Suss E, Barash S, Stavenga D G, Stieve H, Selinger Z, Minke B
Department of Physiology, Hadassah Medical School, Hebrew University, Jerusalem, Israel.
J Gen Physiol. 1989 Sep;94(3):465-91. doi: 10.1085/jgp.94.3.465.
The Drosophila and Lucilia photoreceptor mutants, trp and nss, respond like wild-type flies to a short pulse of intense light or prolonged dim light; however, upon continuous intense illumination, the trp and nss mutants are unable to maintain persistent excitation. This defect manifests itself by a decline of the receptor potential toward baseline during prolonged intense illumination with little change in the shape or amplitude of the quantal responses to single photons (quantum bumps). Previous work on the trp and nss mutants suggests that a negative feedback loop may control the rate of bump production. Chemical agents affecting different steps of the phototransduction cascade were used in conjunction with light to identify a possible branching point of the feedback loop and molecular stages which are affected by the mutation. Fluoride ions, which in the dark both excite and adapt the photoreceptors of wild-type flies, neither excite nor adapt the photoreceptors of the trp and nss mutants. The hydrolysis-resistant analogue, GTP gamma S, which excites the photoreceptors of wild-type flies, resulting in noisy depolarization, markedly reduces the light response of both mutant flies. Intracellular recordings revealed, however, that the inhibitory effect of GTP gamma S on the nss mutant was accompanied neither by any significant depolarization nor by an increase in the noise, and thus was very different from the effect of a dim background light. The combination of inositol trisphosphate and diphosphoglycerate (InsP3 + DPG), which efficiently excites the photoreceptors of wild-type Lucilia, also excites the photoreceptors of nss Lucilia mutant. The InsP3 + DPG together act synergistically with light to accelerate the decline of the response to light in the mutant flies. These results suggest that the fly phototransduction pathway involves a feedback regulatory loop, which branches subsequent to InsP3 production and regulates guanine nucleotide-binding protein (G protein)-phospholipase C activity. A defect in this regulatory loop, which may cause an unusually low level of intracellular Ca2+, severely reduces the triggering of bumps in the mutants during intense prolonged illumination.
果蝇和绿蝇的光感受器突变体trp和nss，在受到短脉冲强光或长时间弱光照射时，其反应与野生型果蝇相似；然而，在持续强光照射下，trp和nss突变体无法维持持续的兴奋状态。这种缺陷表现为在长时间强光照射期间，感受器电位向基线下降，而对单个光子的量子反应（量子突峰）的形状或幅度变化不大。先前对trp和nss突变体的研究表明，负反馈回路可能控制突峰产生的速率。将影响光转导级联不同步骤的化学试剂与光结合使用，以确定反馈回路的可能分支点以及受突变影响的分子阶段。氟离子在黑暗中既能刺激又能使野生型果蝇的光感受器适应，但对trp和nss突变体的光感受器既无刺激作用也无适应作用。抗水解类似物GTPγS能刺激野生型果蝇的光感受器，导致产生嘈杂的去极化，却显著降低了两种突变体果蝇的光反应。然而，细胞内记录显示，GTPγS对nss突变体的抑制作用既没有伴随任何明显的去极化，也没有噪声增加，因此与弱背景光的作用非常不同。肌醇三磷酸和二磷酸甘油酸（InsP3 + DPG）的组合能有效刺激野生型绿蝇的光感受器，也能刺激nss绿蝇突变体的光感受器。InsP3 + DPG与光协同作用，加速突变体果蝇对光反应的下降。这些结果表明，果蝇的光转导途径涉及一个反馈调节回路，该回路在InsP3产生后分支，并调节鸟嘌呤核苷酸结合蛋白（G蛋白）-磷脂酶C的活性。这个调节回路的缺陷可能导致细胞内Ca2+水平异常低，在长时间强光照射期间严重减少突变体中突峰的触发。
