Retinal degeneration in mice expressing the constitutively active G90D rhodopsin mutant

Abstract

Rhodopsin is the G protein-coupled receptor in rod photoreceptor cells that initiates vision upon photon capture. The light receptor is normally locked in an inactive state in the dark by the covalently bound inverse agonist 11-cis retinal. Mutations can render the receptor active even in the absence of light. This constitutive activity can desensitize rod photoreceptor cells and lead to night blindness. A G90D mutation in rhodopsin causes the receptor to be constitutively active and leads to congenital stationary night blindness, which is generally thought to be devoid of retinal degeneration. The constitutively active species responsible for the night blindness phenotype is unclear. Moreover, the classification as a stationary disease devoid of retinal degeneration is also misleading. A transgenic mouse model for congenital stationary night blindness that expresses the G90D rhodopsin mutant was examined to better understand the origin of constitutive activity and the potential for retinal degeneration. Heterozygous mice for the G90D mutation did not exhibit retinal degeneration whereas homozygous mice exhibited progressive retinal degeneration. Only a modest reversal of retinal degeneration was observed when transducin signaling was eliminated genetically, indicating that some of the retinal degeneration occurred in a transducin-independent manner. Biochemical studies on purified rhodopsin from mice indicated that multiple species can potentially contribute to the constitutive activity causing night blindness.

Stationary and Progressive Phenotypes Caused by the p.G90D Mutation in Rhodopsin Gene

“…Some of these are thought to cause only rod dysfunction (CSNB) (e.g., G90D, T94I, A292E, A295V) [ 1 , 2 , 4 , 9 , 10 , 18 , 19 ], while others were associated with retinal degeneration (RP) (e.g., G90V, S186W, D190N, K296E, K296M) [ 1 , 2 , 3 , 4 , 5 , 6 , 20 ]. The p.G90D mutation is located in the middle of the second transmembrane domain [ 21 ] and is thought to results in constitutive activity of the photoreceptor [ 1 , 3 , 6 , 9 , 12 ]. Biochemical studies have shown a blue-shifted absorbance maximum of G90D rhodopsin and a change in hydroxylamine accessibility, indirectly suggesting structural alterations in dark state rhodopsin that usually occur in light-activated wild-type rhodopsin [ 3 ].…”

“…Around 200 mutations in the rhodopsin gene ( RHO ) cause retinal diseases, in most patients in the form of retinitis pigmentosa (RP) [ 2 , 3 , 4 , 5 ], although other phenotypes have also been described, including sector RP [ 6 ], pericentral RP [ 7 ], congenital stationary night blindness (CSNB) [ 1 , 2 , 3 , 4 , 5 ], and retinitis punctata albescens [ 8 ]. Some of these pathogenic variants are thought to cause the photoreceptor to be constitutively active (activity in the absence of light) based on their conformational changes and the accessibility of the chromophore-binding pocket [ 3 , 9 , 10 , 11 ]. Constitutively active mutants have been associated with both RP and CSNB phenotypes [ 3 , 9 , 12 , 13 , 14 ].…”

“…Some of these pathogenic variants are thought to cause the photoreceptor to be constitutively active (activity in the absence of light) based on their conformational changes and the accessibility of the chromophore-binding pocket [ 3 , 9 , 10 , 11 ]. Constitutively active mutants have been associated with both RP and CSNB phenotypes [ 3 , 9 , 12 , 13 , 14 ]. The study, which was conducted in the United Kingdom, included 4236 individuals with inherited retinal disease from 3195 different families, observed the prevalence of RHO in 3,3% of individuals.…”

“…The first (and only) family of seven patients with p.G90D was reported in 1995 by Sieving et al, who described a mostly stationary disease with the possible slight deterioration with age [ 17 ]. Since then, multiple biochemical studies focused on the characteristics of the mutation [ 3 , 9 , 12 , 13 , 14 ]. In vitro experiments suggest that the p.G90D belongs to the group of constitutively active mutants.…”

“…In vitro experiments suggest that the p.G90D belongs to the group of constitutively active mutants. The p.G90D mutant is able to activate transducin in the dark, resulting in a light-adapted state in the dark and the desensitization of rod photoreceptor cells [ 9 ]. The constitutively active G90D mutant also appears to adopt a conformation with impaired ability to bind arrestin [ 10 ].…”