Biochemistry of visual pigment regeneration: the Friedenwald lecture.
Biochemistry of visual pigment regeneration: the Friedenwald lecture.
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发表时间：
2000-02
影响因子：
4.4
通讯作者：
J. C. Saari
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作者：
J. C. Saari
光传导和视觉周期在脊椎动物视觉中起着互补的作用。光转导是由结合到视蛋白的11-顺式-视黄醛的光异构化引发的，并最终导致感光细胞释放神经递质的变化。视觉循环将光异构化产物全反式视黄醛恢复为11顺式构型，并允许漂白的视觉色素再生（图1）。在过去的20年中，光转导的生化机制已被广泛研究，因此，该过程作为一般理解G蛋白偶联受体的范例。相比之下，对视觉循环的分子理解发展得很差，许多关于反应，酶和控制机制的基本问题仍然没有答案。编码三种视觉循环酶的cDNA序列已经发表;然而，对于该循环的其他三种推定酶，包括视黄醇异构酶（异构水解酶），分子信息是不可用的。据报道，视色素的再生是一个缓慢的过程，视紫红质中11-顺式-视黄醛的光异构化非常迅速。事实上，人类完全适应黑暗需要大约40分钟，而视紫红质转化为光视紫红质只需要200 fsec。然而，这不是一个公平的比较，因为很明显，在环境照明水平下，活体眼睛中的光解和再生速率必须相等且符号相反。任何其他情况都不会与视觉兼容，因为视觉色素会迅速消散。Alpern已经证明，对于人类视杆细胞和视锥细胞，在不同的生理照明水平下，存在稳定的漂白视色素水平，其中漂白和再生速率相等且符号相反。在人类中，当显示在半对数图上时，视觉阈值恢复和视色素再生的进展曲线重合。对于这种对数线性关系的分子解释还没有很好的理解，这种关系可能是偶然的，但大多数人都同意，一种光产物是视觉系统脱敏的原因。尽管脱敏中间体的分子身份仍然是一个积极研究的问题，但很明显，视觉周期反应在确定漂白视色素的稳态水平以及视网膜的敏感性方面是重要的。视网膜色素上皮（RPE）在视色素再生中的关键作用从19世纪世纪研究者的研究中显而易见，他们证明解剖的青蛙视网膜只有在与RPE接触时才能再生其漂白的视色素。读者可以参考Marmor和Martin对他们一些有见地的实验的描述。幸运的是，早期的生理学家使用青蛙的眼睛进行实验，因为啮齿动物的眼睛在从动物身上移除时不会再生它们的视觉色素，这将在后面讨论。50年后，Wald使用提取技术证明维生素A参与视觉过程，并制定了包括RPE参与的第一个现代版本的视觉周期（图2）。在Dowling的经典研究之后，RPE在视觉周期中的作用变得更加清楚，22该研究证明了类维生素A在广泛漂白期间从神经视网膜移出并进入RPE，并在黑暗中恢复期间返回。后来，其他研究人员使用提供更高分辨率的技术进行了研究，验证了基本观察结果。伯恩斯坦等人和兰多对RPE的必要性提供了分子解释，证明11-顺式构型的关键酶促再生发生在该组织内。考虑到这一过程的解剖结构，类维生素A在漂白和再生过程中的跨细胞迁移就更加引人注目了（图3）。相对不溶性的类维生素A必须离开视盘膜，扩散通过胞质隔室到达视杆细胞外节的质膜，穿过质膜，扩散通过视网膜下腔到达RPE细胞的质膜，进入该细胞的视觉周期反应，并返回！脊椎动物视杆细胞视觉周期反应的当前工作假设以示意图形式显示在图4中。该图描绘了RPE和视杆光感受器细胞的内膜和质膜以及分隔这两个细胞的光感受器间基质空间（视网膜下空间）。RPE中的视觉循环酶都与细胞膜有关，然而，它们在亚细胞区室的定位来自华盛顿大学医学院眼科和生物化学系，西雅图，华盛顿。部分由美国国立卫生研究院赠款RO 1 EY 02317、EY 01730和EY 09339以及预防失明研究的无限制奖励支持。JCS是预防失明研究的高级科学研究员。1999年8月6日提交出版; 1999年8月30日接受。商业关系政策：N。通讯作者：John C. Saari，Department of Ophthalmology，Box 356485，University of华盛顿，西雅图，WA 98195-6485. jsaari@u.washington.edu
Phototransduction and the visual cycle play complementary roles in vertebrate vision. Phototransduction is initiated by the photoisomerization of 11-cis-retinal bound to opsin and ultimately results in a change in the release of neurotransmitter by photoreceptor cells. The visual cycle restores the product of photoisomerization, all-trans-retinal, to the 11-cis configuration and allows the regeneration of bleached visual pigments (Fig. 1). The biochemical mechanism of phototransduction has been extensively studied during the past 2 decades, and as a result, the process serves as the paradigm for understanding G-protein–coupled receptors in general. In contrast, molecular understanding of the visual cycle is poorly developed, and many fundamental questions regarding reactions, enzymes, and control mechanisms remain unanswered. Sequences of cDNAs encoding three visual cycle enzymes have been published; however, molecular information is unavailable for the other three presumed enzymes of the cycle, including retinol isomerase (isomerohydrolase). It has been reported that regeneration of visual pigments is a slow process and that photoisomerization of 11-cis-retinal in rhodopsin is very rapid. In fact, complete dark adaptation in humans requires approximately 40 minutes, and conversion of rhodopsin to photorhodopsin requires only 200 fsec. However, this is not a fair comparison, because it is clear that the photolysis and regeneration rates in the living eye must be equal and opposite in sign at ambient levels of illumination. Any other situation would not be compatible with vision, because visual pigment would dissipate rapidly. Alpern has demonstrated for human rods and cones that a steady state level of bleached visual pigments, in which the bleach and regeneration rates are equal and of opposite sign, is present at different levels of physiologic illumination. In humans the progress curves for the regain of visual threshold and for the regeneration of visual pigment coincide when displayed on a semi–log plot. The molecular explanation for this log–linear relationship is not well understood, and the relationship may be fortuitous, but most agree that a photoproduct is responsible for desensitization of the visual system. Although the molecular identity of the desensitizing intermediate(s) remains a matter of active investigation, it is clear that visual cycle reactions are important in determining the steady state level of bleached visual pigment and thus the sensitivity of the retina. The critical role of the retinal pigment epithelium (RPE) in visual pigment regeneration is apparent from the studies of 19th century investigators who demonstrated that dissected frog retina could regenerate its bleached visual pigment only when in contact with the RPE. The reader is referred to Marmor and Martin for a depiction of some of their insightful experiments. It is fortunate that the early physiologists used frog eyes for their experiments because rodent eyes do not regenerate their visual pigments when removed from the animal, as will be discussed later. Fifty years later, Wald used extraction techniques to show that vitamin A was involved in the visual process and formulated the first modern version of the visual cycle including the participation of the RPE (Fig. 2). The role of the RPE in the visual cycle became more clear after the classic study by Dowling, 22 which demonstrated movement of retinoid out of the neural retina and into RPE during extensive bleaching and return during recovery in the dark. Later studies by other investigators with techniques offering more resolution verified the fundamental observation. Bernstein et al. and Rando provided a molecular explanation for the necessity of the RPE with the demonstration that the critical enzymatic regeneration of the 11-cis configuration occurred within this tissue. The transcellular migration of the retinoids during bleaching and regeneration is all the more remarkable, considering the anatomy of the journey (Fig. 3). The relatively insoluble retinoid must leave the disc membranes, diffuse through a cytosolic compartment to reach the plasma membrane of the rod outer segment, traverse the plasma membrane, diffuse across the subretinal space to reach the plasma membrane of the RPE cell, enter into the reactions of the visual cycle in this cell, and make the return journey! A current working hypothesis for the reactions of the vertebrate rod visual cycle is shown in schematic form in Figure 4. The figure depicts internal and plasma membranes of the RPE and rod photoreceptor cells and the interphotoreceptor matrix space (subretinal space) separating these two cells. The visual cycle enzymes in RPE are all associated with membranes; however, their localization to subcellular compartFrom the Departments of Ophthalmology and Biochemistry, University of Washington School of Medicine, Seattle, Washington. Supported in part by National Institutes of Health Grants RO1 EY02317, EY01730, and EY09339 and by unrestricted awards from Research to Prevent Blindness. JCS is a Senior Scientific Investigator of Research to Prevent Blindness. Submitted for publication August 6, 1999; accepted August 30, 1999. Commercial relationships policy: N. Corresponding author: John C. Saari, Department of Ophthalmology, Box 356485, University of Washington, Seattle, WA 98195-6485. jsaari@u.washington.edu