Alternative splicing in human retinal mRNA transcripts of an opsin-related protein

Proteopathy Linked to Exon-Skipping Isoform of RGR-Opsin Contributes to the Pathogenesis of Age-Related Macular Degeneration

Purpose

Proteopathy is believed to contribute to age-related macular degeneration (AMD). Much research indicates that AMD begins in the retinal pigment epithelium (RPE), which is associated with formation of extracellular drusen, a clinical hallmark of AMD. Human RPE produces a drusen-associated abnormal protein, the exon Ⅵ-skipping splice isoform of retinal G protein-coupled receptor (RGR-d). In this study, we investigate the detrimental effects of RGR-d on cultured cells and mouse retina.

Methods

ARPE-19 cells were stably infected by lentivirus overexpressing RGR or RGR-d and were treated with MG132, sometimes combined with or without endoplasmic reticulum (ER) stress inducer, tunicamycin. RGR and RGR-d protein expression, degeneration pathway, and potential cytotoxicity were explored. Homozygous RGR-d mice aged 8 or 14 months were fed with a high-fat diet for 3 months and then subjected to ocular examination and histopathology experiments.

Results

We confirm that RGR-d is proteotoxic under various conditions. In ARPE-19 cells, RGR-d is misfolded and almost completely degraded via the ubiquitin-proteasome system. Unlike normal RGR, RGR-d increases ER stress, triggers the unfolded protein response, and exerts potent cytotoxicity. Aged RGR-d mice manifest disrupted RPE cell integrity, apoptotic photoreceptors, choroidal deposition of complement C3, and CD86+CD32+ proinflammatory cell infiltration into retina and RPE-choroid. Furthermore, the AMD-like phenotype of RGR-d mice can be aggravated by a high-fat diet.

Conclusions

Our study confirmed the pathogenicity of the RGR splice isoform and corroborated a significant role of proteopathy in AMD. These findings may contribute to greater comprehension of the multifactorial causes of AMD.

Human RGR Gene and Associated Features of Age-Related Macular Degeneration in Models of Retina-Choriocapillaris Atrophy

Age-related macular degeneration (AMD) is a progressive eye disease and the most common cause of blindness among the elderly. AMD is characterized by early atrophy of the choriocapillaris and retinal pigment epithelium (RPE). Although AMD is a multifactorial disease with many environmental and genetic risk factors, a hallmark of the disease is the origination of extracellular deposits, or drusen, between the RPE and Bruch membrane. Human retinal G-protein-coupled receptor (RGR) gene generates an exon-skipping splice variant of RGR-opsin (RGR-d; NP_001012740) that is a persistent component of small and large drusen. Herein, the findings show that abnormal RGR proteins, including RGR-d, are pathogenic in an animal retina with degeneration of the choriocapillaris, RPE, and photoreceptors. A frameshift truncating mutation resulted in severe retinal degeneration with a continuous band of basal deposits along the Bruch membrane. RGR-d produced less severe disease with choriocapillaris and RPE atrophy, including focal accumulation of abnormal RGR-d protein at the basal boundary of the RPE. Degeneration of the choriocapillaris was marked by a decrease in endothelial CD31 protein and choriocapillaris breakdown at the ultrastructural level. Fundus lesions with patchy depigmentation were characteristic of old RGR-d mice. RGR-d was mislocalized in cultured cells and caused a strong cell growth defect. These results uphold the notion of a potential hidden link between AMD and a high-frequency RGR allele.

Photic generation of 11-cis-retinal in bovine retinal pigment epithelium

Photoisomerization of the 11-cis-retinal chromophore of rod and cone visual pigments to an all-trans-configuration is the initiating event for vision in vertebrates. The regeneration of 11-cis-retinal, necessary for sustained visual function, is an endergonic process normally conducted by specialized enzyme systems. However, 11-cis-retinal also can be formed through reverse photoisomerization from all-trans-retinal. A nonvisual opsin known as retinal pigment epithelium (RPE)-retinal G-protein-coupled receptor (RGR) was previously shown to mediate visual chromophore regeneration in photic conditions, but conflicting results have cast doubt on its role as a photoisomerase. Here, we describe high-level production of 11-cis-retinal from RPE membranes stimulated by illumination at a narrow band of wavelengths. This activity was associated with RGR and enhanced by cellular retinaldehyde-binding protein (CRALBP), which binds the 11-cis-retinal produced by RGR and prevents its re-isomerization to all-trans-retinal. The activity was recapitulated with cells heterologously expressing RGR and with purified recombinant RGR. Using an RGR variant, K255A, we confirmed that a Schiff base linkage at Lys-255 is critical for substrate binding and isomerization. Single-cell RNA-Seq analysis of the retina and RPE tissue confirmed that RGR is expressed in human and bovine RPE and Müller glia, whereas mouse RGR is expressed in RPE but not in Müller glia. These results provide key insights into the mechanisms of physiological retinoid photoisomerization and suggest a novel mechanism by which RGR, in concert with CRALBP, regenerates the visual chromophore in the RPE under sustained light conditions.

Diversity of color vision: not all Australian marsupials are trichromatic

Color vision in marsupials has recently emerged as a particularly interesting case among mammals. It appears that there are both dichromats and trichromats among closely related species. In contrast to primates, marsupials seem to have evolved a different type of trichromacy that is not linked to the X-chromosome. Based on microspectrophotometry and retinal whole-mount immunohistochemistry, four trichromatic marsupial species have been described: quokka, quenda, honey possum, and fat-tailed dunnart. It has, however, been impossible to identify the photopigment of the third cone type, and genetically, all evidence so far suggests that all marsupials are dichromatic. The tammar wallaby is the only Australian marsupial to date for which there is no evidence of a third cone type. To clarify whether the wallaby is indeed a dichromat or trichromatic like other Australian marsupials, we analyzed the number of cone types in the “dichromatic” wallaby and the “trichromatic” dunnart. Employing identical immunohistochemical protocols, we confirmed that the wallaby has only two cone types, whereas 20–25% of cones remained unlabeled by S- and LM-opsin antibodies in the dunnart retina. In addition, we found no evidence to support the hypothesis that the rod photopigment (rod opsin) is expressed in cones which would have explained the absence of a third cone opsin gene. Our study is the first comprehensive and quantitative account of color vision in Australian marsupials where we now know that an unexpected diversity of different color vision systems appears to have evolved.

Accumulation of extracellular RGR-d in Bruch's membrane and close association with drusen at intercapillary regions

Human retinal pigment epithelial (RPE) cells synthesize an extraneous splice isoform of retinal G protein-coupled receptor (RGR). In this study, we analyzed the exon-skipping variant of RGR (RGR-d) that is found in extracellular deposits. RPE-choroid tissue sections were prepared from postmortem human eyes from donors of various ages. RGR-d was analyzed in drusen and Bruch's membrane by immunohistochemical localization. Extracellular RGR-d is present in most drusen, including hard, soft, confluent and early-stage. Initial drusen formation is known to be preferentially associated with the intercapillary regions of Bruch's membrane. We corroborated this significant association of drusen, including early-stage drusen, with the intercapillary regions. The distribution of extracellular RGR-d in Bruch's membrane differs in old and young donors. In older persons, nodes of concentrated RGR-d accumulate at intercapillary loci, predominantly at the lateral edges of the capillaries of the choriocapillaris. RGR-d loci at the lateral capillary wall appear numerous in old, but not young, donors. Intensely immunostained RGR-d loci can be found at the base of early-stage drusen mounds in the older donors and may precede the formation of these drusen.

Exon-skipping variant of RGR opsin in human retina and pigment epithelium

An extraneous exon-skipping mRNA encodes an altered form of a light-absorbing opsin in human retina and pigment epithelium (RPE). The predicted protein variant differs from full-length RPE-retinal G protein-coupled receptor (RGR) by having an in-frame deletion of exon 6, which contains the entire sixth transmembrane domain. To verify that the exon 6-deleted RGR protein (RGR-d) exists in human retinas, we have produced RGR-d antibody probes. In Western blot assays, the RGR-d protein was detected in retinas of a large proportion ( approximately 53%) of individual donors, including patients with age-related macular degeneration (AMD). The relative abundance of RGR-d varied significantly between individuals. The altered protein is expressed in RPE cells and has a more basal subcellular localization that is remarkably different from that of normal RGR opsin. The presence of this exon-skipping variant of RGR in humans may contribute to the progressive derangement of the RPE.

Expression of opsin genes early in ocular development of humans and mice

We have compared the onsets of expression of the classical visual opsins with those of the non-rod, non-cone opsins in foetal and post-natal eye tissue from mice and humans. Mouse Rgr-opsin, peropsin, encephalopsin and melanopsin are all expressed in foetal development by E11.5, unlike the murine rod and cone opsins that exhibit post-natal expression, e.g. P1 for ultraviolet cone opsin and P5 for rod opsin. Human non-rod, non-cone opsins are also all expressed early, by 8.6 weeks post-conception. The implications of these observations are discussed with regard to the possible functions of these opsins at early stages of ocular development.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

C5a receptor activation. Genetic identification of critical residues in four transmembrane helices

Hormones and sensory stimuli activate serpentine receptors, transmembrane switches that relay signals to heterotrimeric guanine nucleotide-binding proteins (G proteins). To understand the switch mechanism, we subjected 93 amino acids in transmembrane helices III, V, VI, and VII of the human chemoattractant C5a receptor to random saturation mutagenesis. A yeast selection identified 121 functioning mutant receptors, containing a total of 523 amino acid substitutions. Conserved hydrophobic residues are located on helix surfaces that face other helices in a modeled seven-helix bundle (Baldwin, J. M., Schertler, G. F., and Unger, V. M. (1997) J. Mol. Biol. 272, 144-164), whereas surfaces predicted to contact the surrounding lipid tolerate many substitutions. Our analysis identified 25 amino acid positions resistant to nonconservative substitutions. These appear to comprise two distinct components of the receptor switch, a surface at or near the extracellular membrane interface and a core cluster in the cytoplasmic half of the bundle. Twenty-one of the 121 mutant receptors exhibit constitutive activity. Amino acids substitutions in these activated receptors predominate in helices III and VI; other activating mutations truncate the receptor near the extracellular end of helix VI. These results identify key elements of a general mechanism for the serpentine receptor switch.

Localization of the human RGR opsin gene to chromosome 10q23

The human RGR gene encodes an opsin protein (retinal G protein-coupled receptor), which is expressed in Müller cells and the retinal pigment epithelium and is thought to play a role in the visual process. To investigate a possible linkage of the RGR gene to retinal dystrophies, the locus of the gene was mapped on human metaphase chromosomes. Genomic and cDNA fragments of the human RGR gene were used as probes for fluorescence in situ hybridization. Analysis of the fluorescence signals on high-resolution banded chromosomes showed that the RGR gene is localized to human chromosome 10q23. This result now provides for the rapid analysis of this gene with respect to inherited diseases of the retina.