PURPOSE. TO Study mechanisms leading to photoreceptor degeneration in mouse
models for autosomal dominant retinitis pigmentosa (adRP) based on the rho
dopsin P23H mutation.
METHODS. Mice of a transgenic line expressing a rhodopsin triple mutant, V2
0G, P23H, and P27L (GHL), were mated with rhodopsin (rho) knockout mice. Li
ttermates of various ages and genotypes (GHL(+)rho(+/+), GHL(+)rho(+/+), an
d GHL(+)rho(-/-)) were examined for outer nuclear layer thickness and outer
segment formation (histology), fate of mutant rhodopsin (immunocytochemist
ry), and photoreceptor function (electroretinogram; ERG).
RESULTS. Mice expressing GHL-rhodopsin in the absence of wild-type rhodopsi
n had severe retinopathy, which was nearly complete by postnatal day (P)30.
GHL-rhodopsin formed homodimers nearly exclusively on sodium dodecyl sulfa
te-polyacrylamide gel electrophoresis gels, whereas wild-type rhodopsin pre
dominantly formed monomers. Expression level of mutant rhodopsin in predege
nerate (P10) GHL+rho-/- retinas was low, approximately 10% to 25% of normal
levels. No elaboration of disc membrane or outer segment formation was obs
erved at any time point examined. The mutant rhodopsin was found mostly in
perinuclear locales (endoplasmic reticulum; ER) as evidenced by colocalizat
ion using the antibodies Rho1D4 and calnexin-NT.
CONCLUSIONS. GHL-rhodopsin dimerizes, localizes to the ER, and fails to tra
nsport and support outer segment formation. Additionally, the mutant protei
n does not support a scotopic ERG a-wave and accelerates photoreceptor dege
neration over that occurring with the rhodopsin knockout alone. These findi
ngs indicate a cytotoxic effect of the mutant protein, probably elicited by
an unfolded protein response.