Beelen Charlotte Johanna, Asteriti Sabrina, Cangiano Lorenzo, Koch Karl-Wilhelm, Dell'Orco Daniele
Department of Neuroscience, Division of Biochemistry, University of Oldenburg, 26111 Oldenburg, Germany.
Department of Translational Research, University of Pisa, Pisa 56123, Italy.
Comput Struct Biotechnol J. 2021 Jun 23;19:3720-3734. doi: 10.1016/j.csbj.2021.06.033. eCollection 2021.
The phototransduction cascade is paradigmatic for signaling pathways initiated by G protein-coupled receptors and is characterized by a fine regulation of photoreceptor sensitivity and electrical response to a broad range of light stimuli. Here, we present a biochemically comprehensive model of phototransduction in mouse rods based on a hybrid stochastic and deterministic mathematical framework, and a quantitatively accurate description of the rod impedance in the dark. The latter, combined with novel patch clamp recordings from rod outer segments, enables the interconversion of dim flash responses between photovoltage and photocurrent and thus direct comparison with the simulations. The model reproduces the salient features of the experimental photoresponses at very dim and bright stimuli, for both normal photoreceptors and those with genetically modified cascade components. Our modelling approach recapitulates a number of recent findings in vertebrate phototransduction. First, our results are in line with the recently established requirement of dimeric activation of PDE6 by transducin and further show that such conditions can be fulfilled at the expense of a significant excess of G protein activated by rhodopsin. Secondly, simulations suggest a crucial role of the recoverin-mediated Ca-feedback on rhodopsin kinase in accelerating the shutoff, when light flashes are delivered in the presence of a light background. Finally, stochastic simulations suggest that transient complexes between dark rhodopsin and transducin formed prior to light stimulation increase the reproducibility of single photon responses. Current limitations of the model are likely associated with the yet unknown mechanisms governing the shutoff of the cascade.
光转导级联反应是由G蛋白偶联受体启动的信号通路的典范，其特点是对光感受器的敏感性和对广泛光刺激的电反应进行精细调节。在此，我们基于混合随机和确定性数学框架，提出了一个小鼠视杆细胞光转导的生化综合模型，以及对暗态下视杆细胞阻抗的定量准确描述。后者与视杆细胞外段的新型膜片钳记录相结合，能够实现光电压和光电流之间暗闪光反应的相互转换，从而直接与模拟结果进行比较。该模型再现了正常光感受器以及具有基因修饰级联成分的光感受器在非常暗和亮刺激下实验光反应的显著特征。我们的建模方法概括了脊椎动物光转导中的一些最新发现。首先，我们的结果符合最近确定的转导蛋白对PDE6二聚体激活的要求，并进一步表明这种情况可以通过以视紫红质激活的大量过量G蛋白为代价来实现。其次，模拟结果表明，当在光背景下施加闪光时，恢复蛋白介导的对视紫红质激酶的钙反馈在加速关闭过程中起关键作用。最后，随机模拟表明，在光刺激之前形成的暗视紫红质和转导蛋白之间的瞬时复合物增加了单光子反应的可重复性。该模型目前的局限性可能与控制级联反应关闭的未知机制有关。
