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Alexander V. Kolesnikov, Ning Zhang, Krzysztof Palczewski, Vladimir Kefalov; Altered Phototransduction in E150K Rod Opsin Mutant Mice. Invest. Ophthalmol. Vis. Sci. 2012;53(14):745.
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© ARVO (1962-2015); The Authors (2016-present)
The E150K missense mutation of rod opsin in humans causes an autosomal recessive form of the blinding disease, retinitis pigmentosa. Located in the C- terminus of the second opsin cytoplasmic loop, this mutation causes a re-distribution of electrostatic charge on its surface. In HEK-293 cells, the E150K substitution leads to major mislocalization of expressed mutant opsin but does not affect its folding, stability, or ability to bind 11-cis-retinal to form visual pigment and activate the G-protein, transducin. To investigate directly how the E150K rod opsin mutation affects rod phototransduction, we characterized the function of mouse knock-in rods expressing this mutant opsin.
We assessed photoreceptor function in 3-5 week-old homo- and heterozygous E150K and WT mice by transretinal ERG recordings after pharmacologically blocking post-photoreceptor components of the light response and by single-cell suction electrode recordings. We then compared the amplitude, kinetics, and photosensitivity of rod responses from WT and mutant mice.
In transretinal recordings, the maximum rod response amplitude in E150K retinas was reduced by 5-fold compared with that in WT retinas. Nevertheless, mutant pigment could still efficiently drive phototransduction in homozygous E150K rods where the activation phase of phototransduction was unaffected, as determined by single-cell recordings. However, we observed two major changes in dim flash photoresponse kinetics in mutant rods: a broadening of the response peak (time-to-peak increased from 139 to 166 ms) and a 2-fold acceleration of the dim flash recovery time constant (τrec). Consistent with the accelerated τrec, the dominant time constant of recovery from saturating flashes (τD) was 2.5-fold faster in mutant photoreceptors suggesting faster transducin/PDE inactivation. This rapid response recovery likely caused the observed 1.7-fold reduced photosensitivity in homozygous E150K cells. Heterozygous E150K rods displayed an intermediate phenotype.
Our findings demonstrate that the E150K mutation of opsin in mice dramatically reduces the maximum rod response amplitude, likely due to substantial photoreceptor degeneration. Notably, the mutant E150K opsin is capable of initiating phototransduction with normal amplification but altered response kinetics.
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