Absence of sigma 1 receptor accelerates photoreceptor cell death in a murine model of retinitis pigmentosa

PURPOSE. Sigma 1 Receptor (Sig1R) is a novel therapeutic target in neurodegenerative diseases, including retinal disease. Sig1R^–/– mice have late-onset retinal degeneration with ganglion cell loss that worsens under stress. Whether Sig1R plays a role in maintaining other retinal neurons is unknown, but was investigated here using rd10 mice, a model of severe photoreceptor degeneration. METHODS. Wild-type, rd10, and rd10/Sig1R^–/– mice were subjected to ERG and spectraldomain optical coherence tomography (SD-OCT) to assess visual function/structure in situ. Retinas imaged microscopically were subjected to morphometric analysis, immunodetection of cones, and analysis of gliosis. Oxidative and endoplasmic reticulum (ER) stress was evaluated at mRNA/protein levels. RESULTS. Photopic ERG responses were reduced significantly in rd10/Sig1R^–/– versus rd10 mice at P28 (31 ± 6 vs. 56 ± 7 μV), indicating accelerated cone loss when Sig1R was absent. At P28, SD-OCT revealed reduced retinal thickness in rd10/Sig1R^–/– mice (60% of WT) versus rd10 (80% of WT). Morphometric analysis disclosed profound photoreceptor nuclei loss in rd10/Sig1R^–/– versus rd10 mice. rd10/Sig1R^–/– mice had 35% and 60% fewer photoreceptors, respectively, at P28 and P35, than rd10. Peanut agglutinin cone labeling decreased significantly; gliosis increased significantly in rd10/Sig1R^–/– versus rd10 mice. At P21, NRF2 levels increased in rd10/Sig1R^–/– mice versus rd10 and downstream antioxidants increased indicating oxidative stress. At P28, ER stress genes/proteins, especially XBP1, a potent transcriptional activator of the unfolded protein response and CHOP, a proapoptotic transcription factor, increased significantly in rd10/Sig1R^–/– mice versus rd10. CONCLUSIONS. Photoreceptor cell degeneration accelerates and cone function diminishes much earlier in rd10/Sig1R^–/– than rd10 mice emphasizing the importance of Sig1R as a modulator of retinal cell survival.