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Keisuke Sakurai, Akishi Onishi, Hiroo Imai, Osamu Chisaka, Takahiro Yamashita, Kei Nakatani, Yoshinori Shichida; Physiological role of M2-group cone visual pigments on photoresponse in chicken green knock-in mice. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2365.
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Rod and cone photoreceptors exhibit photoresponses different from each other and contain similar visual pigments proteins with distinctive molecular properties. Phylogenic analyses on visual pigments indicate that the rhodopsin group diverged from one of the cone visual pigment groups, referred to as M2 (RH2) group. To elucidate physiological significance of the divergence of visual pigments, we have created knock-in mice, where rhodopsin were replaced with chicken green-sensitive cone opsin as a representative of M2 group, and carried out electrophysiological recordings of the photoreceptors.
Knock-in mice, in which chicken green cDNA was introduced into mouse rhodopsin loci, were generated with homologous recombination on mouse ES cells. Using a suction electrode, we recorded membrane currents from singe rod photoreceptors of knock-in and wild-type mice.
With an immunohistochemistry of mice retina, chicken green cone pigments ectopically expressed by homologous recombination were found to be properly localized in rod outer segments. Electrophysiological analysis of suction recordings showed that photoisomerization of visual pigments required for the half-saturating response was 16 R* and 42 R* in wild-type and homozygote, respectively, indicating that the sensitivity as a function of photoisomerization is 0.4-fold lower in homozygote than that in wild-type. The mean amplitude of single-photon response (pA) calculated with an ensemble variance-to-mean ratio was 0.53 in wild-type and 0.22 in homozygote, which is in a good agreement with the result of photoisomerization-response relation. Moreover, the kinetics of dim flash response of homozygote was significantly accelerated as compared with that of wild-type. Whereas time-to-peak was 159 ms in wild-type and 145 ms in homozygote, integration time was 300 ms in wild-type and 189 ms in homozygote.
The expression pattern of chicken green opsin from recombinant allele is apparently under the endogenous regulatory control. The physiological results suggest that the dim-flash kinetics as well as response amplitude may be affected by property of visual pigments.
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