Photoregeneration of rhodopsin and isorhodopsin from metarhodopsin III in the frog retina

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Rhodopsin-mediated light-off-induced protein kinase A activation in mouse rod photoreceptor cells

Light-induced extrasynaptic dopamine release in the retina reduces adenosine 3',5'-cyclic monophosphate (cAMP) in rod photoreceptor cells, which is thought to mediate light-dependent desensitization. However, the fine time course of the cAMP dynamics in rods remains elusive due to technical difficulty. Here, we visualized the spatiotemporal regulation of cAMP-dependent protein kinase (PKA) in mouse rods by two-photon live imaging of retinal explants of PKAchu mice, which express a fluorescent biosensor for PKA. Unexpectedly, in addition to the light-on-induced suppression, we observed prominent light-off-induced PKA activation. This activation required photopic light intensity and was confined to the illuminated rods. The estimated maximum spectral sensitivity of 489 nm and loss of the light-off-induced PKA activation in rod-transducin-knockout retinas strongly suggest the involvement of rhodopsin. In support of this notion, rhodopsin-deficient retinal explants showed only the light-on-induced PKA suppression. Taken together, these results suggest that, upon photopic light stimulation, rhodopsin and dopamine signals are integrated to shape the light-off-induced cAMP production and following PKA activation. This may support the dark adaptation of rods.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

Stability of dark state rhodopsin is mediated by a conserved ion pair in intradiscal loop E-2

The rhodopsin crystal structure reveals that intradiscal loop E-2 covers the 11-cis-retinal, creating a "retinal plug." Recently, we noticed the ends of loop E-2 are linked by an ion pair between residues Arg-177 and Asp-190, near the highly conserved disulfide bond. This ion pair appears biologically significant; it is conserved in almost all vertebrate opsins and may occur in other G-protein-coupled receptors. We report here that the Arg-177/Asp-190 ion pair is critical for the folding and stability of dark state rhodopsin. We find ion pair mutants that regenerate with retinal are functionally and spectrally wild-type-like yet thermally unstable in their dark state because of rapid hydrolysis of the retinal Schiff base linkage. Surprisingly, Arrhenius analysis indicates that the activation energies for the hydrolysis process are similar between the ion pair mutants and wild-type rhodopsin. Furthermore, the ion pair mutants do not show increased reactivity toward hydroxylamine, suggesting that their instability is not caused by an increased exposure to bulk solvent. Our results indicate that the loop E-2 ion pair is important for rhodopsin stability and thus suggest that retinitis pigmentosa observed in patients with Asp-190 mutations may in part be the result of thermally unstable rhodopsin proteins.

Effect of NADPH on formation and decay of human metarhodopsin III at physiological temperatures

Difference absorption spectra were recorded during the formation and decay of metarhodopsin III after sonicated membrane suspensions of rhodopsin were bleached at 37 degrees C. The data were analyzed using SVD, spectral decomposition and global exponential fitting. By comparison of the results in the presence or absence of 70 microM NADPH and those for bovine or human rhodopsin, a single comprehensive scheme was fit to all the data, including reduction of retinal to retinol by the intrinsic retinol dehydrogenase. On the time scale studied the mechanism involves two 382 nm absorbing species and two 468 nm, absorbing species, supporting the notion that human metarhodopsin III is not a homogeneous species. The results confirm that metarhodopsin III forms and persists sufficiently long in the human retina under physiological conditions that it could undergo secondary photoisomerization.

Metarhodopsin III formation and decay kinetics: comparison of bovine and human rhodopsin

Absorbance spectra were recorded at various times after photolysis of hypotonically washed membrane suspensions of human and bovine rhodopsin (pH 7 or 8, T = 20 or 37 degrees C). Global data analysis showed that at 20 degrees C, metarhodopsin III formation was the only process occurring from 1 to 30 min following photolysis. At pH 8, a significant amount of human metarhodopsin I was present before metarhodopsin III formed. At 37 degrees C, two decay processes were seen, formation of metarhodopsin III (lifetime 113 +/- 3 sec bovine, 93 +/- 2 sec human) and formation of a species with the properties of n-retinylidine opsin (lifetime 900 +/- 150 sec bovine, 2000 +/- 350 sec human). A larger fraction of human rhodopsin (approximately 50%) is converted to metarhodopsin III than is the case for bovine rhodopsin (approximately 35%).

Some properties of solubilized human rhodopsin

This investigation involved an examination of some properties of solubilized human rhodopsin. In confirmation of previous work, the spectral maximum was found to be at 493 nm at temperatures 5-10 degrees C below 37.5 degrees C. An increase in temperature to 37.5 degrees C produced only a minor shift of 2-4 nm toward the blue. The opsin displayed the classic and typical stereospecificity of vertebrate visual pigments, regenerating a pigment at 493 nm with 11-cis retinal and an isopigment at 483 nm with 9-cis retinal. No regeneration occurred with either all-trans or 13-cis retinal. The chromophoric photosensitivity of human rhodopsin and of its 11-cis regenerated pigment was found to be the same at 13.2 X 10(-17) cm2; that of the isopigment, at 4.5 X 10(-17) cm2. The long-lived photoproduct of human rhodopsin at 475 nm (metarhodopsin-III) was found to be especially interesting because of its protracted growth following a brief (20 sec) light exposure of the pigment and because of its long decay time even at 27 degrees C and higher. This property (growth and decay of metarhodopsin-III) was studied at temperatures ranging from 1.9 to 37.5 degrees C. Though NH2OH (4.6 X 10(-3) M) was found to speed the decay of metarhodopsin-III, it did not prevent its presence during decay for minutes after the 20-sec bleach. It is clear that the human metarhodopsin-III is indeed a long-lived intermediate of bleaching and evidence from the literature, which is cited, suggests that this product persists for significant periods of time in the retinas of mammals, including that of man. This fact suggests the possible physiological role of metarhodopsin-III in some aspects of vertebrate vision.

Effect of pH on the formation and decay of the metarhodopsins of the frog

1. Suspensions of membrane vesicles were prepared by sonication of dark-adapted frog rod outer segments. The pH was adjusted to one of twelve values between 5.0 and 9.8; the temperature was 15 degrees C. Each preparation was exposed to a single yellow flash of 120 microseconds duration, and rapid slow changes of absorbance were measured at 475 nm wavelength. 2. Rapid changes consist of a transient rise followed by a diphasic decay to a new level of absorbance which is lower than that before the light exposure. The level of absorbance reached at the end of the rapid changes is lower at lower pH. 3. Kinetic analysis reveals that three reactants take part in the rapid reactions, viz. metarhodopsin I and two isochromic forms of metarhodopsin II named metarhodopsin II' and metarhodopsin II". The kinetics of the reversible transition from metarhodopsin I to metarhodopsin II' are not measurably influenced by the pH of the medium. However, the reversible reaction of metarhodopsin II' with metarhodopsin II" is dependent on pH because metarhodopsin II" is encountered either as a protonated or as a deprotonated compound. 4. Slow reactions are due to metarhodopsin I and to metarhodopsin III. A combined quantitative analysis of both rapid and slow reactions involves a reversible reaction between metarhodopsin II' and metarhodopsin III and a hydrolysis of metarhodopsin II' to retinal and opsin. The scheme employed accounts for the pH dependence of all equilibria although only one of the metarhodopsins reacts directly with protons.

Transient light-induced conformational changes in rhodopsin

Arguments are presented which support the possibility that the unfolding of the rhodopsin molecule during photolysis up to the stage of metarhodopsin II is followed by a spontaneous refolding of the protein, once the isomerized retinaldehyde has left its original binding site. Such a transient conformational change might imply a very similar conformation for rhodopsin and opsin, apart from the presence of the chromophore.