitinase AC digestion of fixed retinas revealed significant (P less than 0.025) reduction in silver grains associated with the photoreceptor outer segment layer as compared to controls. These combined biochemical and autoradiographic studies indicate that the retina, possibly the photoreceptors, synthesize at least a portion of the CS-PG present in the IPM of the mouse.

Animals↗

PubMed1991

A 26 kd calcium binding protein from bovine rod outer segments as modulator of photoreceptor guanylate cyclase.

The resynthesis of cGMP in vertebrate photoreceptors by guanylate cyclase is one of the key events leading to the reopening of cGMP-gated channels after photoexcitation. Guanylate cyclase activity in vertebrate rod outer segments is dependent on the free calcium concentration. The basal activity of the enzyme observed at high concentrations of free calcium (greater than 0.5 microM) increases when the free calcium concentration is lowered into the nanomolar range (less than 0.1 microM). This effect of calcium is known to be mediated by a soluble calcium-sensitive protein in a highly cooperative way. We here show that this soluble protein, i.e. the modulator of photoreceptor guanylate cyclase, is a 26 kd protein. Reconstitution of the purified 26 kd protein with washed rod outer segment membranes containing guanylate cyclase revealed a 3- to 4-fold increase of cyclase activity when the free calcium concentration was lowered in the physiological range from 0.5 microM to 4 nM. Guanylate cyclase in whole rod outer segments was stimulated 10-fold in the same calcium range. The activation process in the reconstituted system was similar to the one in the native rod outer segment preparation, it showed a high cooperativity with a Hill coefficient n between 1.4 and 3.5. The half-maximal activation occurred between 110 and 220 nM free calcium. The molar ratio of the modulator to rhodopsin is 1:76 +/- 32. The protein is a calcium binding protein as detected with 45Ca autoradiography. Partial amino acid sequence analysis revealed a 60% homology to visinin from chicken cones.

Amino Acid Sequence↗

PubMed1983

Localization of S-antigen by enzyme-labelled antibody method and electron microscopy.

Localization of S-antigen was studied by a direct enzyme-labeled immunohistochemical method with light and electron microscopes. IgG was prepared from rabbits which were sensitized by swine retinal S-antigen and the IgG was conjugated with horseradish peroxidase (HRP). This HRP-labeled IgG was directly applied to normal rabbit retina and the retina was processed to make slides for light and electron microscopic examinations. In light microscopic findings, a clear-cut positive response was noted in the outer segment of photoreceptor cells which was similar to that of Wacker's observation, and a weak response was observed in the inner segment of the photoreceptor cells and the outer nuclear layer. In electron microscopic findings, immunoprecipitates, which were represented by electron-dense particles, were observed on both sides of the disk membrane of the outer segment with evenly distributed particles on both outer and inner surfaces, showing a pattern symmetric to that of the membrane. These immunoprecipitates were also noted in the plasma membrane of the outer segment but there they did not show a pattern similar to that of the disk membrane. A small amount of pinpoint-shaped immunoprecipitates were also noted in the cytoplasm of the inner segment and the connecting cilia. The immunoprecipitates were observed to gradually decrease in number with the phagocytosis of the outer segment by the retinal pigment epithelium, and they finally disappeared as fragments which were completely phagocytized. These findings may suggest a renewal process of S-antigen and a rationalization of this theory was discussed.

Animals↗

PubMed1988

Active sites of the cyclic GMP phosphodiesterase gamma-subunit of retinal rod outer segments.

Monoclonal antibodies were prepared to the gamma-subunit of the cGMP phosphodiesterase. One of them gamma p-1, suppresses the activation of phosphodiesterase through the alpha-subunit of transducin. The gamma-subunit fragment 24-45 rich in Arg and Lys residues is involved in gamma p-1 binding and is essential for the gamma-subunit interaction with transducin. Carboxypeptidase Y cleaves off seven amino acid residues from the C-terminus of the gamma-subunit resulting in phosphodiesterase activation. Thus, the C-terminal fragment of gamma-subunit participates in phosphodiesterase inhibition.

3',5'-Cyclic-GMP Phosphodiesterases↗

PubMed1993

Photocurrents in retinal rods of pigeons (Columba livia): kinetics and spectral sensitivity.

1. Membrane photocurrents were recorded from outer segments of isolated retinal rods of pigeons (Columba livia), the first such measurements on the photoreceptors of a bird. The amplitude of the response to 20 ms flashes of narrow wavelength bands of light increases linearly with intensity at low photon fluxes and saturates at higher intensities. The maximum (saturating) photocurrent observed in forty-nine rod cells was 50 pA. Larger responses with less variability in the intensity for half-maximal responses were observed when the physiological saline contained 20 mM bicarbonate (in addition to Hepes buffer). 2. The dependence of peak amplitude on intensity is well fitted by an exponential function; it is usually less well fitted by the Michaelis-Menten (Naka-Rushton) equation. 3. In the presence of bicarbonate, the average sensitivity of pigeon rods to dim flashes was 0.56 pA photon-1 microns -2. The effective collecting area per photon was 1.8 microns 2. About 83 +/- 26 (mean +/- S.D.) photoisomerizations were required for a half-saturating response. 4. The response kinetics of rods to dim flashes can be reasonably well described by a series of four to five either Poisson or independent filters. The time to peak, measured from the mid-point of a 20 ms flash, was 319 +/- 83 ms (mean +/- S.D.). The integration time of the response was 851 +/- 86 ms (mean +/- S.D.) with bicarbonate present and 572 +/- 126 ms in the absence of bicarbonate. The responses of pigeon rods appear to be slower than those of mammals at the same temperature. The fraction of current suppressed by a single photoisomerization is smaller in pigeon than in mammalian rods by a factor of at least two. 5. The spectral sensitivity function was measured between 680 and 330 nm. The maximum at about 505 nm (range 497-508 nm) corresponds to the alpha-band of a vertebrate rhodopsin and agrees with previous behavioural measurements of scotopic sensitivity of pigeons as well as the absorption spectrum of extracts of pigeon rhodopsin. There was no pronounced beta-band in the near-ultraviolet wavelengths.

Animals↗

PubMed1987

Patterns of interdisk connections within the lamellar domains of retinal rod outer segment disks: observations relevant to the axial propagation of incisures.

In rod outer segments (ROSs) of the Congo eel salamander (Amphiuma), we have observed a system of cytoplasmic filaments that interconnect the lamellar domains of adjacent disks. In longitudinal sections, these filaments occur in pairs, spaced 18-20 nm apart. Each filament is 9-11 nm in length, and perpendicular to the disk membrane. Such filament pairs display extended axial alignment through the lamellar domains of successive disks, indicating the involvement of trans-membranous elements. In cross-sectional views, these filaments appear as a row of paired, punctate densities with a periodic spacing of 14-16 nm. Such views indicate that these filaments are organized on a two-dimensional crystalline lattice, oriented perpendicular to the disk lamellae. The linear interdisk densities of the terminal loop complex appear to be identical to these cytoplasmic filaments: both are equal in length, and both are organized as two-dimensional assemblies with equivalent orientations and lattice parameters. These shared features suggest a common subset of molecular components and lattice determinants. Where the paired filament assemblies intersect the disks, one observes a characteristic dilatation of the intradisk space. In these regions, the disk is 3-6 nm wider than adjacent lamellar regions, and tends to approximate the width of the terminal loop expansion along the disk perimeter. Within such dilatations, we have not been able to demonstrate crescentic densities corresponding to those found within the terminal loop region. Paired filament arrays are not randomly located within the lamellar domains, but lie parallel to the local disk perimeter. Both single and multiple arrays are commonly observed. Single arrays are recessed about 80 nm from the disk edge. When more than one array is present, the additional arrays lie parallel to the first and are spaced at intervals of about 70 nm. Thus, in three dimensions, the multiple arrays appear as regularly spaced sheets of filaments, parallel to the local disk perimeter and perpendicular to the lamellar domains. The characteristic interval between such filament arrays suggests the presence of 'spacer' molecules that organize the two-dimensional filament lattices in the third dimension. Such a spacing mechanism may also account for the minimum width observed for disk lobule segments defined by extended parallel incisures. The potential role of paired filament arrays in the axial propagation of incisures is discussed.

Animals↗

PubMed1985

Molar absorptivities of bovine retina rod outer segment phospholipids in n-hexane.

The molar absorptivities of the major bovine rod outer segment (ROS) phospholipids and phophatidylcholine 18:1 have been determined at four wavelengths, i.e., 193.5, 196, 200, and 205 nm. The mean standard error is 1.7% at 95% confidence level. The results obtained can therefore be used for the quantitative analysis of bovine ROS phospholipids. Compared with the existing methods, the spectrophotometric determination of these lipids presents the advantages of being rapid, direct, and very sensitive. The importance of stray light in this type of measurement is also discussed.

Animals↗

PubMed1985

Intramembrane fluidity increases when the cyclohexyl ring of retinal binds to rod outer-segment membranes.

The spin-label 5-4',4'-dimethyloxazolidinyl-N-oxy stearate (5-doxyl-stearate) exists in a viscous environment when it is incorporated into bleached rod outer segment membranes (ROSM). Addition of the cyclohexyl ring moiety of retinal to ROSM produced a large increase in intramembrane fluidity. A decrease in both the order parameter S (from 0.5 to 0.14) and the rotational correlation time (from 4.0 to 2.0 nsec) gives an estimate of the magnitude of change. Other ring analogs produced similar fluidity changes. The extent of their action depended on their structural similarity to the cyclohexyl of retinal. The pH range in which the cyclohexyl ring produced the increase in ROSM fluidity was 6.8-9.5.

Animals↗

PubMed1993

Retinal pigment epithelial cell transplants in retinal degeneration slow mice do not rescue photoreceptor cells.

PURPOSE: To determine if retinal pigment epithelial cells are in any way involved in the degeneration of photoreceptor cells in the retinal dystrophy mouse model, retinal degeneration slow (rds); to determine if normal retinal pigment epithelial cell transplants can affect outer segment development in the retina. METHODS: Retinal pigment epithelial cells of neonatal normal pigmented C3H mice were isolated and transplanted into retinas of postnatal day 33 albino rds mice. Then eyes of 4-month-old rds mice, retinal pigment epithelial cell-transplanted and sham and non-treated control mice, were processed for light and electron microscopy and the thickness of the outer nuclear layer were measured and compared. RESULTS: Measurements of outer nuclear layer thickness in the transplant and control groups revealed that normal retinal pigment epithelial cell transplants did not cause photoreceptor cell rescue in rds mice. In addition, outer segments were not seen in retinal pigment epithelial cell-transplanted rds retinas. CONCLUSIONS: This study supports the conclusions of other investigators that the photoreceptor cell is the primary site of the genetic defect that results in retinal dystrophy in the rds mouse model.

Animals↗

PubMed1989

Regulation of cGMP-dependent conductance in cytoplasmic membrane of rod outer segments by transducin.

A preparation of the photoreceptor G-protein, transducin, containing mainly the T alpha-subunit in a GTP-gamma-S-bound form, has been used for perfusion of the intracellular surface of excised patches of rod outer segment cytoplasmic membrane from frog retina. The preparation has been shown to result in the complete suppression of the cGMP-activated ionic conductance of the cytoplasmic membrane patch. The effect is entirely reversible after the protein has been washed out and is not observed in the absence of cGMP. The degree of conductance inhibition depends on the protein concentration, half-maximal inhibition occurring at 1 microM T alpha-GTP-gamma-S.

Animals↗

PubMed1988

Regulation of eye derived growth factor binding to membranes by light, ATP or GTP in photoreceptor outer segments.

Eye derived growth factor II (EDGF II), the retinal form of acidic fibroblast growth factor (aFGF) is present in rod outer segments (ROS) purified in the dark, which display higher EDGF specific activity than all other parts of the retina. EDGF binds to ROS disc membranes upon illumination. This binding is not reversible in the dark. ATP, but not GTP, readily releases EDGF from either dark-adapted or previously bleached ROS. The release of EDGF activity from ROS membranes would require a phosphorylation mechanism since AMP-PNP, an ATP analogue, is not efficient. ROS membranes compete with cellular EDGF receptors of retinal pigment epithelial cells in vitro for the binding of labelled EDGF II, suggesting that they also possess specific binding sites. These data suggest that EDGF II is involved in photoreceptor cell biology.

Adenosine Triphosphate↗

PubMed1995

Isoproterenol inhibits rod outer segment phagocytosis by both cAMP-dependent and independent pathways.

PURPOSE: The authors studied the involvement of cAMP-dependent second messenger systems in the inhibition of rod outer segment (ROS) phagocytosis by isoproterenol (ISO) and forskolin (FSK) using two membrane-permeant analogs of cyclic adenosine monophosphate (cAMP), the Rp and Sp diastereoisomers of cyclic adenosine 3',5' monophosphothioate (cAMPS). Rp-cAMPS is a potent competitive inhibitor of cAMP-dependent protein kinase I and II (PKA I and II), whereas Sp-cAMPS is a potent activator of these enzymes. METHODS: ROS phagocytosis was quantitated in cultured rat RPE cells using a previously described double immunofluorescence assay. RESULTS: Sp-cAMPS showed a dose-dependent inhibition of ROS phagocytosis, whereas 100 microM Rp-cAMPS had no effect on this process. Rp-cAMPS fully prevented the inhibitory effect of Sp-cAMPS and FSK but was able to prevent only partially the inhibition of ROS phagocytosis induced by ISO. Isoproterenol plus FSK showed an additive effect on the inhibition of phagocytosis, suggesting that they act at two independent sites. However, ISO plus Sp-cAMPS or FSK plus Sp-cAMPS showed no additivity. CONCLUSIONS: Results suggest that FSK inhibits ROS phagocytosis by RPE cells through a cAMP-dependent pathway, whereas ISO inhibits ROS phagocytosis by RPE cells through cAMP-dependent and cAMP-independent pathways.

Animals↗

PubMed2003

Aging of cultured retinal pigment epithelial cells: oxidative reactions, lipofuscin formation and blue light damage.

This report reviews our experimental work on cultured retinal pigment epithelial (RPE) cells, fed native or UV-irradiated photoreceptor outer segments (POS). We showed that significantly more lipofuscin (LF) was formed in cells cultured in 40% oxygen than in cells cultured in 8% oxygen, indicating an involvement of oxidative mechanisms in LF formation. The antioxidants alpha-tocopherol, lycopene, zeaxanthin and lutein significantly reduced LF formation. RPE cells high in melanin content exhibited significantly less formation of LF than cells low in or devoid of melanin, suggesting that melanin acts as an effective antioxidant. The phagocytic capacity of LF-loaded RPE cells was significantly reduced compared to that of unloaded control cells, indicating that LF-loaded RPE cells may be unable to serve the photoreceptors sufficiently regarding phagocytosis of shed outer segment tips. Blue light irradiation destabilized lysosomal membranes in LF-loaded RPE cells and significantly reduced the viability of such cells compared to unloaded, irradiated control cells. These results may be of significance in relation to the development of age-related macular degeneration (AMD).

Animals↗

PubMed1981

Agglutination of isolated rod outer segments by lectins.

The interactions of ten lectins with rod outer segment (ROS) plasma membranes and the effects of various haptene sugars were examined by studying agglutination in suspensions of washed ROSs. It is concluded that agglutination sites on the ROS membrane involve receptors that are recognized by eight lectins with haptene sugar preferences that include D-Gal, D-GalNAc, D-Man, and D-GlcNAc. Lectins binding alpha-L-fucose do not induce agglutination. The identification of some receptors and the significance of these results in terms of the known complexities of lectin specificities is discussed.

Agglutination↗

PubMed1990

Identification of a C-terminal protein carboxyl methyltransferase in rat liver membranes utilizing a synthetic farnesyl cysteine-containing peptide substrate.

Polypeptides synthesized in eucaryotic cells with a C-terminal -Cys-Xaa-Xaa-Xaa (-CXXX) sequence are candidates for post-translational modifications that include the removal of the last 3 amino acids and the lipidation and methyl esterification of the cysteinyl residue. To characterize the methylation reaction in vitro, the peptide Leu-Ala-Arg-Tyr-Lys-Cys (LARYKC) and its S-isoprenylated and S-alkylated derivatives were synthesized and assayed as methyl-accepting substrates with subcellular fractions of rat tissues including liver microsomal membranes. While little or no peptide-specific methyltransferase activity was detected in the latter preparation using the unmodified hexapeptide, the C10, C15, and C20 isoprenylated derivatives were substrates with Km values of 389 microM for S-geranyl-LARYKC, 2.2 microM for S-farnesyl-LARYKC, and approximately 10.9 microM for S-geranylgeranyl-LARYKC. The methyl-acceptor activities of a variety of n-alkyl S-derivatives of LARYKC (C8, C10, C13, C15) were also tested; all of these compounds were poorer substrates than the S-geranyl derivative. This enzyme activity uses S-adenosyl-L-methionine as the methyl donor (Km = 2.1 microM) and can be inhibited by S-adenosylhomocysteine (Ki = 9.2 microM), a product of the methylation reaction. The S-farnesyl-LARYKC peptide can inhibit the carboxyl methylation of bovine retinal rod outer segment membrane proteins that was previously shown to occur at the alpha-carboxyl group of C-terminal cysteine residues, demonstrating that the same enzyme can methylate both peptides and proteins. These results suggest that the methyl esterification of proteins containing a C-terminal -CXXX sequence requires not only the removal of the 3 terminal amino acids, but the isoprenylation of the sulfhydryl group as well.

Amino Acid Sequence↗

PubMed1997

Biochemical characterization of cell specific enzymes in light-exposed rat retinas: oxidative loss of all-trans retinol dehydrogenase activity.

PURPOSE: To determine the effect of acute, intense, visible light on the activities of all-trans retinol dehydrogenase (t-RDH) and glutamine synthetase (GS), two oxidatively sensitive enzymes located in the photoreceptors and Müller cells, respectively. METHODS: Male albino rats, previously maintained in a weak cyclic light- or dark-rearing environment, were exposed to intense light (490-580 nm) for as long as 24 hrs. One-half of the experimental animals were pre-treated with the antioxidant 1,3-dimethylthiourea (DMTU), at 500 mg/kg, i.p., 24 hrs before and just before light exposure. Upon sacrifice, retinas were excised for the determination of t-RDH and GS activity, or for the preparation of rod outer segments (ROS). Other light-exposed rats were maintained in darkness for 2 weeks before sacrifice, for rhodopsin determinations. Retinal homogenates were also treated in vitro under oxidizing conditions to compare enzymatic inactivation with the in vivo effects of light exposure. RESULTS: In cyclic light-reared rats 24 hr light exposures resulted in a significant loss of t-RDH activity in retinal homogenates and in isolated ROS. In both the retina and ROS, pretreatment of the animals with DMTU completely prevented the loss of t-RDH activity. As measured by rhodopsin levels 2 weeks after light exposure, DMTU-treated rats exhibited no loss of photoreceptor cells, whereas those not given the antioxidant lost over 50% of their photoreceptors. Retinal GS activity was unchanged by 24 hr intense light exposures. In dark-reared rats 4 hr light exposures did not alter retinal t-RDH or GS activity, despite the loss or approximately 70% of the rhodopsin content or the eye, measured 2 weeks later. When 4 hr light-exposed rats were held in darkness for an additional 20 hrs, a significant loss of retinal t-RDH occurred, but no change in GS activity was measured. In these rats DMTU treatment also prevented the loss of t-RDH activity. In contrast to the lack of an in vivo light effect on retinal GS, oxidation in vitro completely inactivated the enzyme after only 1 hr. CONCLUSIONS: The light-induced loss of t-RDH in both cyclic light- and dark-reared rats is an oxidative and time dependent process that is not strictly photochemical in nature. The loss of rhodopsin and t-RDH activity, but not GS activity, following intense light exposure are manifestations of light's effect on photoreceptor cells without a comparable effect in the adjacent retinal Müller cells. Additional work will be needed to understand the differences in light damage susceptibility between retinal photoreceptors and glial cells and between cyclic light- and dark-reared rats.

Alcohol Oxidoreductases↗

PubMed1989

Lipofuscin-like substances accumulate rapidly in brain, retina and internal organs with cysteine protease inhibition.

The protease inhibitor, leupeptin, has been previously shown to cause an accumulation of lysosomally associated intracytoplasmic dense bodies resembling lipofuscin when administered intraventricularly to the brains of young rats (Ivy et al., Science, 226:985, 1984). These findings support the idea that lipofuscin formation during normal aging involves perturbed proteolytic degradation. In the current study, we delineate more precisely the proteolytic mechanisms in question and examine the effects of protease inhibition on other tissues and species. Four sets of experiments were done. In the first, rats were administered leupeptin (L; an inhibitor of cysteine and some serine proteases), E-64C (a cysteine protease inhibitor), chloroquine (C; a lysosomal enzyme inhibitor), aprotinin (A; a serine protease inhibitor) or physiological saline (S) into the lateral ventricle of the brain at a constant rate for two weeks using an osmotic mini-pump. In the second set, beagle dogs were administered L or S intraventricularly in a similar fashion. In the third set, young rats received intraocular injections of L, E64C, C, A or S from 1 to 9 days at 24 hr intervals. In the fourth set, young rats and mice were administered L, E-64C or S intraperitoneally via an osmotic mini-pump for 2 or more weeks. The tissue of all animals was processed for light and electron microscopy as well as for fluorescence analysis. In rat brain cells (especially hippocampus and cerebellum), substances which by morphological, histochemical and fluorescence emission criteria resemble lipofuscin, accumulated following L, C or E64C, but not A or S treatment. A similar buildup of lipofuscin-like substance occurred in canine brain following L but not S infusion. In retinal pigment epithelial cells, undigested rod outer segment discs accumulated after L, C or E-64C and possibly A, but not after S injections. With increasing survival times, the accumulated substances became morphologically more similar to lipofuscin. Cells of liver (both hepatocytes and non-parenchymal cells) and kidney (mainly renal tubule cells) also accumulated substances resembling lipofuscin in response to L but not S infusion; other internal organs were differentially affected. These results demonstrate 1) that specific inhibition of cysteine proteases is sufficient to cause accumulation of lipofuscin-like substances in brain and retina and 2) that several species and organ systems are similarly affected by protease inhibition. Taken together, these findings are consistent with the idea that defective or decreased proteolysis by lysosomal cysteine proteases is responsible for lipofuscin accumulation during normal aging.

Aging↗

PubMed1995

Ultrastructural features of retinal dystrophy in mutant vitiligo mice.

Ultrastructural features of the retinal pigment epithelium and photoreceptor cells were studied in vitiligo (C57BL/6 mivit/mivit) mice. Eyes from 12-day- to 56-week-old animals were analysed. Abnormal photoreceptors were seen in 12-day-old mice. By 3 weeks malformed outer segments were evident in the posterior and equatorial retina, but normal photoreceptors were present in the periphery. By 28 weeks, a marked gradient in cell loss was evident, with a progressive increase in cell viability along the posterior-peripheral axis. Viable intact photoreceptors were still present in the peripheral retina of 56-week-old mice. Melanosome content varied between adjacent pigment epithelium cells in both the posterior and peripheral retina. In the choroid, however, a steep posterior-peripheral gradient in melanosome content was evident with highest pigmentation in the periphery. In the optic nerve head region abnormal development of photoreceptors was correlated with proliferation of abnormal pigment epithelium cells. Accumulation of rod outer segment debris in the posterior and peripheral subretinal space preceded photoreceptor cell death. Short pigment epithelial microvilli without proper attachment to photoreceptors are suggestive of alterations in pigment-epithelium-photoreceptor interaction, which might affect photoreceptor differentiation and phagocytosis of rod outer segments by pigment epithelium cells.

Animals↗