Nir I, Papermaster D S
Department of Pathology, University of Texas Health Science Center, San Antonio.
Prog Clin Biol Res. 1989;314:251-64.
Opsin is normally sequestered predominantly in the outer segment disc and plasma membranes of adult photoreceptors. Absence of opsin from the inner segment plasma membrane in normal photoreceptors is probably not due to the inability of the inner segment plasma membrane to retain opsin. Rather, in the adult mammalian retina, if opsin is inserted at sites in the apical inner segment plasma membrane, in a fashion comparable to the pathway in amphibians, it is rapidly transported predominantly to the outer segment by unknown mechanisms. Dystrophic rds retinas, lacking an outer segment, display newly synthesized opsin throughout the plasma membrane. If opsin is transported to the inner segment plasma membrane as a specific insertional site, diffusion in the plane of the membrane may redistribute opsin throughout the plasma membrane which encloses the nucleus and the synaptic terminal. Alternatively, opsin may be inserted randomly throughout the entire cell's plasmalemma beneath the cilium. Selective transport to the outer segment may preferentially clear the inner segment of most of its opsin and nearly clear the perikaryal and synaptic terminal's plasmalemma in normal cells. In dystrophic retinas, however, as outer segments degenerate or fail to form, opsin is detected readily in the remaining plasma membrane sites. In the rd mouse, some of the opsin molecules in the inner segment plasma membrane might be newly synthesized while others may arise from molecules which reached the inner segment by back-diffusion from the outer segment at least at early stages in the degeneration while outer segments survive. The opsin in the plasma membrane which envelopes the residual rod nuclei and synaptic terminals in dystrophic retinas may account for the persisting light perception in retinas which have lost both the rod outer and inner segments. Dystrophic retinas, such as the rd mouse and RCS rats and possibly human RP retinas, in which cone nuclei survive long after rods disappear, might retain light perception because of cone photo-pigments in the outer nuclear and outer plexiform layers. To explore these questions further, the localization of other components of the transduction cascade and the determination of the efficiency of their coupling in dystrophic cells is necessary. We need to know where the cyclic GMP-sensitive sodium channels lie in these dystrophic cells and the cellular requirements for proximity of these components to generate a signal. Outer segment-free photoreceptors, bearing opsin in their plasma membranes, resemble other cells which have receptor-mediated alterations in membrane permeability to ions.(ABSTRACT TRUNCATED AT 400 WORDS)
视蛋白通常主要隔离于成年光感受器的外段盘膜和质膜中。正常光感受器内段质膜中缺乏视蛋白，可能并非由于内段质膜无法保留视蛋白。相反，在成年哺乳动物视网膜中，如果视蛋白以与两栖动物途径类似的方式插入顶端内段质膜的位点，它会通过未知机制迅速主要转运至外段。缺乏外段的营养不良性视网膜（rds视网膜）在整个质膜中显示出新合成的视蛋白。如果视蛋白作为特定的插入位点转运至内段质膜，在膜平面内的扩散可能会使视蛋白重新分布于包围细胞核和突触终末的整个质膜中。或者，视蛋白可能随机插入纤毛下方整个细胞的质膜中。向外段的选择性转运可能优先清除内段中的大部分视蛋白，并几乎清除正常细胞中核周和突触终末质膜中的视蛋白。然而，在营养不良性视网膜中，随着外段退化或未能形成，在剩余的质膜位点很容易检测到视蛋白。在rd小鼠中，内段质膜中的一些视蛋白分子可能是新合成的，而其他分子可能至少在退化早期外段尚存时，由从外段反向扩散至内段的分子产生。营养不良性视网膜中包围残留杆状细胞核和突触终末的质膜中的视蛋白，可能解释了在失去杆状外段和内段的视网膜中持续存在的光感知。营养不良性视网膜，如rd小鼠和RCS大鼠，可能还有人类视网膜色素变性（RP）视网膜，其中在杆状细胞消失后很长时间锥体细胞核仍存活，可能由于外核层和外网状层中的锥体细胞光色素而保留光感知。为了进一步探讨这些问题，有必要确定转导级联反应其他成分的定位及其在营养不良性细胞中偶联效率。我们需要知道环鸟苷酸敏感钠通道在这些营养不良性细胞中的位置，以及这些成分产生信号时细胞对其接近程度的要求。质膜中带有视蛋白的无外段光感受器类似于其他具有受体介导的膜对离子通透性改变的细胞。（摘要截短于400字）
