The Experts below are selected from a list of 189 Experts worldwide ranked by ideXlab platform
King-wai Yau - One of the best experts on this subject based on the ideXlab platform.
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Farnesylation of retinal transducin underlies its translocation during Light Adaptation
Neuron, 2005Co-Authors: Hidetoshi Kassai, King-wai Yau, Atsu Aiba, Kazuki Nakao, Kenji Nakamura, Motoya Katsuki, Wei Hong Xiong, Hiroo Imai, Yoshinori Shichida, Yoshinori SatomiAbstract:G proteins are posttranslationally modified by isoprenylation: either farnesylation or geranylgeranylation. The γ subunit of retinal transducin (Tα/Tβγ) is selectively farnesylated, and the farnesylation is required for Light signaling mediated by transducin in rod cells. However, whether and how this selective isoprenylation regulates cellular functions remain poorly understood. Here we report that knockin mice expressing geranylgeranylated Tγ showed normal rod responses to dim flashes under dark-adapted conditions but exhibited impaired properties in Light Adaptation. Of note, geranylgeranylation of Tγ suppressed Light-induced transition of Tβγ from membrane to cytosol, and also attenuated its Light-dependent translocation from the outer segment to the inner region, an event contributing to retinal Light Adaptation. These results indicate that, while the farnesylation of transducin is interchangeable with the geranylgeranylation in terms of the Light signaling, the selective farnesylation is important for visual sensitivity regulation by providing sufficient but not excessive membrane anchoring of Tβγ.
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Light Adaptation in retinal rods of the rabbit and two other nonprimate mammals.
The Journal of general physiology, 1991Co-Authors: Kei Nakatani, T Tamura, King-wai YauAbstract:The responses of rabbit rods to Light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of Light Adaptation. At the response peak, however, the amplitude-intensity relation fell sLightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that Light Adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady Light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of Light Adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background Light intensity. We conclude that rabbit rods adapt to Light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.
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Calcium and Light Adaptation in retinal photoreceptors.
Current opinion in neurobiology, 1991Co-Authors: King-wai YauAbstract:In the past several years there has been great progress in the understanding of the phototransduction process in retinal photoreceptors. Recently, this knowledge has expanded and we now understand the mechanism of background Light Adaptation in these cells and the role that calcium has to play.
Vadim Y. Arshavsky - One of the best experts on this subject based on the ideXlab platform.
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Two Temporal Phases of Light Adaptation in Retinal Rods
The Journal of general physiology, 2002Co-Authors: Peter D. Calvert, Vadim Y. Arshavsky, Victor I. Govardovskii, Clint L. MakinoAbstract:Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady Light. Light Adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of Light Adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie Light Adaptation on the molecular level. We found that Adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1–2-s time constant for the onset of Adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every Light intensity tested. The slow phase was observed only at Light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of Adaptation of ∼3,000-fold. Although the fast Adaptation is likely to originate from the well characterized Ca2+-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow Adaptation is unclear. We tested the hypothesis that the slow Adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow Adaptation phase, this hypothesis was ruled out because the intensity of Light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.
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Onset of Feedback Reactions Underlying Vertebrate Rod Photoreceptor Light Adaptation
The Journal of general physiology, 1998Co-Authors: Peter D. Calvert, Yvette M. Lefebvre, Vadim Y. ArshavskyAbstract:Light Adaptation in vertebrate photoreceptors is thought to be mediated through a number of biochemical feedback reactions that reduce the sensitivity of the photoreceptor and accelerate the kinetics of the photoresponse. Ca2+ plays a major role in this process by regulating several components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca2+. Recently, it was proposed that cGMP may be another messenger of Light Adaptation since it is able to regulate the rate of transducin GTPase and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca2+ and cGMP are followed by the changes in the rates of corresponding enzymatic reactions in frog rod outer segments. Our data indicate that there is a temporal hierarchy among reactions that underlie Light Adaptation. Guanylate cyclase activity and rhodopsin phosphorylation respond to changes in Ca2+ very rapidly, on a subsecond time scale. This enables them to accelerate the falling phase of the flash response and to modulate flash sensitivity during continuous illumination. To the contrary, the acceleration of transducin GTPase, even after significant reduction in cGMP, occurs over several tens of seconds. It is substantially delayed by the slow dissociation of cGMP from the noncatalytic sites for cGMP binding located on cGMP phosphodiesterase. Therefore, cGMP-dependent regulation of transducin GTPase is likely to occur only during prolonged bright illumination.
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How many Light Adaptation mechanisms are there
Behavioral and Brain Sciences, 1995Co-Authors: M. Deric Bownds, Vadim Y. ArshavskyAbstract:The generally positive response to our target article indicates that most of the commentators accept our contention that Light Adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in Adaptation. Finally, additional aspects of cGMP involvement in Adaptation are considered. We discuss each of these points in turn.
Peter D. Calvert - One of the best experts on this subject based on the ideXlab platform.
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Two Temporal Phases of Light Adaptation in Retinal Rods
The Journal of general physiology, 2002Co-Authors: Peter D. Calvert, Vadim Y. Arshavsky, Victor I. Govardovskii, Clint L. MakinoAbstract:Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady Light. Light Adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of Light Adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie Light Adaptation on the molecular level. We found that Adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1–2-s time constant for the onset of Adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every Light intensity tested. The slow phase was observed only at Light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of Adaptation of ∼3,000-fold. Although the fast Adaptation is likely to originate from the well characterized Ca2+-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow Adaptation is unclear. We tested the hypothesis that the slow Adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow Adaptation phase, this hypothesis was ruled out because the intensity of Light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.
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Onset of Feedback Reactions Underlying Vertebrate Rod Photoreceptor Light Adaptation
The Journal of general physiology, 1998Co-Authors: Peter D. Calvert, Yvette M. Lefebvre, Vadim Y. ArshavskyAbstract:Light Adaptation in vertebrate photoreceptors is thought to be mediated through a number of biochemical feedback reactions that reduce the sensitivity of the photoreceptor and accelerate the kinetics of the photoresponse. Ca2+ plays a major role in this process by regulating several components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca2+. Recently, it was proposed that cGMP may be another messenger of Light Adaptation since it is able to regulate the rate of transducin GTPase and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca2+ and cGMP are followed by the changes in the rates of corresponding enzymatic reactions in frog rod outer segments. Our data indicate that there is a temporal hierarchy among reactions that underlie Light Adaptation. Guanylate cyclase activity and rhodopsin phosphorylation respond to changes in Ca2+ very rapidly, on a subsecond time scale. This enables them to accelerate the falling phase of the flash response and to modulate flash sensitivity during continuous illumination. To the contrary, the acceleration of transducin GTPase, even after significant reduction in cGMP, occurs over several tens of seconds. It is substantially delayed by the slow dissociation of cGMP from the noncatalytic sites for cGMP binding located on cGMP phosphodiesterase. Therefore, cGMP-dependent regulation of transducin GTPase is likely to occur only during prolonged bright illumination.
Yoshinori Satomi - One of the best experts on this subject based on the ideXlab platform.
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Farnesylation of retinal transducin underlies its translocation during Light Adaptation
Neuron, 2005Co-Authors: Hidetoshi Kassai, King-wai Yau, Atsu Aiba, Kazuki Nakao, Kenji Nakamura, Motoya Katsuki, Wei Hong Xiong, Hiroo Imai, Yoshinori Shichida, Yoshinori SatomiAbstract:G proteins are posttranslationally modified by isoprenylation: either farnesylation or geranylgeranylation. The γ subunit of retinal transducin (Tα/Tβγ) is selectively farnesylated, and the farnesylation is required for Light signaling mediated by transducin in rod cells. However, whether and how this selective isoprenylation regulates cellular functions remain poorly understood. Here we report that knockin mice expressing geranylgeranylated Tγ showed normal rod responses to dim flashes under dark-adapted conditions but exhibited impaired properties in Light Adaptation. Of note, geranylgeranylation of Tγ suppressed Light-induced transition of Tβγ from membrane to cytosol, and also attenuated its Light-dependent translocation from the outer segment to the inner region, an event contributing to retinal Light Adaptation. These results indicate that, while the farnesylation of transducin is interchangeable with the geranylgeranylation in terms of the Light signaling, the selective farnesylation is important for visual sensitivity regulation by providing sufficient but not excessive membrane anchoring of Tβγ.
Hiroko Terasaki - One of the best experts on this subject based on the ideXlab platform.
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Pikachurin Protein Required for Increase of Cone Electroretinogram B-Wave during Light Adaptation.
PloS one, 2015Co-Authors: Masatoshi Nagaya, Mineo Kondo, Shinji Ueno, Taro Kominami, Ayami Nakanishi, Toshiyuki Koyasu, Takahisa Furukawa, Hiroko TerasakiAbstract:In normal eyes, the amplitude of the b-wave of the photopic ERGs increases during Light Adaptation, but the mechanism causing this increase has not been fully determined. The purpose of this study was to evaluate the contribution of receptoral and post-receptoral components of the retina to this phenomenon. To accomplish this, we examined the ERGs during Light Adaptation in Pikachurin null-mutant (Pika -/-) mice, which have a misalignment of the bipolar cell dendritic tips to the photoreceptor ribbon synapses. After dark-Adaptation, photopic ERGs were recorded from Pika -/- and wild type (WT) mice during the first 9 minutes of Light Adaptation. In some of the mice, post-receptoral components were blocked pharmacologically. The photopic b-waves of WT mice increased by 50% during the 9 min of Light Adaptation as previously reported. On the other hand, the b-waves of the Pika -/- mice decreased by 20% during the same time period. After blocking post-receptoral components, the b-waves were abolished from the WT mice, and the ERGs resembled those of the Pika -/- mice. The extracted post-receptoral component increased during Light Adaptation in the WT mice, but decreased for the first 3 minutes to a plateau in Pika -/- mice. We conclude that the normal synaptic connection between photoreceptor and retinal ON bipolar cells, which is controlled by pikachurin, is required for the ERGs to increase during Light-Adaptation. The contributions of post-receptoral components are essential for the photopic b-wave increase during the Light Adaptation.
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Amplitude increase of the multifocal electroretinogram during Light Adaptation.
Investigative ophthalmology & visual science, 1999Co-Authors: Mineo Kondo, Yozo Miyake, Chang Hua Piao, Atsuhiro Tanikawa, Masayuki Horiguchi, Hiroko TerasakiAbstract:PURPOSE. To determine using the multifocal ERG technique whether there are any regional differences in the increase in the amplitude of cone electroretinograms (ERGs) during Light Adaptation. METHODS. Multifocal ERGs were recorded with the Visual Evoked Response Imaging System from five normal subjects. Thirty-seven hexagonal stimulus elements and a recording time of 60 seconds were used. After 20 minutes of dark Adaptation, multifocal ERGs were repeatedly recorded every 2 minutes over a period of 16 minutes. The amplitudes of the multifocal ERGs at different eccentricities were compared during the 16 minutes of Light Adaptation. RESULTS. During the 16 minutes of Light Adaptation, the summed responses of the multifocal ERGs increased in amplitude an average of 36% and 47% for the negative and positive components, respectively. The magnitude of increase was minimal in the central retina at 22% and was significantly larger in the peripheral retina at 58%. The implicit time was sLightly increased (,4%) with Light Adaptation, but there were no regional differences. CONCLUSIONS. The results demonstrated that there are topographic variations in the amplitude increase of cone ERGs during Light Adaptation. This topographic variation indicates that the mechanism for the increase must be based on known regional differences in the retina. (Invest Ophthalmol Vis Sci. 1999;40:2633‐2637)