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The 9-methyl group of retinal is essential for rapid Meta II decay and phototransduction quenching in red cones.

Estevez ME, Kolesnikov AV, Ala-Laurila P, Crouch RK, Govardovskii VI, Cornwall MC - J. Gen. Physiol. (2009)

Bottom Line: In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group.This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light.These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA. estevez@bu.edu

ABSTRACT
Cone photoreceptors of the vertebrate retina terminate their response to light much faster than rod photoreceptors. However, the molecular mechanisms underlying this rapid response termination in cones are poorly understood. The experiments presented here tested two related hypotheses: first, that the rapid decay rate of metarhodopsin (Meta) II in red-sensitive cones depends on interactions between the 9-methyl group of retinal and the opsin part of the pigment molecule, and second, that rapid Meta II decay is critical for rapid recovery from saturation of red-sensitive cones after exposure to bright light. Microspectrophotometric measurements of pigment photolysis, microfluorometric measurements of retinol production, and single-cell electrophysiological recordings of flash responses of salamander cones were performed to test these hypotheses. In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group. Meta II decay was four to five times slower and subsequent retinol production was three to four times slower in red-sensitive cones lacking the 9-methyl group of retinal. This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light. A mathematical model of the turn-off process of phototransduction revealed that the slower recovery of photoresponse can be explained by slower Meta decay of 9-demethyl visual pigment. These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.

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A comparison of time courses of ROL production derived from microspectrophotometry versus microfluorometry. Data from Figs. 3 (bottom) and 4 have been replotted on the same graph for comparison. Data obtained by microspectrophotometry (MSP) are shown in black: n = 11 cells (native), 9 cells (9-DM), and 11 cells (11-cis). Fluorescence data are shown in gray: n = 6 cells (native), 6 cells (9-DM), and 5 cells (11-cis). Concentration of ROL in microspectrophotometry data are expressed as a fraction of bleached cone pigment. Fluorescence data are scaled for best visual match to microspectrophotometry. Error bars show ± SEM.
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fig5: A comparison of time courses of ROL production derived from microspectrophotometry versus microfluorometry. Data from Figs. 3 (bottom) and 4 have been replotted on the same graph for comparison. Data obtained by microspectrophotometry (MSP) are shown in black: n = 11 cells (native), 9 cells (9-DM), and 11 cells (11-cis). Fluorescence data are shown in gray: n = 6 cells (native), 6 cells (9-DM), and 5 cells (11-cis). Concentration of ROL in microspectrophotometry data are expressed as a fraction of bleached cone pigment. Fluorescence data are scaled for best visual match to microspectrophotometry. Error bars show ± SEM.

Mentions: The analysis of spectral data from cones in the dark-adapted condition (native visual pigment) is complicated by the varying ratio of two visual pigment chromophores (A1 and A2 retinal), which varies seasonally and from animal to animal among larval salamanders but can be determined by fitting the dark-adapted spectra with a combination of A1- and A2-based visual pigment templates (Govardovskii et al., 2000). Because of this A1/A2 mixture in native dark-adapted pigment, experiments throughout the entire study were designed to mostly compare cones bleached and regenerated with 9-DM retinal to cones bleached and regenerated with 11-cis retinal. However, often data from native cones are also illustrated for comparison (e.g., see Figs. 1–5).


The 9-methyl group of retinal is essential for rapid Meta II decay and phototransduction quenching in red cones.

Estevez ME, Kolesnikov AV, Ala-Laurila P, Crouch RK, Govardovskii VI, Cornwall MC - J. Gen. Physiol. (2009)

A comparison of time courses of ROL production derived from microspectrophotometry versus microfluorometry. Data from Figs. 3 (bottom) and 4 have been replotted on the same graph for comparison. Data obtained by microspectrophotometry (MSP) are shown in black: n = 11 cells (native), 9 cells (9-DM), and 11 cells (11-cis). Fluorescence data are shown in gray: n = 6 cells (native), 6 cells (9-DM), and 5 cells (11-cis). Concentration of ROL in microspectrophotometry data are expressed as a fraction of bleached cone pigment. Fluorescence data are scaled for best visual match to microspectrophotometry. Error bars show ± SEM.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2717693&req=5

fig5: A comparison of time courses of ROL production derived from microspectrophotometry versus microfluorometry. Data from Figs. 3 (bottom) and 4 have been replotted on the same graph for comparison. Data obtained by microspectrophotometry (MSP) are shown in black: n = 11 cells (native), 9 cells (9-DM), and 11 cells (11-cis). Fluorescence data are shown in gray: n = 6 cells (native), 6 cells (9-DM), and 5 cells (11-cis). Concentration of ROL in microspectrophotometry data are expressed as a fraction of bleached cone pigment. Fluorescence data are scaled for best visual match to microspectrophotometry. Error bars show ± SEM.
Mentions: The analysis of spectral data from cones in the dark-adapted condition (native visual pigment) is complicated by the varying ratio of two visual pigment chromophores (A1 and A2 retinal), which varies seasonally and from animal to animal among larval salamanders but can be determined by fitting the dark-adapted spectra with a combination of A1- and A2-based visual pigment templates (Govardovskii et al., 2000). Because of this A1/A2 mixture in native dark-adapted pigment, experiments throughout the entire study were designed to mostly compare cones bleached and regenerated with 9-DM retinal to cones bleached and regenerated with 11-cis retinal. However, often data from native cones are also illustrated for comparison (e.g., see Figs. 1–5).

Bottom Line: In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group.This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light.These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA. estevez@bu.edu

ABSTRACT
Cone photoreceptors of the vertebrate retina terminate their response to light much faster than rod photoreceptors. However, the molecular mechanisms underlying this rapid response termination in cones are poorly understood. The experiments presented here tested two related hypotheses: first, that the rapid decay rate of metarhodopsin (Meta) II in red-sensitive cones depends on interactions between the 9-methyl group of retinal and the opsin part of the pigment molecule, and second, that rapid Meta II decay is critical for rapid recovery from saturation of red-sensitive cones after exposure to bright light. Microspectrophotometric measurements of pigment photolysis, microfluorometric measurements of retinol production, and single-cell electrophysiological recordings of flash responses of salamander cones were performed to test these hypotheses. In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group. Meta II decay was four to five times slower and subsequent retinol production was three to four times slower in red-sensitive cones lacking the 9-methyl group of retinal. This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light. A mathematical model of the turn-off process of phototransduction revealed that the slower recovery of photoresponse can be explained by slower Meta decay of 9-demethyl visual pigment. These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.

Show MeSH
Related in: MedlinePlus