Molecular Mechanisms of determination of the photoresponse characteristics of cones

我们的视觉系统由两部分组成：视杆细胞和视锥细胞。这些光感受器的感光性不同，因此杆状体介导黄昏视觉，锥状体介导日光视觉。视杆细胞和视锥细胞不仅在光敏性上有区别，而且在其他反应特征上也有区别。视锥细胞的光响应时间过程要短得多，这大大提高了我们白天视觉的时间分辨率。杆状细胞充满明亮的背景光，对更强的光没有反应。相比之下，视锥细胞不饱和，对非常明亮的光有反应。通过纯化的视杆细胞和视锥细胞，我们研究了视杆细胞和视锥细胞之间这些差异的分子机制。我们之前的研究表明，视色素对转导蛋白的激活效率在视锥细胞中要低得多。在本研究中，我们研究了视杆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，杆状视色素在光的作用下，对视锥转导素的激活效率与杆状视色素相似，因此，视锥视色素对视锥转导素的激活效率较低是由于视锥转导素的效力较低。在短暂的闪光后，锥细胞的光响应恢复要快得多。说明球果中鸟苷酸环化酶的cGMP合成活性较高。为了证明这一点，我们直接测量了鸟苷酸环化酶的活性。结果表明，锥体的活性大约高30倍。GCAP蛋白可增强环化酶活性。我们确定了gcap在杆状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。从这些测定中，我们现在可以定量地了解棒状细胞和视锥细胞中cGMP的合成。

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.