The localization and timing of post-translational modifications of rat rhodopsin

Retinal Degeneration Caused by Rod-Specific Dhdds Ablation Occurs without Concomitant Inhibition of Protein N-Glycosylation

Dehydrodolichyl diphosphate synthase (DHDDS) catalyzes the committed step in dolichol synthesis. Recessive mutations in DHDDS cause retinitis pigmentosa (RP59), resulting in blindness. We hypothesized that rod photoreceptor-specific ablation of Dhdds would cause retinal degeneration due to diminished dolichol-dependent protein N-glycosylation. Dhdds^flx/flx mice were crossed with rod-specific Cre recombinase-expressing (Rho-iCre75) mice to generate rod-specific Dhdds knockout mice (Dhdds^flx/flx iCre⁺). In vivo morphological and electrophysiological evaluation of Dhdds^flx/flx iCre⁺ retinas revealed mild retinal dysfunction at postnatal (PN) 4 weeks, compared with age-matched controls; however, rapid photoreceptor degeneration ensued, resulting in almost complete loss of rods and cones by PN 6 weeks. Retina dolichol levels were markedly decreased by PN 4 weeks in Dhdds^flx/flx iCre⁺ mice, relative to controls; despite this, N-glycosylation of retinal proteins, including opsin (the dominant rod-specific glycoprotein), persisted in Dhdds^flx/flx iCre⁺ mice. These findings challenge the conventional mechanistic view of RP59 as a congenital disorder of glycosylation.

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Deletion of Dhdds in rod cells caused rapid retinal degeneration in mice

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Retinal dolichol levels markedly decreased before onset of degeneration

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Protein N-glycosylation was uncompromised despite Dhdds deletion

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Degeneration also involved gliosis, microglial activation, and phagoptosis

Complex Structural PPT1 Variant Associated with Non-syndromic Canine Retinal Degeneration

Rod and cone photoreceptors are specialized retinal neurons that have a fundamental role in visual perception, capturing light and transducing it into a neuronal signal. Aberrant functioning of rod and/or cone photoreceptors can ultimately lead to progressive degeneration and eventually blindness. In man, many rod and rod-cone degenerative diseases are classified as forms of retinitis pigmentosa (RP). Dogs also have a comparable disease grouping termed progressive retinal atrophy (PRA). These diseases are generally due to single gene defects and follow Mendelian inheritance.We collected 51 DNA samples from Miniature Schnauzers affected by PRA (average age of diagnosis ∼3.9 ±1 years), as well as from 56 clinically normal controls of the same breed (average age ∼6.6 ±2.8 years). Pedigree analysis suggested monogenic autosomal recessive inheritance of PRA. GWAS and homozygosity mapping defined a critical interval in the first 4,796,806 bp of CFA15. Whole genome sequencing of two affected cases, a carrier and a control identified two candidate variants within the critical interval. One was an intronic SNV in HIVEP3, and the other was a complex structural variant consisting of the duplication of exon 5 of the PPT1 gene along with a conversion and insertion (named PPT1_dci). PPT1_dci was confirmed homozygous in a cohort of 22 cases, and 12 more cases were homozygous for the CFA15 haplotype. Additionally, the variant was found homozygous in 6 non-affected dogs of age higher than the average age of onset. The HIVEP3 variant was found heterozygous (n = 4) and homozygous wild-type (n = 1) in cases either homozygous for PPT1_dci or for the mapped CFA15 haplotype. We detected the wildtype and three aberrant PPT1 transcripts in isolated white blood cell mRNA extracted from a PRA case homozygous for PPT1_dci, and the aberrant transcripts involved inclusion of the duplicated exon 5 and novel exons following the activation of cryptic splice sites. No neurological signs were detected among the dogs homozygous for the PPT1_dci variant. Therefore, we propose PPT1_dci as causative for a non-syndromic form of PRA (PRA_PPT1) that shows incomplete penetrance in Miniature Schnauzers, potentially related to the presence of the wild-type transcript. To our knowledge, this is the first case of isolated retinal degeneration associated with a PPT1 variant.

Targeting of mouse guanylate cyclase 1 (Gucy2e) to Xenopus laevis rod outer segments

Photoreceptor guanylate cyclase (GC1) is a transmembrane protein and responsible for synthesis of cGMP, the secondary messenger of phototransduction. It consists of an extracellular domain, a single transmembrane domain, and an intracellular domain. It is unknown how GC1 targets to the outer segments where it resides. To identify a putative GC1 targeting signal, we generated a series of peripheral membrane and transmembrane constructs encoding extracellular and intracellular mouse GC1 fragments fused to EGFP. The constructs were expressed in Xenopus laevis rod photoreceptors under the control of the rhodopsin promoter. We examined the localization of GFP-GC1 fusion proteins containing the complete GC1 sequence, or partial GC1 sequences, which were membrane-associated via either the GC1 transmembrane domain or the rhodopsin C-terminal palmitoyl chains. Full-length GFP-GC1 targeted to the rod outer segment disk rims. As a group, fusion proteins containing the entire cytoplasmic domain of GC1 targeted to the OS, whereas other fusion proteins containing portions of the cytoplasmic or the extracellular domains did not. We conclude that GC1 likely has no single linear peptide-based OS targeting signal. Our results suggest targeting is due to either multiple weak signals in the cytoplasmic domain of GC1, or co-transport to the OS with an accessory protein.

Rhodopsin: the functional significance of asn-linked glycosylation and other post-translational modifications

Rhodopsin, the G-protein coupled receptor in retinal rod photoreceptors, is a highly conserved protein that undergoes several types of post-translational modifications. These modifications are essential to maintain the protein's structure as well as its proper function in the visual transduction cycle. Rhodopsin is N-glycosylated at Asn-2 and Asn-15 in its extracellular N-terminal domain. Mutations within the glycosylation consensus sequences of rhodopsin cause autosomal dominant retinitis pigmentosa, a disease that leads to blindness. Several groups have studied the role of rhodopsin's N-linked glycan chains in protein structure and function using a variety of approaches. These include the generation of a transgenic mouse model, study of a naturally occurring mutant animal model, in vivo pharmacological inhibition of glycosylation, and in vitro analyses using transfected COS-1 cells. These studies have provided insights into the possible role of rhodopsin glycosylation, but have yielded conflicting results.

Induction of cystine-glutamate transporter xc- by human immunodeficiency virus type 1 transactivator protein tat in retinal pigment epithelium

PURPOSE

The transactivator protein Tat encoded by the human immunodeficiency virus-1 (HIV-1) genome reduces glutathione levels in mammalian cells. Because the retina contains large amounts of glutathione, a study was undertaken to determine the influence of Tat on glutathione levels, gamma-glutamyl transpeptidase activity, and the expression and activity of the cystine-glutamate transporter xc- in the human retinal pigment epithelial cell line ARPE-19 and in retina from Tat-transgenic mice.

METHODS

The transport function of xc- was measured as glutamate uptake in the absence of Na+. mRNA levels for xCT and 4F2hc, the two subunits of system xc-, were monitored by RT-PCR and Northern blot and protein levels by Western blot. The expression pattern of xCT and 4F2hc in the mouse retina was analyzed by immunofluorescence.

RESULTS

Expression of Tat in ARPE-19 cells led to a decrease in glutathione levels and an increase in gamma-glutamyl transpeptidase activity. The transport function of xc- was upregulated, and this increase was accompanied by increases in the levels of mRNAs for xCT and 4F2hc and in corresponding protein levels. The influence of Tat on the expression of xc- was independent of the cellular status of glutathione. Most of these findings were confirmed in the retina of Tat-transgenic mice.

CONCLUSIONS

Expression of HIV-1 Tat in the retina decreases glutathione levels and increases gamma-glutamyl transpeptidase activity. Tat also upregulates the expression of system xc-. Glutathione levels may be decreased and the expression of xc- enhanced in the retina of patients with HIV-1 infection, leading to oxidative stress and excitotoxicity.

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.

Depalmitoylation of rhodopsin with hydroxylamine

Intrinsic membrane proteins that to date have been investigated with respect to the function of palmitoylation are the beta-adrenergic receptor, rhodopsin, the alpha 2A-adrenergic receptor, and the influenza virus spike glycoprotein. As described above, the studies have led to differing conclusions with respect to the influence of palmitoylation on physiological activity. The basis of the differences remains unclear, but it may relate at least in part to the membrane environment of the protein during these studies, that is, the presence of a native membrane, the membrane composition of the expression cell line (in the case of mutant proteins), or the absence of membrane (in the case of detergent-purified proteins). For example, in the case of rhodopsin, the composition of the ROS disk membrane differs from that of the rod plasma membrane, and presumably also from the plasma membranes of cell lines in which mutant rhodopsins are expressed. Variation in membrane composition is known to have marked effects on the ability of rhodopsin to mediate the photic activation of PDE. Thus, although Karnik et al. clearly demonstrated the absence of an absolute requirement for palmitate in activating transducin, the influence of detergent on tertiary protein structure may have masked the full effect of the elimination of palmitate on the transducin-activating property of rhodopsin. Alternatively, the differing results obtained in the studies of rhodopsin could be a consequence of differences in amino acid sequences of the proteins studied. The precise functional role of the palmitate groups of rhodopsin remains an important question for further research. It was suggested by Ovchinnikov et al. that the hydrolysis of covalently bound palmitate might occur during the process of rhodopsin bleaching, but more recent data argue against this hypothesis. Experiments using synthetic peptides (representing cytoplasmic loop regions of rhodopsin) to identify the sites of interaction of R* and transducin provide support for an alternative possibility, namely, that palmitoylation and the resulting cytoplasmic loop play a role in the coupling of rhodopsin with transducin. The finding that the binding of transducin to R* occurs independently of the presence of palmitate argues against an essential requirement of palmitoylation on the binding step itself. However, available data indicate an enhancement, by depalmitoylation, of light-dependent GTPase activity in ROS preparations, although not in assays of unpalmitoylated, purified mutant rhodopsins (see above).(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of beta-galactosidase activity in purified bovine retinal rod outer segments

We have identified beta-galactosidase activity in purified bovine rod outer segments (ROS), using rho-nitrophenyl-beta-D-galactopyranoside (PNPG) and chlorophenol red-beta-D-galactopyranoside (CPRG) as substrates. This glycosylhydrolase activity did not appear to represent contamination from other retinal subcellular fractions, based upon the relative specific activities of beta-galactosidase vs. other hydrolases (N-acetyl-beta-glucosaminidase, alpha- and beta-mannosidase, alpha-fucosidase, and acid phosphatase) in bovine retina and ROS homogenates. Using PNPG as a substrate, two pH optima were observed (at 3.5 and 5.5), while the hydrolysis of CPRG exhibited a single, broad pH optimum centered at 5.5. In contrast, hydrolysis of PNPG and CPRG by retinal homogenates exhibited single pH optima, at 3.5 and 5.5., respectively. ROS beta-galactosidase activity increased linearly with time, temperature, and protein concentration, and obeyed Michaelis-Menten kinetics with both substrates. For PNPG, Vmax approximately 88 nmol/h/mg protein and the apparent Km approximately 147 microM. For CPRG, Vmax approximately 33 nmol/h/mg protein and the apparent Km approximately 50 microM. ROS beta-galactosidase activity was affected by carbohydrates and their derivatives: glucose, fucose, sucrose, maltose and N-acetyl-galactosamine were found to stimulate the activity, while D-galactono-gamma-lactone and, to a lesser extent, D-galactose were inhibitory. The enzyme activity also was slightly stimulated by [Cl-] and markedly by dithiothreitol (DTT), while rho-chloro-mercuribenzoic acid (PCMB) and rho-hydroxymercuribenzoic acid (PHMB) inactivated the enzyme. In addition, the enzymatic activity was also found to be differentially sensitive to various anionic and nonionic detergents. However, n-octyl-beta-D-glucoside was slightly stimulatory.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence and foveal enrichment of rod opsin in the "all cone" retina of the American chameleon

The retinal photoreceptors of the eye of the American chameleon, Anolis carolinensis, have been considered to be exclusively cones. Its retina is unusual for possessing two foveas (areas associated with heightened visual acuity), with the major, central fovea deeply incised and very densely packed with photoreceptors. Immunoblotting and light- and electron microscopic-immunocytochemistry, using several opsin monoclonal antibodies previously found specific for rods, demonstrated the presence and localization of this protein in the Anolis retina. This visual pigment appears sparsely in a subpopulation of photoreceptors in the periphery but overwhelmingly in the central fovea. Complementary results with cone-specific antibody and lectin binding corroborated this spatial organization. These results, as well as those with geckos, suggest that photoreceptor morphology is not an accurate guide among the lacertilians to visual pigment content, and that this phylogenetic grouping may constitute a crossroads in vertebrate photoreceptor evolution.

Structural and functional properties of rhodopsin from rod outer segment disk and plasma membrane

The structural and functional properties of bovine rhodopsin from rod outer segment disk and plasma membranes were compared by high performance liquid chromatography (HPLC), mass spectrometric analyses, and in vitro rhodopsin phosphorylation assays. Disk and plasma membranes separated by a ricin gold-dextran affinity perturbation method were treated with trypsin or cyanogen bromide, and the N-terminal and C-terminal rhodopsin peptides were isolated by immunoaffinity chromatography using antirhodopsin monoclonal antibodies coupled to Sepharose. Reverse phase HPLC chromatograms of the C-terminal and N-terminal peptides from disk and plasma membrane rhodopsin were found to be similar. Mass spectrometric, PicoTag, and hexose analyses of the tryptic 1-16 N-terminal peptides further indicated that the post-translational glycosylation of plasma membrane rhodopsin is identical to that of disk membrane rhodopsin. HPLC analysis of soluble peptides obtained from cyanogen bromide and tryptic digestion of immunoaffinity purified rhodopsin also indicated that no significant differences exist between disk and plasma membrane rhodopsin. Light-induced phosphorylation of rhodopsin in disk and plasma membranes were also compared using in vitro phosphorylation assays. Plasma membrane rhodopsin was found to undergo light-dependent, rhodopsin kinase catalyzed phosphorylation to the same extent as disk membrane rhodopsin. These results indicate that the bulk rhodopsin in rod outer segment plasma membranes appears to be identical to rhodopsin in disk membranes in regard to primary structure, post-translational glycosylation and light-dependent phosphorylation. On this basis, it is unlikely that the sorting of rhodopsin between disk and plasma membranes occurs by a mechanism based on differences in structural properties of rhodopsin.

In vitro galactosylation of rhodopsin and opsin: kinetics, properties and characterization

At best, only trace amounts of galactose have been detected as constituents of rhodopsin as analysed by several laboratories. Nevertheless, the enzymatic galactosylation of rhodopsin proceeds readily in vitro, a process which can be catalysed by galactosyltransferases from several sources. Little information is available, however, concerning the properties of the in vitro reaction. We have examined characteristics of the latter process with the hope of shedding light on the capacity of the retina to carry out this reaction. Kinetic properties of the galactosyltransferases of bovine and embryonic chick retinas, bovine milk and rat liver-Golgi were examined using rhodopsin, opsin, N-acetylglucosamine and ovalbumin as exogenous acceptors. All of these studies demonstrated the very limited activity of the galactosyltransferases of the retina as compared to the milk and rat liver systems. The subcellular distribution of the galactosyltransferases of bovine retina was examined. The influence of compounds that might modulate the reaction was also examined. alpha-Lactalbumin, a modifier of the galactosyltransferase in milk, acted as a competitive inhibitor of the galactosylation of opsin. Analogs of vitamin A, shown to inhibit galactosyltransferases in other systems, did not have this effect on the galactosylation of opsin. Similarly, mixing experiments could not demonstrate the presence of endogenous material that inhibited the reaction in the retina. The conformation of the visual pigment was shown to influence the reaction. After bleaching by visible light, opsin was preferred over rhodopsin as an acceptor of galactose by the galactosyltransferases of bovine and embryonic chick retinas and by rat liver. This distinction was only minimally demonstrated by the milk enzyme. The galactosylation of ovalbumin was unaffected by conditions of light or dark by any of the enzymes. While the mode ratio of galactose to rhodopsin after catalysis by the milk enzyme was about 1.6, this ratio was only about 0.01 after reaction with the enzyme from bovine retina. The linkage of galactose in enzymatically galactosylated rhodopsin and opsin was beta(1-4).

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.

Transient hyperglycosylation of rhodopsin with galactose

Rhodopsin's oligosaccharide chains contain predominantly two types of sugar residues: mannose and N-acetylglucosamine. In the present work, bovine and rat rhodopsin were analysed biochemically for the presence of a third sugar, galactose. Treatment of bovine rod outer segments (ROS) with galactose oxidase followed by reduction with tritium-labeled sodium borohydride revealed the presence of existing molecules of galactose on rhodopsin. Rats injected intravitreally with [3H]galactose and [14C]leucine and maintained in darkness were killed 1 hr, 6 hr, 1, 3 or 5 days following the injection. Retinas were collected for subcellular fractionation and rhodopsin from each of the fractions was purified by ConA sepharose chromatography and SDS-PAGE. During the first 6 hr, galactose selectively labeled rhodopsin in the Golgi-enriched fraction resulting in increased [3H]/[14C] ratios in both Golgi and ROS. The data suggested that trimming was occurring at the transition from Golgi to ROS. Furthermore, a decrease in isotope ratio in the ROS between 6 hr and 1 day suggested further trimming of rhodopsin after membrane assembly in the ROS. Additional in vivo experiments demonstrated existing molecules of galactose on rhodopsin's oligosaccharide chain using lectin affinity chromatography. Rats injected intravitreally with [35S]methionine were dark-adapted for 2 hr. Following subcellular fractionation of retinas, ConA purified rhodopsin from ROS was applied to one of two additional lectin columns: Ricinus communis agglutinin (RCA) or Griffonia simplicifolia I (GSA). Eight to nine percent of the labeled rhodopsin was bound to and eluted from RCA, whereas none bound to GSA, indicating the presence of a beta-galactoside. The RCA agarose eluted protein co-electrophoresed with a rhodopsin standard and was light sensitive. Galactose was shown to be the terminal sugar on this subset of rhodopsin and was not capped by neuraminic acid. Binding of rhodopsin's oligosaccharide to RCA was abolished by pre-treatment with beta-galactosidase. Decreased binding of rhodopsin to RCA was observed following intravitreal injection of castanospermine but not swainsonine. Of those two inhibitors of glycoprotein trimming, only castanospermine would be expected to prevent the addition of galactose to the oligosaccharide. The association of galactose with rat rhodopsin appeared to be a transient one. At 2 hr, 8-9% of rhodopsin contained galactose, at 6 hr only 2.2% had galactose and by 24 hr less than 1% did. The galactose was trimmed from rhodopsin's oligosaccharide presumably after its role was complete. Separation of rhodopsin of the plasma membranes from rhodopsin of discs indicated that 75% of the galactose-containing rhodopsin was in the plasma membrane and only 25% was in the discs. These findings suggested a possible role for galactose in new disc formation with subsequent removal after the discs are sealed.