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

Center for Neuroscience, University of California, Davis, CA 95618, USA.
Vision research (Impact Factor: 1.82). 02/2011; 51(4):447-58. DOI: 10.1016/j.visres.2010.12.017
Source: PubMed


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 (Opn1sw(Neo/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 Opn1sw(Neo/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 Opn1sw(Neo/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 Opn1sw(Neo/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.

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    • "In Figure 1A,B, we show the predicted average flash response profiles of cones at different retinal latitudes (1–4: mixed S/M cones) and that of the small but distinct population of pure S cones sparsely distributed throughout the retina. To generate these profiles, we: (1) used hyperbolic saturation functions (Nikonov et al., 2006); (2) assumed the flash sensitivity of the normalized response of S-and M-cones in G t α −/− mice, recorded with suction pipette and without a rod-saturating background (Table 1 in Nikonov et al., 2006: 0.042% photons −1 ·µm 2 , equivalent to a half-saturating flash strength of 2381 ph·µm −2 ), to be valid for S-and M-cones in wild type mice; moreover, we assumed that all cones express the same total amount of opsin; (3) the typical fraction of M-opsin expressed by cones as a function of their retinal latitude (i.e., the average among cones within each horizontal slice of retina) was taken as the average of the values predicted by two recently published quantitative models (Daniele et al., 2011; Wang et al., 2011) obtained from mice in which rod responses were absent or suppressed; since these models extend to slightly different retinal latitudes (±2.5 mm and ±2.0 mm, respectively), we normalized their ranges prior to averaging ; the relevant parameters of the cones shown in Figure 1, given as %M-opsin/position, are 1: 64%/dorsalmost, 2: 24%/dorsal third, 3: 4.7%/ventral third, 4: 1.2%/ventralmost, S: 0%/ubiquitous; (4) the absorbance of M-opsin at 365 nm (peak of the β-band) was taken as 20% of maximum, while for S opsin, its absorbance at 520 nm was taken to be 4 log-units below maximum (based on Govardovskii et al., 2000). The kinetics protocol (Figure 1C) was designed to detect rod input in cones by exploiting the high sensitivity of rods and their slow recovery after a saturating flash. "
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