The Gain of Rod Phototransduction

Leskov, Ilya; Klenchin, Vadim A.; Handy, Jason W.; Whitlock, Gary; Govardovskii, V. I.; Bownds, M D; Lamb, Trevor D; Pugh, Edward N.; Arshavsky, Vadim Y.

doi:10.1016/s0896-6273(00)00063-5

Cited by 163 publications

(79 citation statements)

References 45 publications

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“…Characteristics of the light-stimulated PDE activity in turn depend on the rate and extent of rhodopsin activation to MII, MII activation of G t , and the resulting activation of the PDE by G t␣ . A recently published model relates the generation of the ERG a-wave to the kinetic and equilibrium parameters associated with the various steps in the visual transduction pathway (26). The model explicitly relates the amplitude of the ERG a-wave to the number of activated rhodopsin molecules and the subsequent number of G proteins and PDE catalytic subunits activated, whereas the rate of generation of the a-wave is associated with the kinetics of coupling of these proteins in the signaling pathway.…”

Section: Discussionmentioning

confidence: 99%

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

Niu

Mitchell

Lim

et al. 2004

Journal of Biological Chemistry

211

151

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The fatty acid (FA) docosahexaenoic acid (DHA, 22: 6n-3) is highly enriched in membrane phospholipids of the central nervous system and retina. Loss of DHA because of n-3 FA deficiency leads to suboptimal function in learning, memory, olfactory-based discrimination, spatial learning, and visual acuity. G protein-coupled receptor (GPCR) signal transduction is a common signaling motif in these neuronal pathways. Here we investigated the effect of n-3 FA deficiency on GPCR signaling in retinal rod outer segment (ROS) membranes isolated from rats raised on n-3-adequate or -deficient diets. ROS membranes of second generation n-3 FA-deficient rats had ϳ80% less DHA than n-3-adequate rats. DHA was replaced by docosapentaenoic acid (22:5n-6), an n-6 FA. This replacement correlated with desensitization of visual signaling in n-3 FA-deficient ROS, as evidenced by reduced rhodopsin activation, rhodopsintransducin (G t ) coupling, cGMP phosphodiesterase activity, and slower formation of metarhodopsin II (MII) and the MII-G t complex relative to n-3 FA-adequate ROS. ROS membranes from n-3 FA-deficient rats exhibited a higher degree of phospholipid acyl chain order relative to n-3 FA-adequate rats. These findings reported here provide an explanation for the reduced amplitude and delayed response of the electroretinogram a-wave observed in n-3 FA deficiency in rodents and nonhuman primates. Because members of the GPCR family are widespread in signaling pathways in the nervous system, the effect of reduced GPCR signaling due to the loss of membrane DHA may serve as an explanation for the suboptimal neural signaling observed in n-3 FA deficiency.

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Section: Discussionmentioning

confidence: 99%

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

Niu

Mitchell

Lim

et al. 2004

Journal of Biological Chemistry

211

151

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“…The visual phototransduction cascade is the most quantitatively characterized G protein signaling system (24). The initial rate of G protein activation was estimated to be Ϸ100 G proteins per second per activated rhodopsin (25), which is Ϸ1,000-fold faster than the yeast rate, which is 0.1 G proteins per second per activated receptor. Likewise, the rates of G protein deactivation were Ϸ10-fold faster than the corresponding yeast values: the GTPase activity of transducin was 1 s Ϫ1 in the presence of RGS9 and 0.02 s Ϫ1 in the absence of RGS9 (26).…”

Section: Discussionmentioning

confidence: 99%

A quantitative characterization of the yeast heterotrimeric G protein cycle

Kitano

2003

Proc. Natl. Acad. Sci. U.S.A.

171

188

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The yeast mating response is one of the best understood heterotrimeric G protein signaling pathways. Yet, most descriptions of this system have been qualitative. We have quantitatively characterized the heterotrimeric G protein cycle in yeast based on direct in vivo measurements. We used fluorescence resonance energy transfer to monitor the association state of cyan fluorescent protein (CFP)-G␣ and G␤␥-yellow fluorescent protein (YFP), and we found that receptor-mediated G protein activation produced a loss of fluorescence resonance energy transfer. Quantitative time course and dose-response data were obtained for both wild-type and mutant cells possessing an altered pheromone response. These results paint a quantitative portrait of how regulators such as Sst2p and the C-terminal tail of ␣-factor receptor modulate the kinetics and sensitivity of G protein signaling. We have explored critical features of the dynamics including the rapid rise and subsequent decline of active G proteins during the early response, and the relationship between the G protein activation dose-response curve and the downstream dose-response curves for cell-cycle arrest and transcriptional induction. Fitting the data to a mathematical model produced estimates of the in vivo rates of heterotrimeric G protein activation and deactivation in yeast. Living organisms detect and respond to a variety of environmental cues through heterotrimeric G protein signal transduction pathways. Genetic or pharmacologic perturbation of these systems in humans can have significant medical consequences (1). Many pharmaceutical agents are directed against components of the G protein activation͞deactivation cycle (1). Achieving a quantitative understanding of this cycle and of the relationship between G protein activation and downstream physiology can aid in drug development.One of the best studied heterotrimeric G protein signaling systems is the yeast mating response in Saccharomyces cerevisiae (2, 3). Haploid yeast cells respond to a peptide pheromone from their partner by undergoing a series of events (e.g., cell-cycle arrest, synthesis of new proteins) in preparation for mating. In a cells, the pheromone ␣-factor, secreted by ␣ cells, activates the G protein coupled ␣-factor receptor (Ste2p). Most of the components of this pathway have been identified, including the receptors Ste2p and Ste3p (a-factor receptor), the G protein subunits Gpa1p (G␣), Ste4p (G␤), and Ste18p (G␥), and the G protein regulator (RGS) Sst2p.Yeast geneticists have isolated mutants possessing altered sensitivity to the pheromone ␣-factor (4). In particular, the following all confer an ␣-factor supersensitive phenotype: the deletion of BAR1 (bar1⌬), which encodes a secreted protease that degrades ␣-factor, the deletion of SST2 (sst2⌬), and the removal of the C-terminal tail of ␣-factor receptor STE2 (ste2 300⌬). It is important to provide a quantitative description of how these mutations affect the G protein cycle to complement the existing qualitative understanding.Until recently, the ...

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“…A model, relating the biochemical events in the visual transduction pathway with the neural response, as measured by the electroretinograms, was recently published (34). In this model, the response at any time after a light stimulus is directly proportional to the concentration of activated rhodopsin molecules, i.e.…”

Section: Discussionmentioning

confidence: 99%

Optimization of Receptor-G Protein Coupling by Bilayer Lipid Composition II

Niu

Mitchell

Litman

2001

Journal of Biological Chemistry

106

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The visual transduction system was used as a model to investigate the effects of membrane lipid composition on receptor-G protein coupling. Rhodopsin was reconstituted into large, unilamellar phospholipid vesicles with varying acyl chain unsaturation, with and without cholesterol. The association constant (K a ) for metarhodopsin II (MII) and transducin (G t ) binding was determined by monitoring MII-G t complex formation spectrophotometrically. At 20°C, in pH 7.5 isotonic buffer, the strongest MII-G t binding was observed in 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0,22: 6PC), whereas the weakest binding was in 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (18:0,18:1PC) with 30 mol% cholesterol. Increasing acyl chain unsaturation from 18:0,18:1PC to 18:0,22:6PC resulted in a 3-fold increase in K a . The inclusion of 30 mol% cholesterol in the membrane reduced K a in both 18:0,22:6PC and 18:0,18: 1PC. These findings demonstrate that membrane compositions can alter the signaling cascade by changing protein-protein interactions occurring predominantly in the hydrophilic region of the proteins, external to the lipid bilayer. These findings, if extended to other members of the superfamily of G protein-coupled receptors, suggest that a loss in efficiency of receptor-G protein binding is a contributing factor to the loss of cognitive skills, odor and spatial discrimination, and visual function associated with n-3 fatty acid deficiency.The G protein-coupled motif is a fundamental mode of cell signaling, utilized in vision, taste, olfaction, and a variety of neurotransmitter systems. The receptors for these systems are integral membrane proteins, embedded in a lipid matrix. Neuronal and retinal tissues and the olfactory bulb contain high levels of the n-3 polyunsaturated acyl chain derived from docosahexaenoic acid (22:6n-3) 1 in their cell membrane phospholipids (1, 2). Approximately 50% of the acyl chains in the phospholipids of the ROS disc membrane consist of 22:6n-3 (1). The physiological significance of 22:6n-3 is demonstrated by the impaired visual response (3), learning deficits (2), loss of odor discrimination (4), and reduced spatial learning (5) associated with n-3 fatty acid deficiency. In all cases where acyl chain analysis was carried out, the 22:6n-3 content of membrane phospholipids was dramatically reduced in the n-3-deficient animals where it was replaced by 22:5n-6 (5). These findings suggest that the high levels of 22:6n-3 in membrane phospholipids play a critical role in various membrane-associated signaling pathways. A common thread in several of these processes is the ubiquitous motif of G protein-coupled signaling systems. However, molecular mechanisms linking 22:6n-3 phospholipids with essential physiological functions remain to be clarified. The study described herein aims to elucidate such mechanisms by investigating the effect of membrane lipid composition on G protein-coupled signal transduction. In G protein-coupled systems, the receptor activates an effector protein t...

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The Gain of Rod Phototransduction

Abstract: and transforms into the active conformation metarhodopsin II (R*). The signal is then amplified by the reactions illustrated schematically in Figure 1A to generate the electrical response: the suppression of circulating current that results from closure of cGMP-activated

Cited by 163 publications

References 45 publications

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

A quantitative characterization of the yeast heterotrimeric G protein cycle

Optimization of Receptor-G Protein Coupling by Bilayer Lipid Composition II

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