Critical processes which characterize the photocurrent of retinal rod outer segments to flash stimuli

Molecular properties of rhodopsin and rod function

Signal transduction in rod cells begins with photon absorption by rhodopsin and leads to the generation of an electrical response. The response profile is determined by the molecular properties of the phototransduction components. To examine how the molecular properties of rhodopsin correlate with the rod-response profile, we have generated a knock-in mouse with rhodopsin replaced by its E122Q mutant, which exhibits properties different from those of wild-type (WT) rhodopsin. Knock-in mouse rods with E122Q rhodopsin exhibited a photosensitivity about 70% of WT. Correspondingly, their single-photon response had an amplitude about 80% of WT, and a rate of decline from peak about 1.3 times of WT. The overall 30% lower photosensitivity of mutant rods can be explained by a lower pigment photosensitivity (0.9) and the smaller single-photon response (0.8). The slower decline of the response, however, did not correlate with the 10-fold shorter lifetime of the meta-II state of E122Q rhodopsin. This shorter lifetime became evident in the recovery phase of rod cells only when arrestin was absent. Simulation analysis of the photoresponse profile indicated that the slower decline and the smaller amplitude of the single-photon response can both be explained by the shift in the meta-I/meta-II equilibrium of E122Q rhodopsin toward meta-I. The difference in meta-III lifetime between WT and E122Q mutant became obvious in the recovery phase of the dark current after moderate photobleaching of rod cells. Thus, the present study clearly reveals how the molecular properties of rhodopsin affect the amplitude, shape, and kinetics of the rod response.

Strength of Ca(2+) binding to retinal lipid membranes: consequences for lipid organization

There is evidence that membranes of rod outer segment (ROS) disks are a high-affinity Ca(2+) binding site. We were interested to see if the high occurrence of sixfold unsaturated docosahexaenoic acid in ROS lipids influences Ca(2+)-membrane interaction. Ca(2+) binding to polyunsaturated model membranes that mimic the lipid composition of ROS was studied by microelectrophoresis and (2)H NMR. Ca(2+) association constants of polyunsaturated membranes were found to be a factor of approximately 2 smaller than constants of monounsaturated membranes. Furthermore, strength of Ca(2+) binding to monounsaturated membranes increased with the addition of cholesterol, while binding to polyunsaturated lipids was unaffected. The data suggest that the lipid phosphate groups of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) in PC/PE/PS (4:4:1, mol/mol) are primary targets for Ca(2+). Negatively charged serine in PS controls Ca (2+) binding by lowering the electric surface potential and elevating cation concentration at the membrane/water interface. The influence of hydrocarbon chain unsaturation on Ca(2+) binding is secondary compared to membrane PS content. Order parameter analysis of individual lipids in the mixture revealed that Ca(2+) ions did not trigger lateral phase separation of lipid species as long as all lipids remained liquid-crystalline. However, depending on temperature and hydrocarbon chain unsaturation, the lipid with the highest chain melting temperature converted to the gel state, as observed for the monounsaturated phosphatidylethanolamine (PE) in PC/PE/PS (4:4:1, mol/mol) at 25 degrees C.

Modeling and analysis of spatio-temporal change in [Ca2+]i in a retinal rod outer segment

The change in [Ca2+]i, in a retinal rod outer segment to flash and step stimuli was simulated. The present model included inward and outward calcium fluxes through cation channels and an Na-Ca-K exchanger, respectively, calcium buffers, and diffusion through the interdiskal space of cytoplasm and incisures. Under control conditions (with the diffusion constant for calcium (DCa) of 10(-10) m2/s and the total concentration ([B]t) of 240 microM) the decrease in [Ca2+]i to flash stimuli was found to be localized around the edge of a disk irrespective of the presence or absence of incisures. Homogeneous but limited degree of decrease in [Ca2+]i was seen under a larger DCa of 10(-9) m2/s with no incisure. For the step illumination by which all the cation channels on a plasma membrane were closed, the decrease in [Ca2+]i around the edge of a disk was quick, while that at the center of an interdiskal space was slow (50% of the resting level at 5 s after the onset of the step illumination). These results indicate that the calcium feedback on guanylate cyclase and possibly on S-modulin in response to flash stimuli proceeds only around the edge of disk membranes or on the plasma membrane. This implicates localized mechanisms for the signal transduction and early phase of adaptation within rod outer segments.

Transduction steps which characterize retinal cone current responses to flash stimuli

There are three prominent characteristics in current responses of retinal cones to flash stimuli when compared with those of rods. First, the time-to-peak of the photocurrent is two to four times faster than for the rod. Second, the photosensitivity of the cone is more than one order of magnitude lower than that of the rod. Third, current responses of cones resemble characteristic under-shoot in the recovery phase. At present, however, it is not known what kinds of mechanisms underlie these characteristics. In the present study, critical steps which characterize cone responses have been investigated by changing rate constants in the model for the signal transduction scheme of cone outer segments under the assumption that the scheme is basically the same as for rods. The computer simulations have shown that rate constants were divided into three categories: those which do not appreciably change photocurrent, those which change only photosensitivity, and those which change both sensitivity and speed. Furthermore, rate constants which characterize undershoot were identified. From these simulations it has been revealed that characteristics of the cone response could be reproduced by changing the rate constants. The critical ones are rate of rhodopsin deactivation, rate of GTP hydrolysis, and deactivation rate of activated phosphodiesterase. These results suggest possibilities that the signal transduction mechanisms in cones are basically the same as those for rods, while rate constants are different.