Sato Shinya, Frederiksen Rikard, Cornwall M Carter, Kefalov Vladimir J
Department of Ophthalmology and Visual Sciences,Washington University School of Medicine,Saint Louis,Missouri 63110.
Department of Physiology and Biophysics,Boston University School of Medicine,Boston,MA 02118.
Vis Neurosci. 2017 Jan;34:E004. doi: 10.1017/S0952523817000013.
Vertebrate rod and cone photoreceptors require continuous supply of chromophore for regenerating their visual pigments after photoactivation. Cones, which mediate our daytime vision, demand a particularly rapid supply of 11-cis retinal chromophore in order to maintain their function in bright light. An important contribution to this process is thought to be the chromophore precursor 11-cis retinol, which is supplied to cones from Müller cells in the retina and subsequently oxidized to 11-cis retinal as part of the retina visual cycle. However, the molecular identity of the cis retinol oxidase in cones remains unclear. Here, as a first step in characterizing this enzymatic reaction, we sought to determine the subcellular localization of this activity in salamander red cones. We found that the onset of dark adaptation of isolated salamander red cones was substantially faster when exposing directly their outer vs. their inner segment to 9-cis retinol, an analogue of 11-cis retinol. In contrast, this difference was not observed when treating the outer vs. inner segment with 9-cis retinal, a chromophore analogue which can directly support pigment regeneration. These results suggest, surprisingly, that the cis-retinol oxidation occurs in the outer segments of cone photoreceptors. Confirming this notion, pigment regeneration with exogenously added 9-cis retinol was directly observed in the truncated outer segments of cones, but not in rods. We conclude that the enzymatic machinery required for the oxidation of recycled cis retinol as part of the retina visual cycle is present in the outer segments of cones.
脊椎动物的视杆和视锥光感受器在光激活后需要持续供应生色团以再生其视觉色素。介导我们日间视觉的视锥细胞，需要特别快速地供应11-顺式视黄醛生色团，以便在强光下维持其功能。人们认为，对这一过程的一个重要贡献是生色团前体11-顺式视黄醇，它从视网膜中的穆勒细胞供应给视锥细胞，随后被氧化为11-顺式视黄醛，作为视网膜视觉循环的一部分。然而，视锥细胞中顺式视黄醇氧化酶的分子身份仍不清楚。在这里，作为表征这种酶促反应的第一步，我们试图确定这种活性在蝾螈红色视锥细胞中的亚细胞定位。我们发现，当将分离的蝾螈红色视锥细胞的外段与内段直接暴露于11-顺式视黄醇的类似物9-顺式视黄醇时，暗适应的开始要快得多。相比之下，在用生色团类似物9-顺式视黄醛处理外段与内段时，没有观察到这种差异，9-顺式视黄醛可以直接支持色素再生。令人惊讶的是，这些结果表明顺式视黄醇氧化发生在视锥光感受器的外段。证实这一观点的是，在视锥细胞的截短外段中直接观察到了外源性添加的9-顺式视黄醇的色素再生，但在视杆细胞中没有观察到。我们得出结论，作为视网膜视觉循环一部分的再循环顺式视黄醇氧化所需的酶机制存在于视锥细胞的外段。
