Alterations in the photoactivation pathway of rhodopsin mutants associated with retinitis pigmentosa

Abstract

The visual photoreceptor rhodopsin undergoes a series of conformational changes upon light activation, eventually leading to the active metarhodopsin II conformation, which is able to bind and activate the G‐protein, transducin. We have previously shown that mutant rhodopsins G51V and G89D, associated with retinitis pigmentosa, present photobleaching patterns characterized by the formation of altered photointermediates whose nature remained obscure. Our current detailed UV–visible spectroscopic analysis, together with functional characterization, indicate that these mutations influence the relative stability of the different metarhodopsin photointermediates by altering their equilibria and maintaining the receptor in a nonfunctional light‐induced conformation that may be toxic to photoreceptor cells. We propose that G51V and G89D shift the equilibrium from metarhodopsin I towards an intermediate, recently named as metarhodopsin Ib, proposed to interact with transducin without activating it. This may be one of the causes contributing to the molecular mechanisms underlying cell death associated with some retinitis pigmentosa mutations.

Retinitis Pigmentosa Mutants Provide Insight into the Role of the N-terminal Cap in Rhodopsin Folding, Structure, and Function

“…Furthermore, the light-activated mutant pigments had abnormal MII decay profiles with two kinetic components. Similar MII decay profiles have been described previously for other rhodopsin mutants, including those that cause ADRP (50,51). Although the molecular basis of the double-component decay profile is not known, it could be due to incomplete photoconversion of the mutant pigments to MII (Fig.…”

Molecular Mechanisms of Disease for Mutations at Gly-90 in Rhodopsin

“…The clinical phenotype of the visual disorders associated with both mutations, CSNB with G90D and RP with G90V, must be a consequence of differences in the dark-inactive structural conformation of the mutant receptor, which is thermally unstable in the case of the G90V mutation. This lower stability, alone or in combination with the formation of altered activation photointermediates upon illumination (29), could affect the rhodopsin cycle of intermediates, altering the normal rhodopsin turnover and prompting receptor malfunction and accumulation, which could trigger photoreceptor cell death associated with RP.…”

“…In contrast, G90V photobleaching behavior is altered, with formation of a distinguishable photointermediate upon illumination. This altered photointermediate formation has been proposed to affect the metarhodopsin conformation equilibria and to be one of the potential molecular defects associated with some RP mutants (29).…”

Differential Light-induced Responses in Sectorial Inherited Retinal Degeneration

“…7C). To determine whether this could be a differential feature of sector RP mutants, retinal release was compared with that of the G51V RP mutant; position Gly-51 is located one helical turn below Asn-55, and G51V also showed a different photobleaching behavior when compared with WT similar to N55K (45). In this case, however, the G51V MetaII assay showed a similar behavior to WT (data not shown).…”

“…Previous reports on mice carrying a transgene with the P23H mutation involved housing the animals under overhead light or in the dark, and higher photoreceptor degeneration was observed in the inferior retina (38). e Gt activation initial rate was calculated as previously described (45). In all cases, mean values and standard error were obtained from three independent experiments.…”