Molecular Mechanisms of determination of the photoresponse characteristics of cones

我们的视觉系统由两部分组成，杆和锥。这些光感受器在它们的光敏感性方面不同，因此杆细胞介导微光视觉，而锥细胞介导日光视觉。视杆细胞和视锥细胞不仅在光敏感性上有区别，而且在其他反应特性上也有区别。视锥细胞的光响应时间过程更短，这大大提高了我们白天视觉的时间分辨率。视杆细胞被明亮的背景光饱和，对更强烈的光线没有反应。相比之下，视锥细胞不饱和，对非常明亮的光线有反应。我们利用纯化的视杆细胞和视锥细胞研究了视杆细胞和视锥细胞之间差异的分子机制。在本研究中，我们研究了视杆细胞视色素对视锥细胞和视杆细胞转导蛋白的激活作用。结果表明，视杆细胞在光刺激下激活视锥细胞转导蛋白的效率与视杆细胞转导蛋白的效率相当，提示视锥细胞转导蛋白的激活效率低是由于视锥细胞转导蛋白的活性低所致，短暂闪光后视锥细胞光反应的恢复要快得多。这表明鸟苷酸环化酶的cGMP合成活性在视锥细胞中更高。为了证明这一点，我们直接测量鸟苷酸环化酶活性。结果表明，活性在球果中高约30倍。GCAP蛋白增强环化酶活性。我们确定了GCAP在视杆细胞和视锥细胞中的亚型表达模式，并确定了这些GCAP的表达量。通过这些测定，我们现在以定量的方式了解了视杆细胞和视锥细胞中的cGMP合成。

Our visual system consists of two components, rods and cones. These photoreceptors differ in their light-sensitivity so that rods mediate twilight vision and cones mediate daylight vision. Rods and cones are not only distinguished in the light-sensitivity but also in other response characteristics. The photoresponse time course is much briefer in cones, which improves the time resolution of our daylight vision greatly. Rods are saturated with bright background light and do not respond to more intense light. In contrast, cones are not saturated and respond to very bright light. By using purified rods and cones, we examined the molecular mechanisms underlying these differences between rods and cones.We previously showed that, the activation efficiency of transducin by visual pigment is much lower in cones. In the present study, we examined the activation of cone and rod transducin by rod visual pigment. The result showed that rod visual pigment, when it is activated by light, activated cone transducin at a similar efficiency as rod transducin, The result, therefore suggested that the low activation efficiency of cone transducin by cone visual pigment is due to low potency of cone transducin.The recovery of photoresponse after a brief light flash is much faster in cones. This suggests that the cGMP synthetic activity of guanylate cyclase is higher in cones. To prove it, we measured directly the guanylate cyclase activity. The result showed that the activity is approximately 30 times higher in cones. The cyclase activity is enhanced by GCAP proteins. We determined the subtype expression patterns of GCAPs in rods and cones, and determined the amount of expression of these GCAPs. From these determinations, we now understand the cGMP synthesis in rods and cones in a quantitative way.