Retinal S antigen identified as the 48K protein regulating light-dependent phosphodiesterase in rods

Experimental autoimmune uveoretinitis: idiotypic regulation and disease suppression

Experimental autoimmune uveoretinitis (EAU), a mostly T-cell dependent disease, was induced in laboratory animals by a single immunization with retinal extract or purified S-antigen in complete Freund's adjuvant. It can be prevented or suppressed by injections of either the autoantigen or monoclonal antibodies against the autoantigen. The suppression of EAU by these antibodies was associated with an anti-idiotypic antibody response. The inhibition of the pathogenic immune response by the antigen or the antibodies could be explained, according to Jerne's hypothesis of immunoregulation, by a disturbance of the network of idiotype and anti-idiotype interactions.

Immunological detection of arrestin, a phototransduction regulatory protein, in the cytosol of nucleated erythrocytes

Cytosolic extracts of trout and turkey erythrocytes were tested for their immunoreactivity with polyclonal and monoclonal antibodies to retinal arrestin (S-antigen), a cytosolic protein of photoreceptor cells involved in the desensitization of rhodopsin. After adsorption or immunoaffinity chromatography of the extracts, these antibodies specifically recognized a protein having a molecular weight similar to that of retinal arrestin. Because the G-protein-mediated transduction systems, such as visual and beta-adrenergic systems, display a high degree of structural and functional homology, the presence of arrestin-like proteins in non-photosensitive cells suggests that these proteins are involved in the transduction of chemical signals, with a possible role in receptor desensitization.

Melanotic ganglioglioma of the pineal region

A melanotic ganglioglioma was biopsied in the pineal region of a 12-month-old girl who preoperatively underwent a ventriculo-peritoneal shunt for hydrocephalus and postoperatively received radiotherapy. The tumor was subtotally excised when the girl was 7 years and 4 months of age. Histologically, it demonstrated mature neurons in disorganized clusters and in well-differentiated cerebrum-like tissue, rare binucleated neurons, glia similar to normal gray matter, and bands of fibrous tissue containing heavily pigmented cells. Ultrastructurally, melanosomes of stages I to IV were identified in the pigmented cells. An origin involving retinal differentiation of the primitive pineal gland was not supported; the tumor was negative for both retinal S-antigen (MAbA9-C6) and cellular retinal-binding protein (CRALBP). This report demonstrates the ability of a cerebral neoplasm to contain neurons, glia, and melanin-containing cells; all of which are neuroectodermally derived.

Arrestin of bovine photoreceptors reveals strong ATP binding

The soluble protein arrestin (also named 48K-protein or retinal-S-antigen) is involved in controlling light-dependent transducin and cGMP phosphodiesterase activity in retinal rods. It is also known for its ability to induce autoimmune reactions in the eye causing the eye disease uveitis. We report here a rapid binding of ATP to arrestin with KA = 2 x 10(21) (l/mol)3 and a coordination number n = 3. This ATP binding to arrestin supports the notion of a nucleotide exchange which initiates the rapid inhibitory action of this enzyme during the primary step of vertebrate phototransduction.

Rat T-cell lines specific to a nonimmunodominant determinant of a retinal protein (IRBP) produce uveoretinitis and pinealitis

Rat lymphocyte lines were established, with specificity toward two synthetic peptides derived from the interphotoreceptor retinoid-binding protein (IRBP), which specifically localizes in the retina and pineal gland. One of the peptides, R4, is immunopathogenic, producing experimental autoimmune uveoretinitis (EAU) and pinealitis (EAP) in immunized rats, while the other peptide, R3, exhibits no detectable immunopathogenicity in rats. The cell lines carry surface markers specific for the helper/inducer subset of T-lymphocytes. When tested by the proliferation assay, the line cells demonstrated major histocompatibility-restricted vigorous responses against the immunizing (homologous) peptide, but failed to recognize the intact IRBP molecule. This finding is in line with other data indicating that peptides R3 and R4 are nonimmunodominant determinants of IRBP for the Lewis rat. Yet, the cell lines specific for R4 were highly immunopathogenic, producing EAU and EAP in naive rats at numbers as low as 0.25 x 10(6), with histopathological changes similar to those induced by active immunization with this peptide. The immunological capacity of the cell lines was further demonstrated by the finding that spleen cells from recipient rats of these lines responded well against the homologous peptides. The uniqueness of this system, in which lymphocytes specific toward a nondominant determinant are immunopathogenic, is underscored and the possible mechanisms of disease induction are discussed.

Lymphocyte proliferative responses of patients with ocular toxoplasmosis to parasite and retinal antigens

In vitro lymphocyte proliferative responses to purified toxoplasma antigens and the retinal S-antigen were evaluated in 40 patients with ocular toxoplasmosis; 16 (40%) had a positive in vitro response to the retinal S-antigen. The proliferative responses to the 2G11 (p22) toxoplasma membrane antigen approached that of a crude antigen preparation of Toxoplasma gondii, whereas the response to the 1E11 immunodominant p30 membrane antigen was considerably less striking. HLA typing was also performed, and none correlated to S-antigen or toxoplasma reactivity. The evidence strongly suggests that patients with a clearly infectious ocular disorder may have part of their disease mediated by a putative autoimmune mechanism. An immune response to the 2G11 toxoplasma antigen may increase the risk of developing ocular disease or a substrain of toxoplasma with 2G11 as its immunodominant membrane antigen may have a greater affinity for the retina.

Immunoreactive S-antigen in cerebrospinal fluid: a marker of pineal parenchymal tumors?

This investigation evaluated the possibility that the occurrence of S-antigen in cerebrospinal fluid (CSF) might be used as a preoperative marker of pineal parenchymal tumors (pineoblastoma and pineocytoma). Such a marker could provide a means of preoperatively differentiating these neoplasms from pineal region tumors of other origin. The S-antigen, also known as the 48-kD protein or arrestin, is a highly antigenic protein originally found in the retina and pineal gland. In the retinal photoreceptors and submammalian pineal photoreceptors the protein is thought to be involved in phototransduction; its function in the mammalian pinealocyte is unknown. S-Antigen immunoreactivity also occurs in certain neoplastic cells of retinoblastomas, pineocytomas, pineoblastomas, and cerebellar medulloblastomas. This study included a group of 13 patients with tumors of the pineal region. Samples of CSF were obtained preoperatively and analyzed for the S-antigen using western blot technology. Tumor biopsy material was classified according to conventional neurohistological criteria and was also examined by immunocytochemical techniques for the presence of the S-antigen. S-Antigen immunoreactivity was found in the preoperative CSF of the one patient found to have pineocytoma; tumor tissue removed from this patient was the only neoplastic tissue examined in this study which contained S-Antigen immunoreactive tumor cells. Furthermore, hydroxyindole-O-methyltransferase activity was detectable in the pineocytoma but not in three other pineal tumors, and melatonin levels in the CSF of the pineocytoma patient were the highest in the patient group examined. These preliminary results suggest that testing for S-antigen in CSF might be useful in characterizing and treating tumors of the pineal region and, when identified in conjunction with other markers, it might also help to better define pineal parenchymal tumors. This study needs confirmation with a larger number of patients. If this approach is eventually found to be a reliable predictor of pineal cell tumors, it may supplant the need for surgical biopsies before initiating appropriate adjunctive therapy.

Sequence homology between yeast histone H3 and uveitopathogenic site of S-antigen: lymphocyte cross-reaction and adoptive transfer of the disease

Experimental autoimmune uveitis (EAU) serves as an animal model of ocular inflammation. The disease is caused by the immunization of microgram amounts of a soluble retinal protein, designated S-antigen, in susceptible animal strains, including primates. We induced EAU and experimental autoimmune pinealitis (EAP) in Lewis rats with a small synthetic peptide corresponding to amino acid positions 106-121 in yeast histone H3. This peptide contains five consecutive amino acids identical to a uveitopathogenic site (peptide M) in human S-antigen. Lymph node or mononuclear cells from different species of animals immunized either with histone H3 or with peptide M showed significant cross-reaction as measured by in vitro lymphocyte mitogenesis assay using [3H]thymidine. Also, we adoptively transferred the EAU and EAP in naive rats by immune lymph node cells. These findings support the fact that selected bacterial, viral, or fungal proteins with amino acid sequence homologies to normal retinal proteins are uveitopathogenic and, as such, provide a basis for autoimmune inflammatory diseases.

Kinetics, binding constant, and activation energy of the 48-kDa protein-rhodopsin complex by extra-metarhodopsin II

We have found that the 48-kDa protein (or S-antigen 48k) of the rod photoreceptor enhances the light-induced formation of the photoproduct metarhodopsin II (MII) from prephosphorylated rhodopsin. The effect is analogous to the known enhancement of MII (extra-MII) that results from selective interaction of MII with G-protein. We have determined some parameters of the MII-48k interaction by measuring the extra-MII absorption change induced by the 48-kDa protein. The amplitude saturation yields a dissociation constant for the MII-48k complex on the order of 50 nM. At the technical limit of these measurements, 13.7 degrees C and 12 microM 48-kDa protein, we find a rate of 2.3 s-1 for formation of the 48k-MII complex. Extrapolation of these values to cellular conditions yields an occupation time of phosphorylated MII by 48k less than 200 ms. This is short compared to estimated rates of phosphorylation. The temperature dependence of the MII-48k formation rate is very high (Q10 for 5 degrees C/15 degrees C = 9-10). The related Arrhenius activation energy (165 kJ mol-1) is correspondingly high and indicates a considerable transient chemical change during the binding process.

ADP-ribosylation of bovine S-antigen by cholera toxin

The S-antigen (alias 48K protein or arrestin) of bovine rod photoreceptors contains two stretches of amino acid sequence homologous to the ADP-ribosylation sites of the alpha subunit of transducin (Ta). We have found that cholera toxin transfers the ADP-ribosyl group from NAD to purified bovine S-antigen as well as to S-antigen in rod outer segment membranes, while Bordetella pertussis toxin is unable to catalyze the transfer reaction efficiently. Under the same conditions, both toxins catalyzed ADP-ribosylation of Ta in rod outer segments. The ADP-ribosylation of S-antigen by cholera toxin indicates that S-antigen not only exhibits sequence homology with the ADP-ribosylation sites of Ta, but it must also resemble Ta in the tertiary structure of the domain which determines the susceptibility of S-antigen to the catalytic action of cholera toxin. These results suggest that S-antigen may function as a competitor of Ta in some stage of the cGMP cascade of visual transduction.

Epitope mapping of bovine retinal S-antigen with monoclonal antibodies

We examined the binding of seven murine monoclonal antibodies raised to S-antigen, an immunopathogenic, 404 residue photoreceptor cell autoantigen which induces experimental autoimmune uveoretinitis. S-antigen has also been identified as arrestin, a protein involved in the regulation of phototransduction. One additional monoclonal antibody (C10C10), raised to a synthetic peptide (peptide N) corresponding to residues 281 to 302 in bovine S-antigen, was also studied. In preliminary studies we examined the specificity of the antibody response to bovine S-antigen in sera from Balb/c mice. Western blots of mouse sera on the cyanogen bromide digest of bovine S-antigen demonstrated that all animals produced antibody which recognized epitopes within the C-terminal cyanogen bromide peptide designated CB46. Mice of the H-2d haplotype, including the Balb/c strain often used to produce monoclonal antibodies, showed little activity to cyanogen bromide peptides other than CB46. Also, all seven of the monoclonals raised to S-antigen are specific for epitopes in the CB46 peptide. The epitopes recognized by the monoclonal antibodies could be grouped into four distinct sites defined by peptides AE-1 (A2G5), peptide AA (PDS-1), peptide 19-OV (A9C6), and peptide 199 (BDS-1,2,3 and 4). The mono-clonal antibody, C10C10, raised to peptide N recognizes an epitope in the N peptide and binds to a larger cyanogen bromide peptide designated CB123 as well as intact S-antigen. Fine mapping of these epitopes was done with various subpeptides. None of the antibodies bound the known immunopathogenic peptide, peptide M, which resides in CB123 although the C10C10 antibody binds a peptide adjacent to peptide M.

The sequence of human retinal S-antigen reveals similarities with alpha-transducin

The complete amino acid sequence of human retinal S-antigen (48 kDa protein), a retinal protein involved in the visual process has been determined by cDNA sequencing. The largest cDNA was 1590 base pairs (bp) and it contained an entire coding sequence. The similarity of nucleotide sequence between the human and bovine is approximately 80%. The predicted amino acid sequence indicates that human S-antigen has 405 residues and its molecular mass is 45050 Da. The amino acid sequence homology between human and bovine is 81%. There is no overall sequence similarity between S-antigen and other proteins listed in the National Biomedical Research Foundation (NBRF) protein data base. However, local regions of sequence homology with alpha-transducin (T alpha) are apparent including the putative rhodopsin binding and phosphoryl binding sites. In addition, human S-antigen has sequences identical to bovine uveitopathogenic sites, indicating that some types of human uveitis may in part be related to the animal model of experimental autoimmune uveitis (EAU).

Effects of continuous light exposure on the rat retina and pineal gland

We examined the effects of intense continuous light (400 ftc for 10 days) on the morphology and immunoreactivity of the rat retina and pineal gland. The light treatment caused severe degeneration in the retina, with loss of most photoreceptor cells, and produced a marked decrease in S-antigen immunoreactivity in this organ. Unlike the retina, the light treatment had minimal effects on the histological structure of the pineal gland. However, pineal glands of light-treated rats exhibited a substantial reduction in their S-antigen immunoactivity, as demonstrated immunohistochemically and quantitatively verified by the rocket immunoelectrophoresis technique: pineal glands of light exposed rats had approximately two-thirds of the S-antigen immunoactivity of the untreated controls. Light treatment was also found to reduce both uveoretinitis and pinealitis in rats immunized with S-antigen. Yet pinealitis was not affected by light exposure in rats, as they were enucleated before treatment. This study thus provides new information to support the notion that the mammalian pineal gland is directly associated with light detection by the retina.

Effect of denaturization on the immunogenicity and uveitogenicity of retinal S-antigen

Previous studies have shown that alteration of pH or temperature of retina extract can affect its complement fixing reactivity with anti-S-antigen serum. To examine the effect of pH or heat on the immunogenicity and uveitogenicity of purified bovine S-antigen, guinea-pigs were injected with pH- or heat-treated S-antigen and evaluated for clinical and histopathological signs of uveoretinitis, histopathology of pineal gland, serum and intraocular S-antigen antibody reactivity, and S-antigen skin test reactivity. Guinea-pigs that received pH 7 or pH 10 treated S-antigen responded as did those that received untreated S-antigen. Guinea-pigs injected with pH 4 or heat-treated S-antigen exhibited lower incidence, later onset and less severe uveitis than those that received untreated S-antigen. Systemic responses of skin test reactivity to S-antigen were not different from those of the control group; pineal gland involvement and serum anti-S antibody reactivity were slightly reduced. Skin test responses of animals receiving treated antigen were less to the treated (injected) antigen than to untreated S-antigen. In addition, antibody responses of guinea-pigs receiving pH 4 or heat-treated antigen were less to the treated (injected) antigen than to untreated S-antigen. These results suggest that the sites on the S-antigen molecule responsible for various aspects of pathogenicity and immunogenicity do not have the same sensitivity to physical/chemical treatment and may reside on different parts of the molecule. Furthermore, the reactive sites especially for antibody and skin test reactivity, may be continuous sites.

Temporal disparity in pineal and retinal ontogeny

The development of photoreceptors and two putative neurotransmitter systems in the pineal organ and retina was studied during embryogenesis in the three-spined stickleback Gasterosteus aculeatus L. The investigation was performed by aid of immunocytochemistry using well characterized antisera to the retinal proteins alpha-transducin (TD alpha) and S-antigen (SA) (photoreceptor-markers), antisera against L-glutamic acid decarboxylase (GAD), gamma-aminobutyric acid (GABA), choline-O-acetyltransferase (ChAT) and with acetylcholinesterase (AChE) histochemistry (neurotransmitter-markers). It was possible to set up the following developmental time-table concerning the first appearance of positive immuno- and enzyme-reactive cells in the pineal organ and retina: I AChE-activity and TD alpha- and SA-immunoreactive cells in the pineal organ; II GAD- and GABA-immunoreactive cells in the pineal organ and retina; ChAT immunoreactivity and AChE activity in the retina; III hatching; IV SA-immunoreactive cells in the retina. The obtained results provide good evidence that while photoreceptor cells develop much earlier in the pineal organ than in the retina, neurons develop simultaneously in the pineal organ and retina.

The photoreceptor-specific 33 kDa phosphoprotein of mammalian retina: generation of monospecific antibodies and localization by immunocytochemistry

The distribution in mouse retina of a 33,000 Da phosphoprotein (33 kDa) that complexes with the beta/gamma subunits of transducin (T beta gamma) and undergoes light-induced dephosphorylation was determined by immunocytochemistry. An antiserum containing antibodies for the 33 kDa protein and beta-transducin of mouse and bovine retinas was generated against the purified 33 kDa-T beta gamma complex from bovine retina. The antiserum reacts with beta-transducin derived from either 33 kDa-T beta gamma complex or transducin complex (T alpha beta gamma), but not with the alpha- or gamma-transducin. It also reacts with both the phosphorylated and unphosphorylated form of the 33 kDa-T beta gamma complex. Antibodies, monospecific for the 33 kDa and beta-transducin subunits respectively, were purified from the antiserum by immunoadsorption and used in immunocytochemical analysis of the respective antigens. The 33 kDa protein was found to be associated exclusively with the photoreceptor cells of the retinas, with the most intense staining in the inner and outer segments' layers and lighter staining in the synaptic terminal layers. beta-Transducin also is found in the photoreceptors, but some T beta immunoreactivity exists within the inner plexiform layer. The specific localization of the 33 kDa protein together with its light-modulated phosphorylation suggest that the 33 kDa-T beta gamma complex is involved in light-regulated activities of the rod photoreceptor cells.

Immune mechanisms in autoimmune ocular disease

Aberrant expression of Class II MHC antigens (Ia) by non-immune cells is considered to be an important mechanism in the pathogenesis of autoimmune disease processes including those affecting the eye. It is suggested that circulating autoreactive T-cells are directed to their target organ as a result of aberrant expression of Ia antigens by the vascular endothelium of that organ. This hypothesis was tested in this study using two different models of severe ocular inflammation, induced by either S-antigen or bovine serum albumin (BSA). The retinal vascular endothelium becomes Ia + in S-antigen induced inflammation but not in inflammation induced by BSA. The accumulation in the eye of a T-cell line, ThS, specific for an ocular antigen (S-antigen), was compared in the two types of ocular inflammation and compared to that of another T-cell line, ThP, specific for a non-ocular antigen (PPD). In S-antigen induced inflammation, there was much greater accumulation of ThS than ThP whereas in BSA induced inflammation, both T-cell lines accumulated to the same extent but more than in uninflamed eyes. These results suggest that when the retinal vascular endothelium expresses Ia antigens during an inflammatory process, autoreactive T-cells will be specifically retained in the eye as a result of this and perpetuate the autoimmune destructive process.

Experimental posterior uveitis. II. Electroretinographic studies

Experimental posterior uveitis was induced by the inoculation of retinal S-antigen into black hooded Lister rats. The time course of the disease was monitored by electroretinography (ERG), and the ERG changes were correlated with clinical signs and underlying pathological damage. The ERG became supernormal in the third week after inoculation, with some loss of temporal resolution (lowered ERG flicker fusion frequency), though the disease was not clinically manifest at this time and histological examination was normal. The ERG became subnormal after 21 days as clinical signs of disease began to appear. This subnormality was associated with focal photoreceptor necrosis, the degree of destruction being proportional to the reduction in the ERG. The ERG recovered with resolution of disease in most cases, though return to control values was uncommon. We suggest that the initial supernormal ERG reflects an underlying biochemical change mediated by the action of anti retinal S-antigen antibodies.

Immunochemistry of the outer retina

The immunochemistry of the outer retina is discussed with particular reference to photoreceptor cells, the retinal pigment epithelium and the interphotoreceptor space. The antigens identified and the techniques utilised are summarised.

Structural and functional relationships of guanosine triphosphate binding proteins

Information available at present documents the existence of three well-defined classes of guanine nucleotide binding proteins functioning as signal transducers: Gs and Gi which stimulate and inhibit adenylate cyclase, respectively, and transducin which transmits and amplifies the signal from light-activated rhodopsin to cGMP-dependent phosphodiesterase in ROS membranes. Go is a fourth member of this family. Its function is the least known among GTP binding signal transducing proteins. The family of G proteins has a number of properties in common. All are heterotrimers consisting of three subunits, alpha, beta, and gamma. Each of the subunits may be heterogeneous depending on species and tissue of origin and may be posttranslationally modified covalently. The alpha subunits vary in size from 39 to 52 kDa. The sequences for Gs alpha and transducin alpha have 42% overall homology and those of Gi alpha and Gs alpha 43%, whereas those of Gi alpha and transducin alpha have a higher degree (68%) of homology. All alpha subunits bind guanine nucleotides and are ADP-ribosylated by either pertussis toxin (Gi, transducin, Go) or cholera toxin (Gs, Gi, transducin). Thus, transducin and Gi, which have the highest degree of sequence homology, are also ADP-ribosylated by both toxins. The beta subunits have molecular weights of 36 and 35 kDa, respectively. While Gs, Gi, and Go contain a mixture of both, transducin contains only the larger (36-kDa) beta-polypeptide. The relationship of the 36- and the 35-kDa beta subunits is not defined. Although the complete sequence of the 36-kDa beta subunit of transducin has been deduced from the cDNA sequence, complete sequences of other beta subunits are not yet available so that detailed comparisons cannot be made at present. However, the proteolytic profiles of each class of the beta subunits of different G proteins are indistinguishable. The gamma subunit of bovine transducin has been completely sequenced. It has a Mr of 8400. Again complete sequences of other gamma subunits are not yet available. While the gamma subunits of Gs, Gi, and Go have identical electrophoretic mobility in SDS gels, they differ significantly in this respect from the gamma subunit of transducin. Moreover, crossover experiments point to functional differences between gamma subunits from G protein and transducin complexes. In addition, a role for beta, gamma in anchoring guanine nucleotide binding proteins to membranes has been postulated.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen-directed retention of an autoimmune T-cell line

We have used the T-cell-mediated, organ-specific autoimmune disease model of experimental autoimmune uveoretinitis (EAU) in the Lewis rat to study antigen-directed retention of autoimmune T helper cells in the target organ. We have compared the migration into the eye of two T-helper-cell lines: ThS, specific for retinal S antigen (S-Ag), that is uveitogenic to normal syngenic recipients, and ThP, specific to purified protein derivative of tuberculin (PPD), that is non-uveitogenic. The retention of adoptively transferred 51Cr-labeled ThS and ThP was studied up to the stage of disease induction in unprimed animals, during the acute stage of EAU induced by active immunization with S-Ag, and during the acute stage of a uveitis induced by a nonocular antigen (bovine serum albumin, BSA). Low numbers of cells from the two lymphocyte lines were detected in the eyes of unprimed animals, with no obvious increase of ThS over ThP, despite induction of EAU in the recipient animals by the injected ThS cells. In S-Ag-induced EAU many more ThS accumulated in the eye than ThP. In BSA uveitis both T-cell lines accumulated in the eye to the same extent, but more than in control noninflamed eyes. These results demonstrate the presence of increased antigen-specific retention of circulating autoimmune T helper lymphocytes during the acute stage of an ocular antigen-specific, but not ocular antigen nonspecific, inflammation. Since detectable accumulation of ThS cells in the eye was not a prerequisite for the induction of EAU, this phenomenon appears to be the result, rather than the cause, of the autoimmune process.

Response of guinea pigs to the synthetic peptide-M of the uveitogenic, retinal S-antigen: antisera immunofluorescent reactivity on normal guinea pig retina

S-antigen can elicit an experimental autoimmune uveitis (EAU) and pinealitis (EAP) in experimental animals. The sera of these animals have immunohistochemical reactivity with the photoreceptor cells of normal retina and pinealocytes. Lewis rats injected with the synthetic peptide-M corresponding to a specific sequence of S-antigen also develop an EAU and EAP. In this study we have investigated the immunohistochemical reactivity pattern of sera of guinea pigs injected with peptide-M. We found reactivity in the area of Muller's cells of normal guinea pig retina. Some of the sera showed weak reactivity with retina photoreceptors cells and pinealocytes. These patterns of reactivity are not seen in control sera of uninjected or saline in adjuvant injected guinea pigs. These results are consistent with observations of experimental and human uveitides.

Antibodies against retinal photoreceptor-specific proteins reveal axonal projections from the photosensory pineal organ in teleosts

With the aid of specific antisera to the retinal proteins S-antigen and alpha-transducin and to the rhodopsin apoprotein opsin, we have labeled various cell populations in the pineal organ, parapineal organ, habenular nucleus, and subcommissural organ in two teleost species: the rainbow trout and the European minnow. Although these proteins are associated with photoreceptor functions, not only photoreceptor cells but also the majority of parenchymal cells in the pineal organ were immunoreactive. Immunoreactive cells with dendrite- and axonlike processes were observed also in the parapineal organ and the habenular nucleus. Furthermore, S-antigen-immunoreactive, long, axonal processes were observed in the pineal organ and could be traced from the pineal organ to the habenular nucleus and to the pretectal area. In the light of recent HRP electron microscopical and immunocytochemical studies we propose (1) that not only the classical pineal photoreceptor cells of poikilothermic vertebrates but also other types of CSF-contacting neurons may be the phylogenetic ancestors of mammalian pinealocytes, and (2) a close interrelationship between the pineal organ and the limbic system, effectuated by the direct projections from pineal photoreceptors/CSF-contacting neurons/pinealocytes to the habenular nucleus, and by displaced "pinealocytelike" elements scattered in the habenular nucleus.

Electron immunocytochemical localization of retinal S-antigen with a rat monoclonal antibody

This report describes the ultrastructural localization of S-antigen in pig and human retinas by means of a highly specific rat IgG monoclonal antibody, S2.4.C5, followed by a secondary antibody adsorbed to colloidal gold. This monoclonal antibody gave definitive staining with negligible background. The protein was detected in both the rod outer and inner segments. Connecting cilia and the rod outer segment disc membranes were labelled. The outer segment plasma membrane was not obviously labelled. Cones were labelled at background level. S-Antigen was not detected in any other cells of the neural retina. The fate of S-antigen was also followed to the pigment epithelial phagosomes where intracytoplasmic ROS debris was stained with the antibody. No label, however, was detected in the choroid, suggesting that trans-cellular transport of the S-antigen did not occur.

Uveitis induced in primates by IRBP: humoral and cellular immune responses

In a previous publication, we have reported that immunization with bovine interphotoreceptor retinoid-binding protein (IRBP) produces severe uveitis in monkeys. The present study further examined the uveitogenicity of IRBP and analysed the immune responses in the immunized monkeys. Uveitis developed in monkeys immunized with bovine IRBP at doses as low as 10 micrograms Kg-1 body wt. In contrast, no disease was detected in monkeys immunized with monkey IRBP at the total dose of 100 micrograms Kg-1. Serum antibodies were measured in the immunized monkeys by the enzyme-linked immunosorbent assay, while cellular immunity was determined by skin hypersensitivity and the lymphocyte mitotic reaction in culture. No humoral or cellular immune responses were detected in monkeys immunized with monkey IRBP. Monkeys immunized with bovine IRBP produced antibodies which cross-reacted well with monkey IRBP. On the other hand, these monkeys did not react with monkey IRBP by skin hypersensitivity and their lymphocytes responded in culture to this protein only when tested by a highly sensitive procedure. In contrast, significant responses of the same cells to bovine IRBP were obtained at all tested conditions. These data are interpreted to suggest that the pathogenic cellular immune responses in vivo detect cross-reactivity with this autologous ocular antigen more efficiently than most conventional laboratory tests.

Primary and secondary structure of bovine retinal S antigen (48-kDa protein)

The complete amino acid sequence of bovine S antigen (48-kDa protein) has been determined by cDNA and partial amino acid sequencing. A 1623-base-pair (bp) cDNA contains an open reading frame coding for a protein of 404 amino acids (45,275 Da). Tryptic peptides and cyanogen bromide peptides of native bovine S antigen were purified and partially sequenced. All of these peptides were accounted for in the long open reading frame. Searching of the National Biomedical Research Foundation data bank revealed no extensive sequence homology between S antigen and other proteins. However, there are local regions of sequence similarity with alpha transducin, including the sites subject to ADP-ribosylation by Bordetella pertussis and cholera toxins and the phosphoryl binding-sites. Secondary structure prediction and circular dichroic spectroscopy show that S antigen is composed predominantly of beta-sheet conformation. Acid-catalyzed methanolysis suggests the presence of low levels of carbohydrate in the molecule.

S-antigen: characterization of a pathogenic epitope which mediates experimental autoimmune uveitis and pinealitis in Lewis rats

S-antigen (48K protein) is a photoreceptor cell protein highly pathogenic for the induction of experimental autoimmune uveitis (EAU) and intimately involved in the visual process. EAU is characterized, in part, as a T-cell mediated autoimmune disease which results in a severe inflammation of the uveal tract, and pineal gland. In order to determine specific sites in S-antigen responsible for its pathogenicity we synthesized twenty-three peptides, corresponding to the entire 404 amino acid sequence, and tested each peptide for its ability to induce EAU in Lewis rats. One peptide, peptide M (18 amino acids in length), was found to be highly pathogenic and consistently induced an EAU that was identical to the disease caused by native S-antigen. Clinically, the disease that developed in the eye was characterized by iris and pericorneal hyperemia, followed by inflammatory exudates in the anterior and vitreous chambers. Histopathologically a severe inflammatory response was observed which resulted in the complete destruction of the photoreceptor cell layer of the retina. In order to more fully characterize this pathogenic site, 14 additional smaller peptides (eight to eighteen amino acids in length) corresponding to the left and right portions of peptide M were synthesized. Of these peptides, peptide M16L, M15L, and M12L induced EAU, further localizing this pathogenic site to a small well-characterized region of S-antigen consisting of twelve amino acids. In addition, animals with ocular inflammatory disease had an associated pinealitis characterized by a lymphocytic infiltration of the subcapsular and central area of the pineal gland. The significance of these findings and the relationship of S-antigen in the pathogenesis of EAU and other autoimmune diseases is discussed.

Experimental autoimmune uveoretinitis (EAU) induced by retinal interphotoreceptor retinoid-binding protein (IRBP): differences between EAU induced by IRBP and by S-antigen

Rats immunized with the retinal interphotoreceptor retinoid-binding protein (IRBP) develop an inflammatory eye disease, "experimental autoimmune uveoretinitis" (EAU). The ocular changes which characterize the EAU induced by IRBP resemble those seen in rats which develop EAU by immunization with another retinal protein, S-antigen (S-Ag). Yet, the two antigens do not cross-react antigenically and the two diseases differ by several features: At low doses (less than or equal to 4 micrograms/rat), IRBP was more uveitogenic in Lewis rats than was S-Ag, inducing disease more reproducibly and with earlier onset time. On the other hand, at higher doses (greater than or equal to 20 micrograms/rat) the disease induced by S-Ag was more severe than that induced by the same doses of IRBP. Rats of various inbred strains differed in their susceptibility to EAU induced by these two antigens. In particular, BN rats were more susceptible to IRBP-induced EAU than to the S-Ag-induced disease, while WF and RCS-rdy+ rats developed severe EAU when immunized with S-Ag but showed minimal or no ocular change when immunized with IRBP.

Effect of light-adaptation on the binding of 48-kDa protein (S-antigen) to photoreceptor cell membranes

During the process of light-adaptation, a part of retinal S-antigen ("48-kDa protein") is bound to the photoreceptor cell membranes. This fraction can be isolated by first extracting the soluble S-antigen with isotonic buffer and subsequently extracting the bound S-antigen with detergent. In this way we found that light-adaptation to 250 lx or more induces a maximum binding of 62% of total S-antigen within 2 minutes in rat retina in vivo. At low light intensity (50 lx) this process lasts 15 minutes, while at 5 lx only 30% of S-antigen is bound. Presumably the number of available phosphorylated (bleached) rhodopsin molecules is the limiting factor in time and quantity. Dark-adaptation causes an initial rapid release of S-antigen during the first 5 minutes, but it takes more than 2 hours to reach the minimum level of about 10% bound S-antigen. The rates of binding of S-antigen in the light and of release of S-antigen in the dark are compared to other phenomena of light and dark adaptation.

Prevention of experimental autoimmune uveoretinitis by active immunization with autoantigen-specific monoclonal antibodies

Preimmunization of Lewis rats with anti-S antigen (S-Ag) monoclonal antibodies (mAb) led to protection against experimental autoimmune uveoretinitis (EAU) induced by this retinal autoantigen. High titers of anti-idiotypic (anti-Id) antibodies were raised against three mouse mAb, S2D2 (IgG2b), S6H8 (IgG2a) and S7D6 (IgG1), directed at S-Ag. An almost complete prevention was observed in S2D2 mAb-immunized animals while a partial protection was achieved with S6H8 and S7D6 mAb. No detectable anti-Id antibody nor disease prevention was observed in rats immunized with the mAb S9E2 (IgG2a) which only recognizes bovine and sheep S-Ag, or with control mAb of the same isotypes irrelevant to S-Ag. The mAb treatment did not modify the level of the whole polyclonal antibody response to S-Ag. These results suggest an important role in the pathogenesis of EAU for the epitopes recognized by S2D2-S6H8 and S7D6 in the S-Ag molecule. The success of anti-Id immunization for autoimmune disease suppression may depend on the identification of relevant epitopes.

The retinal phototransduction process: enzymatic cascade and regulation

Among cellular systems performing the transduction of an external stimulus, phototransduction in vertebrate rod cells is a unique case which allows convergent approaches to electrophysiological, biophysical and biochemical analyses. The framework of the molecular processes involved in the corresponding enzymatic cascade is now elucidated and can be considered as a model for G protein mediated transductions. We present here the main features of this cascade, its amplification and regulation properties. The mode of stimulation by the aluminofluoride ion is particularly addressed.

Retinoblastoma. Immunohistochemistry and cell differentiation

Tumor from eight enucleated eyes was analyzed by immunohistochemistry, using a panel of specific antibodies including interphotoreceptor retinoid-binding protein (IRBP), S-antigen (S-Ag), opsin, neuron-specific enolase (NSE), glial fibrillary acidic protein (GFAP), laminin, and vimentin. In addition, immunoelectron microscopy and enzyme-linked immunosorbent assay (ELISA) for IRBP were performed. Immunohistochemical staining disclosed the most pronounced labeling of tumor cells with NSE and IRBP antibodies. A correlation was found between the degree of tumor differentiation and amount of IRBP, a protein specifically synthesized by photoreceptor cells. Moderate labeling of the better differentiated tumors was also observed with antibodies against S-Ag and focal labeling in a few tumors with opsin antibodies. Anti-GFAP labeling was limited to a smaller number of reactive glial cells and perivascular glial cells. These data indicate the essential neuronal nature of retinoblastoma tumor cells in situ as well as at least partial photoreceptor-like features, as shown by the presence of recognized photoreceptor cell markers (IRBP, S-Ag, opsin). Tissue culture studies using the human Y-79 retinoblastoma cell line also demonstrate that the tumor cells are primitive multipotential retinoblasts capable of at least partial differentiation along neuronal, glial, or pigment epithelial cell lines.

Eye diseases and proteins controlling visual transduction

Retinal S antigen is a soluble protein found in abundance in photoreceptor cells. Immunization of laboratory animals with this antigen in adjuvant induces experimental autoimmune uveoretinitis. Cellular immunity plays a major role in this condition. Autoimmune responses toward retinal S antigen are often observed in patients with retinal inflammatory disease, however, these responses are usually secondary to local tissue damage. The S antigen is identical to the 48 K protein characterized in rod outer segments by its light-dependent binding to the disk membrane in the presence of ATP. This protein binds specifically to photoexcited and phosphorylated rhodopsin, and quenches the activity of the light-dependent cGMP-phosphodiesterase, most probably because it competes with transducin. There is no evidence for any direct inactivation of phosphodiesterase by 48 K protein. In view of the numerous similarities between the photoreceptor enzyme cascade and hormone-activated cyclase systems, a related protein could be involved in the desensitization of hormonal systems.

IRBP from bovine retina is poorly uveitogenic in guinea pigs and is identical to A-antigen

Retinal interphotoreceptor retinoid-binding protein (IRBP) is a potent uveitogen in Lewis rats, producing experimental autoimmune uveitis (EAU) reproducibly at doses lower than those of S-antigen (S-Ag). In contrast, IRBP was found to be poorly uveitogenic in three strains of guinea pigs, inducing only minor changes in a small proportion of these animals. On the other hand, S-Ag was found to induce EAU in the majority of immunized guinea pigs, with changes more severe than those induced by IRBP. Unlike the difference in their uveitogenicity, IRBP and S-Ag induced similar levels of specific immune responses in the immunized guinea pigs. The poor uveitogenicity of IRBP in guinea pigs resembles that of A-antigen (A-Ag). The two proteins are also similar in other features and were found in this study to be antigenically identical. It is proposed that IRBP and A-Ag are one and the same protein.

Molecular cloning of the S-antigen cDNA from bovine retina

S-antigen is the major component of the soluble proteins in the retina and the pineal, and is thought to play a regulatory role in the visual cycle. It induces experimental autoimmune uveitis (E.A.U.) in various species of laboratory animals. We have cloned the cDNA of S-antigen from bovine retina, containing the entire coding region, and determined its nucleotide sequence. The sequence consists of a 1212 nucleotide region that specific the amino acid sequence, a 368 nucleotide 5' untranslated region and a 80 nucleotide 3' untranslated region. The previously reported partial amino acid sequence defining the carboxy terminal moiety of the polypeptide, fits into our sequence. Homologies were found between the predicted amino acid sequence of S-antigen and that of alpha transducin.

Immunoelectron microscopic localization of photoreceptor-specific markers in the monkey retina

Antibodies for several molecules that function in the visual process were used to localize these molecules in primate rod and cone cells. These antibodies (monoclonal or polyclonal) were prepared against Interphotoreceptor Retinoid-binding Protein (IRBP), S-antigen (S-Ag), opsin, alpha-transducin and also against cyclic GMP (cGMP). Lowicryl-embedded tissues were labeled with secondary antibodies linked to colloidal gold. Although IRBP is predominantly an extracellular protein, the relatively small amount found intracellularly was localized mainly in rods, with little in cones. Opsin, S-Ag and cGMP were found mainly in rod cell outer segments. A polyclonal antiserum raised against transducin-alpha purified from rod outer segments predominantly labeled rod cells, but an antiserum against the carboxyterminal decapeptide of transducin-alpha labeled both rod and cone cells. Thus, most of these specialized molecules are present predominantly in rod cells, confirming major differences in components of the visual cycle in rods and cones.

Deactivation of photoactivated rhodopsin by rhodopsin-kinase and arrestin

Photoactivated rhodopsin (R) catalyses, by repetitively interacting with many copies of a guanosine nucleotide binding protein (transducin), the amplified binding of GTP to transducin molecules which then activate cyclic GMP phosphodiesterase. Electrophysiologists recently have shown that cyclic GMP keeps ion channels in the plasma membrane of the rod outer segment open in darkness, and that light-induced hydrolysis of cyclic GMP leads to closure of the channels and therefore to hyperpolarization of the rod cell. Photoactivated rhodopsin interacts not only with transducin, but with two more proteins: a protein kinase that specifically phosphorylates R (in contrast to dark-adapted rhodopsin) at multiple sites; and an abundant soluble protein of 48 KDal (called 48 K-protein, S-antigen, or arrestin) that specifically binds to phosphorylated R. Phosphorylation partially suppresses the ability of R to catalyze transducin-mediated phosphodiesterase activation even in the absence of arrestin. Binding of arrestin to the phosphorylated R potentiates this inhibitory effect, most probably because arrestin competes with transducin for binding on the phosphorylated R. Phosphorylation, in conjunction with arrestin binding, therefore appears to be a mechanism that terminates the active state of the receptor, R.

S-antigen: identification of the MAbA9-C6 monoclonal antibody binding site and the uveitopathogenic sites

The location of the monoclonal antibody, MAbA9-C6, binding site and two uveitopathogenic sites in S-antigen have been determined. Using cyanogen bromide, S-antigen was cleaved into nine peptides, designated C1 to C9. MAbA9-C6 bound selectively to one large peptide designated C5, consisting of 122 amino acids. Six peptides (20 to 22 amino acids in length) designated 2,3,K,L,N and M, corresponding to the entire length of peptide C5, were synthesized chemically. In a radioimmunoassay and a dot-binding immunoassay, MAbA9-C6 bound selectively to one of the six peptides, peptide 3, indicating that this region of peptide C5 contains the MAbA9-C6 binding site. Twelve smaller peptides (ten amino acids in length), corresponding to the amino acid sequence of peptide 3, were synthesized and used in a competitive inhibition binding assay. These studies localized the MAbA9-C6 binding site to a small region within peptide 3. In addition, peptide K and peptide M were highly pathogenic for the induction of experimental auto-immune uveitis (EAU). Clinical and histological evidence of a severe uveo-retinitis, indistinguishable from that seen with native S-antigen, was documented in Lewis rats immunized with the synthetic peptides (50 micrograms), 11-12 days following immunization. Our results show that the MAbA9-C6 binding site and the two uveitopathogenic sites lie in close proximity to each other within the region of S-antigen corresponding to peptide C5. Furthermore, microcomputer analysis of the average hydrophilicity/hydrophobicity values of the amino acid sequence corresponding to peptide C5 shows that the MAbA9-C6 binding site and one uveitopathogenic site (peptide K) lie on the adjacent peaks. The significance of these findings and their relationship to the role of S-antigen in the pathogenesis of EAU and the phototransduction of vision is discussed.

Characterization of immunologically active cyanogen bromide peptide fragments of bovine and human retinal S-antigen

Peptides which account for most, if not all, of the cyanogen bromide (CNBr)-generated peptide fragments of bovine retinal S-antigen have been identified and examined for their immunoreactivity with antisera raised to bovine and human S-antigen and with immune lymphocytes further selected twice in vitro with either bovine or human S-antigen. Amino-acid sequencing of a large fragment of S-antigen missing a small N-terminal peptide revealed the location of three overlapping CNBr peptides near the N-terminus. Amino-acid sequencing of several other CNBr peptides has allowed their position in a partial DNA-predicted sequence of the carboxy terminal half of the antigen to be determined. The total CNBr digest of human S-antigen was also prepared and compared with the fragments of the bovine antigen. Sera from rats immunized with bovine or human S-antigen were similar in their specificity in the enzyme-linked immunosorbent assay (ELISA) for purified bovine peptides except for the CB21 peptide which was not significantly bound by anti-human S-antigen sera. All of the other bovine peptides recognized by anti-bovine S-antigen sera were also bound by antibodies in the sera raised to the human antigen. The CNBr peptides of human and bovine S-antigen were extracted from gel slices and also assayed in the ELISA. Peptides of bovine S-antigen purified by HPLC were tested for their ability to stimulate an in vitro proliferative response in lymphocytes from Lewis rats immunized with either bovine or human S-antigen. Only quantitative differences in the proliferative response to human vs. bovine S-antigen and CNBr peptides were found. Methodology for the purification and analysis of the peptides is presented as well as the properties of the peptides.

Rapid affinity purification of retinal arrestin (48 kDa protein) via its light-dependent binding to phosphorylated rhodopsin

Arrestin (also named '48 kDa protein' or 'S-antigen') is a soluble protein involved in controlling light-dependent cGMP phosphodiesterase activity in retinal rods, and is also known for its ability to induce autoimmune uveitis of the eye. We report a rapid and simple purification method based on the property of arrestin to bind specifically and reversibly to illuminated and phosphorylated rhodopsin [(1984) FEBS Lett. 176, 473-478]. This method does not require column chromatography and yields about 2-4 mg purified arrestin from 15 bovine retinas. Pure arrestin can be resolved by isoelectric focusing into at least 10 distinct bands, all of which stain with a monoclonal antibody specific for S-antigen.

A 48 kDa protein arrests cGMP phosphodiesterase activation in retinal rod disk membranes

Photolyzed rhodopsin (R) catalyzes GTP-binding to alpha-transducins (T alpha); T alpha X GTPs then activate cGMP phosphodiesterase (PDE). PDE activation is arrested by ATP in two ways: (i) initial velocity is suppressed, and (ii) PDE velocity rapidly returns to preactivation levels (turnoff). Arrestin (a 48 kDa protein) markedly enhances turnoff while not affecting initial velocity. Arrestin in the presence of ATP achieves rapid turnoff by directly inhibiting activated PDE, as indicated by its ability to inhibit the direct activation of PDE by T alpha X GMP--PNP (guanylyl-imidodiphosphate). Double reciprocal plots reveal a competition between arrestins and activated transducins for sites on PDE. Blocking R phosphorylation blocks initial velocity suppression but does not disturb rapid turnoff. Our data suggest a 2-fold mechanism for PDE deactivation: (i) formation of T alpha X GTPs is suppressed by R phosphorylation, while (ii) activation of PDE by T alpha X GTPs is competitively inhibited by arrestins when ATP is present.

Quantitative determination of S-antigen in human ocular tissues, aqueous humour and serum

Retinal S-antigen is thought to play an important role in the immunopathogenesis of uveitis. To investigate whether S-antigen is a sequestered antigen confined to the retina, a sensitive ELISA was developed to determine the levels of this protein in various human ocular tissues, aqueous humour and serum. The ELISA was performed by incubating S-antigen-containing samples with solid-phase bound immunospecific rabbit anti human S-antigen F(ab')2 fragments and then incubating the bound S-antigen with mouse anti bovine S-antigen serum and the bound mouse antibodies with peroxidase-labelled rabbit anti mouse IgG; the peroxidase activity is developed with ABTS. This method was demonstrated to be highly sensitive and specific: S-antigen could be measured at concentrations of 5-10 nanograms per ml, irrespective of whether it was present in buffer, undiluted whole serum of tissue extracts. Human retinas were shown to contain approximately 1.2 mg immunoreactive S-antigen per retina. Of the other human ocular tissues, only the vitreous and choroid (including pigment epithelium) contained small amounts of S-antigen. Low levels of S-antigen could also be detected in the aqueous humour of two out of seven patients with posterior uveitis. No immunoreactive S-antigen could be detected in the serum of either healthy individuals or patients with uveitis. Sera collected from diabetic patients 15 minutes after extensive laser photocoagulation also did not contain immunoreactive S-antigen. Preliminary experiments with rats to study the clearance of intravenously injected S-antigen from the circulation indicated that the relatively short half-life (+/- 30 min) of circulating S-antigen might account for the absence of detectable S-antigen in the patient sera.

Pineal-retinal molecular relationships: distribution of "S-antigen" in the pineal complex

The immunocytochemical localization of S-antigen, a specific protein first discovered in retinal photoreceptors, was studied in the pineal complex of vertebrates (eel, pike, frog, lizard, passerines, mouse, hamster) using monoclonal antibody immunofluorescence. S-antigen immunoreactivity was demonstrated concurrently in retinal photoreceptors and in most pineal phototransducers of all species, i.e. in pineal cells of the receptor series (cone-like, modified photoreceptor cells, pinealocytes) and in cone-like photoreceptors of the frog frontal organ and lizard parietal eye. The labelling was distributed either in all compartments of these cells, or restricted to outer segments. The functional significance of the S-antigen as well as some phylogenetic and ontogenic implication of this marker are discussed.

The opsin family of proteins

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