Variants of
rhodopsin, a complex of 11-cis
retinal and
opsin, cause
retinitis pigmentosa (RP), a degenerative disease of the retina. Trafficking defects due to
rhodopsin misfolding have been proposed as the most likely basis of the disease, but other potentially overlapping mechanisms may also apply. Pharmacological
therapies for RP must target the major disease mechanism and contend with overlap, if it occurs. To this end, we have explored the molecular basis of
rhodopsin RP in the context of pharmacological rescue with 11-cis
retinal. Stable inducible cell lines were constructed to express wild-type
opsin; the pathogenic variants T4R, T17M, P23A, P23H, P23L, and C110Y; or the nonpathogenic variants F220L and A299S. Pharmacological rescue was measured as the fold increase in
rhodopsin or
opsin levels upon addition of 11-cis
retinal during
opsin expression. Only Pro23 and T17M variants were rescued significantly. C110Y
opsin was produced at low levels and did not yield
rhodopsin, whereas the T4R, F220L, and A299S
proteins reached near-wild-type levels and changed little with 11-cis
retinal. All of the mutant
rhodopsins exhibited misfolding, which increased over a broad range in the order F220L, A299S, T4R, T17M, P23A, P23H, P23L, as determined by decreased thermal stability in the dark and increased
hydroxylamine sensitivity. Pharmacological rescue increased as misfolding decreased, but was limited for the least misfolded variants. Significantly, pathogenic variants also showed abnormal photobleaching behavior, including an increased ratio of
metarhodopsin-I-like species to
metarhodopsin-II-like species and aberrant photoproduct accumulation with prolonged illumination. These results, combined with an analysis of published biochemical and clinical studies, suggest that many
rhodopsin variants cause disease by affecting both biosynthesis and photoactivity. We conclude that pharmacological rescue is promising as a broadly effective
therapy for
rhodopsin RP, particularly if implemented in a way that minimizes the photoactivity of the
mutant proteins.