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

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.

Expression of Retinal G Protein-Coupled Receptor, a Member of the Opsin Family, in Human Skin Cells and Its Mediation of the Cellular Functions of Keratinocytes

Background: Photoreceptive proteins play critical physiological roles in human skin cells. The retinal G protein-coupled receptor (RGR) is a photoisomerase in the human retina, but its expression and cellular functions in human skin cells have not been reported.

Objectives: We aimed to detect RGR expression in various skin cells and evaluate its regulation of the cellular functions of keratinocytes.

Methods: The expression, distribution, and subcellular location of the RGR in normal human epidermal keratinocytes and cells with pathological conditions including psoriasis, seborrheic keratosis, and squamous cell carcinoma were determined using microscopic tools (immunohistochemical staining, immunofluorescence staining, and immunoelectron microscopy) and Western blotting (WB). The protein levels of the RGR in primary human melanocytes, keratinocytes, and fibroblasts isolated from the neonatal foreskin were measured by WB. The expression and subcellular localization of the RGR in these cells were detected by immunofluorescence staining under a fluorescence microscope and laser scanning confocal microscope. Additionally, the levels of RGR expression in normal keratinocytes exposed to ultraviolet (UV)-A or total ultraviolet radiation (UVR) in the presence or absence of all-trans-retinal were measured by WB. Furthermore, the effects of the RGR on human keratinocyte functions including proliferation, migration, and apoptosis were evaluated using the Cell Counting Kit 8, wound healing, and Transwell assays after reducing the RGR mRNA level in keratinocytes using small interfering RNA technology.

Results: The RGR was primarily located in the epidermal basal and spinous layers and skin appendages. Its expression increased in psoriatic lesions, seborrheic keratosis, and squamous cell carcinoma. Confocal microscopy showed that the RGR was located in the cell membrane and nucleus of keratinocytes, melanocytes, and fibroblasts. Keratinocytes had a higher expression of the RGR than melanocytes and fibroblasts, as well as nuclear expression, according to nuclear/cytoplasmic fractionation. Colloidal gold immunoelectron microscopy technology further confirmed that the RGR is mainly located in the nucleoplasm and mitochondria and is scattered in the cytoplasm and other organelles in the epidermal keratinocytes. Notably, RGR knockdown in keratinocytes led to the inhibition of cell proliferation and migration, augmenting cell apoptosis.

Conclusions: This study is the first to demonstrate the presence of RGR in the human skin. Our findings indicate that the RGR may play a critical role in the physiological function of epidermal keratinocytes.