Retinitis pigmentosa (RP) is an inherited, degenerative eye disease that causes visual impairment and eventual blindness. RP is caused by mutations in eye photoreceptor rhodopsin. Zhang et al. engineered mice with RP-associated rhodopsin mutations. Here they used Serial Block-Face Scanning Electron Microscopy (SBFSEM) to generate high-resolution 3D images of retinas from mice with two mutated copies of the rhodopsin gene. Brown arrows point to abnormal disc inclusions in the retinal pigment epithelium (RPE) and by brown regions in the RPE cells (cells are false-colored to distinguish individual cells; nuclei are shown in blue). The lower portion of the image is a reconstructed RPE cell depicting abnormal stacking of rhodopsin (normal receptors in green, mutant receptor in red). A single mutant receptor disrupts the organization of normal receptors expressed in the same RPE cell. Mice with a single mutated copy of the rhodopsin gene exhibited disorganized retinal structures and altered photoreceptor positioning that contributed to a delayed-onset, mild form of retinal degeneration.
The pathophysiology of the E150K mutation in the rod opsin gene associated with autosomal recessive retinitis pigmentosa (arRP) has yet to be determined. We generated knock-in mice carrying a single nucleotide change in exon 2 of the rod opsin gene resulting in the E150K mutation. This novel mouse model displayed severe retinal degeneration affecting rhodopsin’s stabilization of rod outer segments (ROS). Homozygous E150K (KK) mice exhibited early-onset retinal degeneration, with disorganized ROS structures, autofluorescent deposits in the subretinal space, and aberrant photoreceptor phagocytosis. Heterozygous (EK) mice displayed a delayed-onset milder retinal degeneration. Further, mutant receptors were mislocalized to the inner segments and perinuclear region. Though KK mouse rods displayed markedly decreased phototransduction, biochemical studies of the mutant rhodopsin revealed only minimally affected chromophore binding and G protein activation. Ablation of the chromophore by crossing KK mice with mice lacking the critical visual cycle protein LRAT slowed retinal degeneration, whereas blocking phototransduction by crossing KK mice with GNAT1-deficient mice slightly accelerated this process. This study highlights the importance of proper higher-order organization of rhodopsin in the native tissue and provides information about the signaling properties of this mutant rhodopsin. Additionally, these results suggest that patients heterozygous for the E150K mutation should be periodically reevaluated for delayed-onset retinal degeneration.
Ning Zhang, Alexander V. Kolesnikov, Beata Jastrzebska, Debarshi Mustafi, Osamu Sawada, Tadao Maeda, Christel Genoud, Andreas Engel, Vladimir J. Kefalov, Krzysztof Palczewski