Photoreceptor cell-specific localization of S-antigen in retina

Visual Arrestin 1 acts as a modulator for N-ethylmaleimide-sensitive factor in the photoreceptor synapse

In the G-protein-coupled receptor phototransduction cascade, visual Arrestin 1 (Arr1) binds to and deactivates phosphorylated light-activated opsins, a process that is critical for effective recovery and normal vision. In this report, we discovered a novel synaptic interaction between Arr1 and N-ethylmaleimide-sensitive factor (NSF) that is enhanced in a dark environment when mouse photoreceptors are depolarized and the rate of exocytosis is elevated. In the photoreceptor synapse, NSF functions to sustain a higher rate of exocytosis, in addition to the compensatory endocytosis to retrieve and to recycle vesicle membrane and synaptic proteins. Not only does Arr1 bind to the junction of NSF N-terminal and its first ATPase domains in an ATP-dependent manner in vitro, but Arr1 also enhances both NSF ATPase and NSF disassembly activities. In in vivo experiments in mouse retinas with the Arr1 gene knocked out, the expression levels of NSF and other synapse-enriched components, including vGLUT1 (vesicular glutamate transporter 1), EAAT5 (excitatory amino acid transporter 5), and VAMP2 (vesicle-associated membrane protein 2), are markedly reduced, which leads to a substantial decrease in the exocytosis rate with FM1-43. Thus, we propose that the Arr1 and NSF interaction is important for modulating normal synaptic function in mouse photoreceptors. This study demonstrates a vital alternative function for Arr1 in the photoreceptor synapse and provides key insights into the potential molecular mechanisms of inherited retinal diseases, such as Oguchi disease and Arr1-associated retinitis pigmentosa.

Differential expression of photoreceptor-specific proteins during disease and degeneration in the progressive rod-cone degeneration (prcd) retina

Progressive rod-cone degeneration (prcd) is a late-onset hereditary retinal degeneration characterized by normal development of photoreceptors prior to degeneration and death of visual cells. We reported previously that expression of opsin mRNA and protein decreases prior to visual cell degeneration. To examine the specificity of this reduction, we have used immunocytochemistry to correlate photoreceptor-specific protein expression with visual cell disease progression. Eyes from light-adapted age-matched control and prcd-affected dogs were fixed in paraformaldehyde, embedded in diethylene glycol distearate (DGD) wax, and reacted with antibodies specific to interphotoreceptor retinoid-binding protein (IRBP), S-antigen, opsin, phosducin, gamma-phosphodiesterase (gamma-PDE), and beta 1-transducin. While IRBP expression did not change with disease progression, immunoreactivity to other proteins varied. For S-antigen and opsin, immunoreactivity decreased dramatically with the transition from photoreceptor disease to degeneration; gamma-PDE immunolabeling in rods also decreased, but the reduction was less abrupt. However, for two other proteins (phosducin and beta 1-transducin), immunoreactivity increased initially and was redistributed (particularly to the rod outer segment) in early disease (stage 1). Our results show that there is a differential expression of photoreceptor-specific proteins with disease and degeneration that is not uniform for all the gene products examined; expression can be decreased, altered in distribution or remain unchanged. It is clear that the decrease of opsin expression described previously is not an isolated phenomenon in the progression of prcd, but is part of a more generalized degenerative process which eventually culminates in cell death.

Idiopathic retinal degeneration in the dog: differential patterns of [3H]uridine incorporation and HIOMT-like immunoreactivity in surviving photoreceptors

Photoreceptor cell pathology was investigated in an 8-yr-old mixed-breed dog which had displayed visual symptoms of 1 month duration. An electroretinogram detected no light-evoked responses. Light and electron microscopic features showed marked thinning and atrophy of the outer both the tapetal and non-tapetal retina appeared to be involved. In the non-tapetal region, a majority of the rod inner segments were missing, while scattered mitochondria-filled stubby inner segments of cones were readily identified. Inner segments of both rods and cones were observed in the tapetal region. Photoreceptor outer segments were completely absent from the affected retina, and no outer segment debris was observed between the photoreceptor layer and the retinal pigmented epithelium (RPE). Autoradiographic analysis of 3-mm retinal disks from the degenerate retina following incubation with [3H]uridine indicated that only 61% +/- 13 S.D. of the remaining nuclei of rod photoreceptors were undergoing RNA synthesis, whereas more than 99% of cone nuclei incorporated the label. Normal and degenerate retina were also analysed for localization of hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity. While the normal retina showed immunoreactivity in both rod and cone photoreceptors with more intense immunoreactivity present in cones, the degenerate retina showed HIOMT-like immunoreactivity only in the remaining cone photoreceptors. The results of this study of idiopathic photoreceptor degeneration of the canine retina suggest that although both photoreceptor types are involved, rods are more severely affected than cones.

Development and degeneration of retina in rds mutant mice: ultraimmunohistochemical localization of S-antigen

In the developing photoreceptor cells of the homozygous rds mutant mice S-antigen is localized over the ciliary protrusion as in the control mice, and to a lesser extent over the inner segments, perikaryal cytoplasma and the cell terminals. As the outer segments develop in the normal retina, the discs become increasingly immunoreactive. In the rds/rds retina the outer segments fail to develop but small membrane bound vesicles, immunoreactive for S-antigen are extruded and phagocytized by the retinal pigment epithelium. In the retina of older mutant mice, as the photoreceptor cells degenerate slowly, the surviving cells continue to show persistent immunoreactivity for S-antigen in the different regions of the photoreceptor cells. In the heterozygotes the outer segments are reduced and appear abnormal, but the localization of S-antigen is similar to normal. In the receptor region of the normal retina and in the deviant membranous structures in the mutant retina the localization of S-antigen is similar to that of opsin. However, some differences in the subcellular localization of these two photoreceptor specific proteins have been observed. It is concluded that the rds gene acts subsequent to the synthesis of these proteins and possibly at the site of disc assembly.

Reduced level of interphotoreceptor retinoid-binding protein (IRBP), a possible cause for retinal degeneration in the Abyssinian cat

Retinae of Abyssinian cats homozygous for a retinal degeneration gene, and normal controls, have been investigated using antibodies directed against opsin, transducin alpha (TD-alpha), S-antigen (48K protein), interphotoreceptor retinoid-binding protein (IRBP), and cone outer segments. IRBP-immunoreactivity (IR) is much reduced at stage 2 of the disease in affected retinae; later massive photoreceptor cell death occurs. In cats, at a late stage of the disease, the retina exhibits few S-antigen-IR cells in the peripheral part of the retina whereas, in the central part, some patches of cells exhibiting opsin-IR, TD-alpha-IR, and S-antigen-IR are present in remnants of the outer nuclear layer (ONL). No IRBP-IR is detectable at this stage. The form and size of the majority of these remaining cells, however, does not resemble that of normal photoreceptors. No, or only rudimentary, inner and outer segments are present; long bifurcating basal protrusions often occur. These cells, which could be remains of cone elements, are S-antigen immunoreactive. Double labelling for different retina-specific proteins reveals a co-localization of opsin, TD-alpha and S-antigen in some, but not all, remaining photoreceptor elements. Cells exhibiting opsin-IR also show TD-alpha-IR and S-antigen-IR located within the entire cell and its protrusions. In control retinae and retinae at early stages of the disease, immunoreactions are comparable with all antibodies used. However, TD-alpha-IR is less intensive in the photoreceptor terminals. S-antigen-IR cones are most frequently present in the peripheral retina. Reduction of IRBP at an early stage of the disease could be one of the factors leading to photoreceptor cell death at later stages.

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.

Immunohistochemical characterization of human retinoblastomas in situ with multiple markers

We studied paraffin-embedded specimens from 18 surgically enucleated eyes with retinoblastoma by peroxidase-antiperoxidase immunohistochemistry with antibodies against glial fibrillary acidic protein, S-100 protein, Leu 7 epitopes, neuron-specific enolase, the 200-kilodalton subunit of the neurofilament triplet polypeptide, and retinal S-antigen. We found that (1) glial fibrillary acidic protein, S-100 protein, and Leu 7 epitopes were detected only in well-differentiated glial cells that were interpreted as reactive and not neoplastic, (2) undifferentiated neoplastic cells expressed both neuron-specific enolase and retinal S-antigen immunoreactivity, and (3) differentiated cells forming Flexner-Wintersteiner rosettes were found to express neuron-specific enolase, retinal S-antigen, and, occasionally, neurofilament protein. These results support the view that retinoblastomas are composed of neuron-committed cells and favor the origin of these tumors from photoreceptor progenitor cells. We did not find any morphologic or immunohistochemical evidence of glial differentiation from tumor cells that would support the concept that retinoblastoma arises from a primitive neuroectodermal cell capable of divergent differentiation along neuronal and glial lines.

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.

Codistribution of S-antigen and opsin in light-adapted retinal rods

The localization of S-antigen in ultra-cryosections of rat retina has been studied and compared with that of opsin by immunoelectron microscopy. Monospecific antisera to both proteins, and Protein A-coated gold particles of two sizes were used for (double) labeling. In the light-adapted retina, S-antigen showed codistribution with opsin in the rod outer segments. S-antigen labeling was diminished in dark-adapted retina. In retina pigment epithelium, multi-layered bodies were found containing S-antigen and rhodopsin representing spent rod outer segment fragments. In addition to theoretical considerations, there is indication that S-antigen is closely associated with the cytoplasmic surface of rod outer segment disc membranes. It seems moreover likely that S-antigen and opsin are closely associated in the illuminated retina. An important reason for this assumption was the observation that double labeling experiments showed interference of the antibodies to both antigens probably due to steric hinderance. The present results are complementary to, and in agreement with biochemical data from literature and point to an important function of S-antigen in the process of phototransduction.

Induction of experimental autoimmune uveoretinitis and pinealitis by IRBP. Comparison to uveoretinitis induced by S-antigen and opsin

Microgram quantities bovine IRBP (interphotoreceptor retinoid binding protein) injected in Freund's complete adjuvant induced severe autoimmune uveoretinitis and pinealitis in Lewis rats. At low doses the onset was accelerated and intensified by co-injection of Hemophilus pertussis bacteria. Wistar, BN and PVG rats were less susceptible, while the eyes of athymic, nude rats did not respond. The disease developed similar to but faster than S-antigen-induced uveoretinitis, while its onset was one day earlier and the reactions were slightly more severe. As distinct from these two types of uveoretinitis, opsin (in much higher doses) caused milder reactions in the anterior segment, while retinitis dominated. In each type of inflammation the photoreceptor cell layer was totally destroyed. All three ocular diseases were inhibited by cyclosporine treatment, which indicates that T cell-dependent mechanisms are essential for the development.