Peretz A, Sandler C, Kirschfeld K, Hardie R C, Minke B
Department of Physiology, Hadassah Medical School, Hebrew University, Jerusalem, Israel.
J Gen Physiol. 1994 Dec;104(6):1057-77. doi: 10.1085/jgp.104.6.1057.
Invertebrate photoreceptors use the inositol-lipid signaling cascade for phototransduction. A useful approach to dissect this pathway and its regulation has been provided by the isolation of Drosophila visual mutants. We measured extracellular changes of Ca2+ [delta Ca2+]o in Drosophila retina using Ca(2+)-selective microelectrodes in both the transient receptor potential (trp) mutant, in which the calcium permeability of the light-sensitive channels is greatly diminished and in the inactivation-but-no-afterpotential C (inaC) mutant which lacks photoreceptor-specific protein kinase C (PKC). Illumination induced a decrease in extracellular [Ca2+] with kinetics and magnitude that changed with light intensity. Compared to wild-type, the light-induced decrease in [Ca2+]o (the Ca2+ signal) was diminished in trp but significantly enhanced in inaC. The enhanced Ca2+ signal was diminished in the double mutant inaC;trp indicating that the effect of the trp mutation overrides the enhancement observed in the absence of eye-PKC. We suggest that the decrease in [Ca2+]o reflects light-induced Ca2+ influx into the photoreceptors and that the trp mutation blocks a large fraction of this Ca2+ influx, while the absence of eye specific PKC leads to enhancement of light-induced Ca2+ influx. This suggestion was supported by Ca2+ measurements in isolated ommatidia loaded with the fluorescent Ca2+ indicator, Ca Green-5N, which indicated an approximately threefold larger light-induced increase in cellular Ca2+ in inaC relative to WT. Our observations are consistent with the hypothesis that TRP is a light activated Ca2+ channel and that the increased Ca2+ influx observed in the absence of PKC is mediated mainly via the TRP channel.
无脊椎动物的光感受器利用肌醇脂质信号级联进行光转导。果蝇视觉突变体的分离为剖析这一信号通路及其调控提供了一种有用的方法。我们使用钙离子选择性微电极，在瞬时受体电位（trp）突变体（其光敏感通道的钙通透性大大降低）和缺乏光感受器特异性蛋白激酶C（PKC）的失活但无后电位C（inaC）突变体的果蝇视网膜中，测量了细胞外Ca2+的变化（[δCa2+]o）。光照诱导细胞外[Ca2+]降低，其动力学和幅度随光强度而变化。与野生型相比，trp突变体中光诱导的[Ca2+]o降低（Ca2+信号）减弱，但在inaC突变体中显著增强。在inaC;trp双突变体中，增强的Ca2+信号减弱，这表明trp突变的影响超过了在没有眼PKC时观察到的增强作用。我们认为，[Ca2+]o的降低反映了光诱导的Ca2+流入光感受器，trp突变阻断了大部分这种Ca2+流入，而缺乏眼特异性PKC则导致光诱导的Ca2+流入增强。用荧光Ca2+指示剂Ca Green-5N加载的分离小眼的Ca2+测量结果支持了这一观点，该结果表明，相对于野生型，inaC中光诱导的细胞内Ca2+增加约三倍。我们的观察结果与以下假设一致：TRP是一种光激活的Ca2+通道，在没有PKC的情况下观察到的Ca2+流入增加主要是通过TRP通道介导的。
