Elsevier

Vision Research

Volume 51, Issue 4, 23 February 2011, Pages 447-458
Vision Research

A mouse M-opsin monochromat: Retinal cone photoreceptors have increased M-opsin expression when S-opsin is knocked out

https://doi.org/10.1016/j.visres.2010.12.017Get rights and content
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Abstract

Mouse cone photoreceptors, like those of most mammals including humans, express cone opsins derived from two ancient families: S-opsin (gene Opn1sw) and M-opsin (gene Opn1mw). Most C57Bl/6 mouse cones co-express both opsins, but in dorso-ventral counter-gradients, with M-opsin dominant in the dorsal retina and S-opsin in the ventral retina, and S-opsin 4-fold greater overall. We created a mouse lacking S-opsin expression by the insertion of a Neomycin selection cassette between the third and fourth exons of the Opn1sw gene (Opn1swNeo/Neo). In strong contrast to published results characterizing mice lacking rhodopsin (Rho−/−) in which retinal rods undergo cell death by 2.5 months, cones of the Opn1swNeo/Neo mouse remain viable for at least 1.5 yrs, even though many ventral cones do not form outer segments, as revealed by high resolution immunohistochemistry and electron microscopy. Suction pipette recordings revealed that functional ventral cones of the Opn1swNeo/Neo mouse not only phototransduce light with normal kinetics, but are more sensitive to mid-wavelength light than their WT counterparts. Quantitative Western blot analysis revealed the basis of the heightened sensitivity to be increased M-opsin expression. Because S- and M-opsin transcripts must compete for the same translational machinery in cones where they are co-expressed, elimination of S-opsin mRNA in ventral Opn1swNeo/Neo cones likely increases M-opsin expression by relieving competition for translational machinery, revealing an important consequence of eliminating a dominant transcript. Overall, our results reveal a striking capacity for cone photoreceptors to function with much reduced opsin expression, and to remain viable in the absence of an outer segment.

Research highlights

► Targeted insertion of NeoR between ex 3 & 4 of Opn1sw created an S-Opsin null mouse. ► Increased mid-wave sensitivity in ventral cones of S-opsin null suggests elevated M-Opsin. ► Western blots reveal M-opsin expression elevated 1.6-fold above WT level in the S-opsin null. ► The increased M-opsin may result from elimination of competition between S- and M-opsin mRNA for ribosomes.

Keywords

Opsin
Cone survival
Phototransduction
Color vision

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