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W. Mao, K. J. Miyagishima, H. Moaven, A. Sampath, J. Chen; Distinct Isoforms of Transducin-Alpha Subunits Contribute to the Different Light Sensitivities in Rods and Cones. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1676. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Rod photoreceptors are approximately 100 times more sensitive to a single photon than cones. Transducin, a heterotrimeric G-protein involved in phototransduction cascade, consists of different alpha subunits in rods and cones. We hypothesized that cell-specific transducin-alpha may be one underlying factor for the differences in light sensitivity between rods and cones.
The mouse rod opsin promoter was used to drive expression of mouse cone transducin-alpha (cTα) in rod cells of rod transducin-alpha knockout (rTα-/-) mice. We examined the distribution of cTα in both dark-adapted and light-exposed rods by immunocytochemistry. Suction electrode recordings from single rods and ERGs were performed to compare the sensitivity of flash responses between transgenic and wild-type mice.
Expression of cTα in rod photoreceptors was observed in two independent lines of transgenic mice. The expression levels were 15%-20% when compared to endogenous rTα from wildtype mice. Ectopically expressed cTα exhibited similar pattern of light-driven translocation in rods as wildtype rTα. Whereas T beta-gamma showed diffused localization pattern in the rTα-/- rods and does not translocate in response to light exposure, expression of cTα in these rods restored the polarized distribution of T beta-gamma to the outer segment in the dark as well as their ability to translocate to the inner segment in response to light exposure. Suction electrode recordings showed that flash responses from cTα rods were ~100-fold less sensitive and recovered faster when compared to wild-type rods.
Our data suggest that cTα can interact with rod T beta-gamma and thereby restoring their polarized localization pattern and their ability to translocate in response to light. Exchanging endogenous rTα with cTα results a substantial decrease of light sensitivity even when the reduced expression level was taken into account. Therefore, transduction gain exhibited by cell-type specific G-proteins may play a role in the phenotypic difference between rods and cones.
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