Bisegna Paolo, Caruso Giovanni, Andreucci Daniele, Shen Lixin, Gurevich Vsevolod V, Hamm Heidi E, DiBenedetto Emmanuele
Department of Civil Engineering, University of Rome, Tor Vergata, Italy.
Biophys J. 2008 May 1;94(9):3363-83. doi: 10.1529/biophysj.107.114058.
The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately. We show that the activation cascade is responsible for generation of variability, whereas diffusion of the second messengers is responsible for its suppression. Randomness of the activation site contributes at early times to the coefficient of variation of the photoresponse, whereas the Brownian path of a photoisomerized rhodopsin (Rh*) has a negligible effect. The major driver of variability is the turnoff mechanism of Rh*, which occurs essentially within the first 2-4 phosphorylated states of Rh*. Theoretically increasing the number of steps to quenching does not significantly decrease the corresponding coefficient of variation of the effector, in agreement with the biochemical limitations on the phosphorylated states of the receptor. Diffusion of the second messengers in the cytosol acts as a suppressor of the variability generated by the activation cascade. Calcium feedback has a negligible regulatory effect on the photocurrent variability. A comparative variability analysis has been conducted for the phototransduction in mouse and salamander, including a study of the effects of their anatomical differences such as incisures and photoreceptors geometry on variability generation and suppression.
尽管激活级联反应存在许多随机成分，包括随机激活位点、激活受体的随机游走以及其在随机步数中的淬灭，但脊椎动物光转导中的单光子反应具有高度可重复性。在这里，我们使用先前生成并经过测试的时空数学和计算模型来确定变异性降低的可能机制。该模型允许将过程分为多个模块，并分别分析它们的影响。我们表明，激活级联反应负责变异性的产生，而第二信使的扩散负责变异性的抑制。激活位点的随机性在早期对光反应的变异系数有贡献，而光异构化视紫红质（Rh*）的布朗运动路径影响可忽略不计。变异性的主要驱动因素是Rh的关闭机制，这基本上发生在Rh的前2-4个磷酸化状态内。理论上增加淬灭步骤的数量并不会显著降低效应器相应的变异系数，这与受体磷酸化状态的生化限制一致。第二信使在细胞质中的扩散起到了抑制激活级联反应产生的变异性的作用。钙反馈对光电流变异性的调节作用可忽略不计。我们对小鼠和蝾螈的光转导进行了比较变异性分析，包括研究它们的解剖差异（如切迹和光感受器几何形状）对变异性产生和抑制的影响。
