Mutations of the Opsin Gene (Y102H and I307N) Lead to Light-induced Degeneration of Photoreceptors and Constitutive Activation of Phototransduction in Mice

Abstract

Mutations in the Rhodopsin (Rho) gene can lead to autosomal dominant retinitis pigmentosa (RP) in humans. Transgenic mouse models with mutations in Rho have been developed to study the disease. However, it is difficult to know the source of the photoreceptor (PR) degeneration in these transgenic models because overexpression of wild type (WT) Rho alone can lead to PR degeneration. Here, we report two chemically mutagenized mouse models carrying point mutations in Rho (Tvrm1 with an Y102H mutation and Tvrm4 with an I307N mutation). Both mutants express normal levels of rhodopsin that localize to the PR outer segments and do not exhibit PR degeneration when raised in ambient mouse room lighting; however, severe PR degeneration is observed after short exposures to bright light. Both mutations also cause a delay in recovery following bleaching. This defect might be due to a slower rate of chromophore binding by the mutant opsins compared with the WT form, and an increased rate of transducin activation by the unbound mutant opsins, which leads to a constitutive activation of the phototransduction cascade as revealed by in vitro biochemical assays. The mutant-free opsins produced by the respective mutant Rho genes appear to be more toxic to PRs, as Tvrm1 and Tvrm4 mutants lacking the 11-cis chromophore degenerate faster than mice expressing WT opsin that also lack the chromophore. Because of their phenotypic similarity to humans with B1 Rho mutations, these mutants will be important tools in examining mechanisms underlying Rho-induced RP and for testing therapeutic strategies.Rhodopsin is a light sensitive G-protein-coupled receptor composed of a membrane-bound opsin, encoded by the rhodopsin gene (Rho), and a covalently bound, light-sensitive chromophore, 11-cis-retinal. Upon light exposure, 11-cis-retinal is isomerized to all-trans-retinal, which induces a conformational change in rhodopsin to yield its active form, metarhodopsin II (R*).2 R* is deactivated via phosphorylation by rhodopsin kinase and binding to arrestin. In parallel, all-trans-retinal is released from R* and recycled through the visual cycle, to form 11-cisretinal, which regenerates rhodopsin in the rod outer segment. The release of chromophore from R* can also lead to high levels of free opsin in the retina. Free opsin can activate the phototransduction cascade, albeit at a lower rate than R*, and can potentially lead to constitutive activation of transduction after it is phosphorylated and forms a complex with arrestin (1, 2), a phenomena associated with photoreceptor degeneration (1).The maintenance of rod photoreceptors is critically dependent on normal levels of rhodopsin. Rod degeneration is observed in Rho Ϫ/Ϫ mice (3-5) and the human disorder retinitis pigmentosa (RP) caused by Rho mutations is characterized by progressive rod degeneration (6). More than 100 point mutations in rhodopsin collectively account for ϳ25% of autosomal dominant RP as well as some forms of autosomal recessive RP (7,8). These observations have spurred the development...

Assessment of visual function and retinal structure following acute light exposure in the light sensitive T4R rhodopsin mutant dog

“…However, while experimental light-induced damage in WT rodents is typically triggered by exposure to high intensity (≥ 1,100 lux) green or full spectrum white light (≥ 3,000 lux) for periods ranging from hours to days,(47–50) there is evidence for specific RHO mutations (P23H, S344ter, T17M, Y102H, I307N, T4R) conferring an increased sensitivity to light (for review see SI Table S1). (19, 21, 22, 24) Only few studies have investigated in these models of RHO -ADRP the effect of short duration light exposures. In the hT17M transgenic mouse, 2.5 minutes of illumination with 5,000 lux of white light triggers acute photoreceptor death, while limited sensitivity to a 2-fold higher intensity is found in a P23H mutant line.…”

“…(12) In support of this hypothesis are a limited number of case reports of RHO -ADRP patients with an occupational history of exposure to bright illumination and the occurrence of sectoral/altitudinal disease. (13, 14) Furthermore, vulnerability to light has been demonstrated in several animal models of RHO -ADRP including: transgenic X. laevis expressing human P23H, T4K, T17M, N2S/N15S RHO mutations; (15–17) mice with P23H, (18) T17M, (19, 20) Y102H, (21) I307N (21) mutations; the P23H, (22–24) and S334ter (23) rats; and the T4R dog (25). Within this group of animal models, there is a spectrum of sensitivity to light, with, at one extreme, the T17M RHO mouse and the T4R RHO dog.…”

Pharmacological approaches to retinitis pigmentosa: A laboratory perspective

“…It is known that mutated rhodopsin molecules might trigger apoptosis via accumulation in the inner segments and stress of the endoplasmic reticulum (Mendes et al, 2010), the site of ceramide synthesis. Tvrm4 mice, first described by the laboratory of P. Nishina, at Jackson Laboratories, portray severe mutations of the rhodopsin C-terminus resulting in aggressive degeneration phenotypes, to which rhodopsin constitutiveactivation is thought to contribute (Budzynski et al, 2010) (Fig. 8).…”

Signaling States of Rhodopsin in Rod Disk Membranes Lacking Transducin βγ-Complex

“…These half-time values are in good agreement with published results of 8.2 Ϯ 0.5 minutes for bovine rod disc membranes 32 and for purified mouse rhodopsin. 52 The time courses of meta III formation in wild-type and Gt␥-KO RDM are shown in Figure 3B. The data were fitted with single exponential functions that yielded time constants of meta III formation of 10.1 Ϯ 0.6 minutes and 9.5 Ϯ 0.7 minutes for wild-type and Gt␥-KO RDM, correspondingly.…”